The Horsehead Project:
SKETCHING THE HORSEHEAD
...being a somewhat unorthodox approach to deep sky object sketching as data, and for confirmation of the observer's perception of faintest and most challenging objects, rather than a method for achieving 'artistic results' to publish in like company. An analysis of why we have found some Horsehead sketches on the web to be perplexingly unreal, with mistaken proportions and relative size compared to the asserted true fields and marker stars sketched. - SRW, 2012
Eyepiece Sketching Helps Train One's Perceptions
Sketching the Horsehead and other faint nebulae, in ways that serve to help preserve the individual's 'experience in the moment', can improve one's viewing perceptions and deep sky observing techniques. No matter how crude the sketch, something concrete that one creates to document an observation can at least be of use to the person who makes it (if not public viewers over the Net.) There are many discussions on the Net -- as subdivisions of interest on popular astroforums -- that allow public posts of sketches, so that friends and colleagues may contribute what one hopes are constructive suggestions. Or, the beginner may use these for good examples from practiced astro artists, and to ask questions. We would rather not give specific links to such venues; merely try entering the phrase "astronomical sketching forum" in any effective internet search engine, and you immediately find the URLs
The present author feels that it's important to stress that artistic talent is not a pre-requisite. The important thing is to try to put on paper your perceptions of (a) the stars you can see in your eyepiece field -- their relative brightness (indicated by the size of their circular forms, the typical method employed by star charts to indicate stellar magnitude) -- and their relative positions; (b) the differences of contrast -- the full dynamic range between light and dark, and especially the difference between the 'luminous ground' or general interstellar medium, and where YOU see a nebula 'fading' in or out from this general level; and (c) the density of nebulosity, indicated by how strongly you draw the 'glow'.
As shown in a photograph of a typical sketch-card that master-sketcher Jaakko Saloranta supplied us, one naturally has to work in 'negative' mode, drawing brights as being darks using a sketching pencil on white paper. This is the better way of distinctly showing where you distinguish between 'something' as opposed to 'nothing' (the general sky luminous ground.) Then, when you have finished, it's easy, if you have the equipment, to scan the sketch and invert it: to a black background with light above the dark, representing the stars as vivid white points, and the nebulosity as either delicate marks or perhaps a smudged gentle gradation.
Don't worry if this process does not seem to reveal to you that you have a 'gift' or particular talent. You aren't starting out to compete with highly trained sketchers, but instead to record your own private experiences, so that you may look at them later, in the daytime, and compare them to plots by astronomy star chart programs, actual photos or digital sky images, and perhaps also to other experienced amateurs' 'artistic' sketches.
"A picture is worth a thousand words."
Not only will you get better in recording your perceptions accurately, as time goes on; but also you will improve your perceptual skills. To give a relevant remark from fictional literature that is often true in real life: "You see, but you do not observe" (Sherlock Holmes to Dr. Watson, in "The Scandal in Bohemia".) The acutely-trained master detective could SEE and then PERCEIVE clues; Dr. Watson could only SEE...but he was often quite oblivious to the significance of what he saw.
Furthermore, what one thinks he or she "sees" may not be at all accurate. What prompted us to create and post this article was encountering a Horsehead sketch by an artist we won't name; the 'neck' of B33 was drawn -- when we carefully measured it against the actual full FOV of the eyepiece/inexpensive small scope employed -- about 50% longer than figures given by professional astronomical catalogues, or our own measurements of the latest state of the art multiband digital images. The telescope employed was, shall we say vaguely, less than 8" aperture (used not in a deeply rural venue but rather just on the outskirts of a very large city!)
This exaggerated recreation of the Horsehead's dimensions fails to agree with any authoritative references or with the best sketches of world-class experts; one can only assume that the cause of the misperception was a combination of misunderstanding the optical FOV of a cheap starter-grade instrument, plus the "averted imagination" or "received wisdom" that persuaded the artist to 'see' the virtually impossible-to-perceive under such circumstances. For, superior and accurate drawings of the Horsehead, done with scopes of two or three times the aperture, show nothing approaching the alleged size and dimensional relationships. Yet, we found quite a bit of 'attaboy' praise from fellow observers on the forum, who (we suspect) must have shared the same naivete. Whereas, other drawings by the same artist, depicting not a challenging dark nebula like B33, but rather commonplace Messier objects, seemed relatively credible, if nevertheless tending to show "more" than common experiences widely recorded with the same aperture: so perhaps the observer suffered somewhat from a systematic error of struggling too hard to 'see' details shown by deeply exposed photographs that had caused bias and preconception.
If one intends the drawings of faint celestial objects to be something more than "pretty pictures", then surely it is important to develop not only a precise knowledge of telescope limiting magnitude and FOV performance, but also the personal ability to avoid being biased by existing photographic images -- frequently made with wavelengths that the dark adapted eye cannot even perceive! -- and to resist embellishment. As to the former, we have posted an article about self-testing one's personal dimmest perception in a telescope here: "How Faint Can You Go?"; and regarding the latter problem of avoiding delusion, this list of suggestions for "Dealing with Delusion". We have also written a short tutorial on "EYEPIECE TRUE FIELD:
Calculations versus Drift Measurements."
Some Online References About Astronomical Sketching
Our purpose in this article is not to give a step-by-step account of method, but rather to focus on some things that "artistic" people may not care much about: the technical performance of astronomical optics in producing a field of view of the sky: how it may be distorted by the equipment you use, and ways you can check this using simple processes to predict the magnification/field of your eyepieces, and to compare results against standards (charts, deep sky images.) The Horsehead is a particular subject of our study, and because it is so hard to quantify (often requiring averted vision) there is a large variation in its size attributes, judging from many published drawings. "Draw what you see" is probably the safest advice, but "what you see" might be a poor impression, affected by your lack of viewing technique, or incorrect expectations that are based on some opinion of an 'expert' who has contributed casual remarks on a forum.
If you are already doing some form of sketching, you needn't start at the basics; but if you haven't tried to do it, here are some online tutorials:
• Want it all summarized on one page? Look no further than Sketching at the Telescope, a brief article on the large astronomical website of Math Heijen.
• Another very quick read which packs much information into few words is the article Sketch the Skies and Improve your Eyes by Mark Mark Deprest of the club "University Lowbrow Astronomers" (who are all the antithesis of 'lowbrow' in the pejorative sense.)
• Carol Lakomiak, well known astrosketching columnist of BBC's Sky at Night magazine, presents basic advice on how to render shading in this tutorial on the website of the Royal Astronomical Society of Canada (which hosts an AstroSketchers Group.)
• Sketching Tutorial by Buddy Barbee, featuring some very good astro drawings -- if you copy them into a graphics program and reverse the image colors so that they are shown in black on white; otherwise they may be very hard to see on most computer displays.
• Jaakko Saloranta has produced a Tutorial for Astronomical Drawing that shows how he created a highly detailed and realistic sketch of open cluster NGC 1664. Unfortunately his old website is now offline, but we reconstructed his article from his original text plus later posts, using archived backup images. Read it here.
• Drawing the deepsky by Fred Hissink: very straightforward and concise information, which -- rather strangely, given the topic -- tends to be presented only in words, unaccompanied by pictorial examples. He also has separate galleries of personal images.
Hissink has given an interesting commentary on dark nebulae, part of which we don't agree with and will not generally do ourselves, as we highlight in bold: "Drawing a dark nebula means drawing a lot of stars to show the appearance of the object. But, thatís a terrible task, especially when the field is crowded with bright and faint lights. Some observers prefer to print the starfield and draw the nebula behind the eyepiece. Itís a good option so you can save time for drawing the dark nebula. If youíre prepared to draw the field stars at the eyepiece, look closely at the distribution of the stars in the field. Pay attention to the dark areas in the field and try to discover the shape of the dark nebula." But... in our opinion, Fred's suggestion of drawing on a pre-printed star chart cannot possibly help you to learn how to perceive and place the stars and object accurately, and it won't prevent 'false impressions' or 'manufactured data' as you will be working with a biased expectation, based on the chart -- though one can at least use this technique from time to time, as the author has done here, in order to determine precisely which faint stars near "Hoag's Object" could be detected.
• Sketching DSO's using the Mellish Technique, as summarized and described by Alexander Massey, on the astro-forum "IceInSpace". This is a particular method of making 'photorealistic' white markings on black paper which the present author finds rather controversial: for instance, the creator of the technique -- Scott Mellish -- asserts that "if you need averted vision to see a detail, then you should use averted vision to see it on your sketch". He apparently proposes using a soft brush, very light pressure, and looking away from the sketched result as you work. We should think that unless you are observing something as faint as the Horsehead nebula IN AN UTTERLY DARK ENVIRONMENT with virtually NO artificial lights -- so dark that you can't walk around without banging into things -- that looking away from the paper is likely to start degrading dark adaptation, so that further glimpses of the Horsehead with anything but a gigantic aperture telescope under the most efficient viewing conditions will prevent you from holding repeatable detail. His sketch of emission nebulosity in M-16 is pleasing (if you invert it to negative mode of white background; otherwise it's so very dim that little beyond the core is barely viewable on most monitors--and the amount of nebulosity he records, with a 12.5" scope, is quite a bit less than the present author can usually discern in a 4" scope.) So we offer this article with the caveat that you could try the technique, but that perhaps it might not always work out in practice. Perhaps one might prefer to start first with more conventional methods.
Models of accurate sketching technique (just two examples out of many one could choose):
• (1) "Realistic" mode (white stars, black background): Astronomical Sketching Resources including tutorials, videos, and gallery, on the "Belt of Venus" website by Arizona sketching expert Jeremy Perez;
NOTE: previously we suggested here, as examples of 'realism', the work of Japanese artist Kiminori Ikebe, but unfortunately his fine (and quite old) website is now offline; and the Internet Archive Wayback Machine has a text-only copy, leaving out his multitudes of superb astro drawings. - 8.1.14
• (2) "Cartographical" mode (black stars on white background): Jaakko Saloranta of Vantaa, Finland (who formerly had at least four different websites on line with over 1100 astronomical sketches and object reports, up to spring 2013. His specific observations and drawings continue to be available by means of the Deep Sky Archive, which has an accessible diretory hierarchy of his sketch folders at this URL. Many of his sketches have been incorporated into our deep sky object articles on our own observing blog.) We have reconstructed from archival backups his Astronomical Drawing Tutorial.
We are now pleased to report that Jaakko's main website IS indeed back online, once again being maintained and updated with new experiences and drawings: at http://www.kolumbus.fi/jaakko.saloranta/. He has also restored his blogspot website, Breath on a Mirror, containing his interesting articles as deep-sky observing columnist for the Finnish astronomy magazine "Tähdet ja avarus" (Stars and Space).
If you study the work of these two artists, you will be working from our suggested models of how to produce accurate sketches, and how to present them in a rendering that will look intelligible on anybody's computer monitor.
Advice Sometimes Left Out of Sketching Tutorials
Since we would rather address the private sketcher than one who wishes to publish her works for other amateurs -- you may, if you like; but it's always wise to get long experience before doing so, to avoid the pathological criticisms! -- it is perhaps important to focus on one problem that is not well understood by some, who haven't done a thorough mathematical analysis of their telescope and eyepiece collection.
To wit: some sketches we find on the net are not done and reported credibly, because the stated magnification and eyepiece field of view (sometimes called "true field" -- valid only if it's indeed "true"!) don't always match up with the actual angular dimensions, obtained from accurate references. This is caused, we intuit, because some amateurs have not done a careful compilation of the optical performance of their equipment, and therefore do not really know the so-called 'true field'; haven't fully conceptualized the size of particular astronomical objects (i. e., their apparent angular diameter); and cannot relate that to the true field they are perceiving through any of their eyepieces.
It is absolutely essential to make a list of all your eyepieces, for each scope you use, and to record the achieved FOV (field of view) for each one.
An astronomical eyepiece of any quality -- above those practically worthless ones in $50 'toystore' scopes -- will be sold with manufacturer's information of two critical parameters: focal length (in millimeters, mm), and apparent field (in degrees.) There are innumerable tutorials on the net explaining this; we've created a very simple one in our beginners' section which includes a summary of typical designs and their specifications, plus a rudimentary chart of eyepiece lens configurations, here.
The Horsehead Project experimenters have found that sometimes mfr. eyepiece specs are not exactly correct; these devices are, after all, commercial mass-produced products, and those which are in the lower priced range may have a somewhat wider range of tolerance than costly premium models. You might tend to expect deviations of as much as 10-15% in the focal length and apparent field. This is not 'tragic' or anything of a scandal; but outlying values may affect the difference between ACTUAL true field, and ESTIMATED true field (or, as we prefer to call the latter, avoiding the word 'true', the estimated eyepiece visual field.)
Similarly, only the most expensive telescopes have accurate focal length figures supplied by the maker, good to the millimeter! Low-end mass-produced instruments use objectives that are machine-ground and polished, some not even being hand-figured and fine corrected to eliminate curvature and surface errors. This may sound bad; but the progress made in this regard, during the last generation, has been remarkable. Sometimes inexpensive telescopes have surprisingly good optics, easily achieving 'diffraction limited' performance when properly aligned. But, any given scope might deviate slightly from the published focal length: perhaps by as much as 5 to 10%.
Magnification (plus all the interactively- related parameters) will vary a bit with some eyepieces in Schmidt-Cassegrain scopes, set up a bit differently than as tested and supplied by the maker: for example, if you add a diagonal with a longer path, or an external focuser. The focus is determined by changing the actual focal length (moving the primary mirror) which will make a few percent difference in your result. Some oculars also require a considerably different in/out focal travel compared to the approximate 'norm' (if one can come up with any sort of consensus) so users of SCT's and related complex scopes might be advised to star-drift test their eyepiece, check the resulting field, and then try to work backwards, mathematically, to determine the precise magnification: if you care. We confess that we use our C-11 relatively infrequently, so we just tend to accept the predicted calculations from published specs. We have noticed a very slight difference of just a few percent in field width with some oculars, compared to predicted calculations: perhaps 2-4% worst case, nothing to concern us about. The human eye/brain probably cannot possibly judge, by simply glimpsing into an eyepiece without intensely checking star positions at the edge of the field against references, if the true field is, say, 45 arcminutes...or 49 arcminutes.
All these deviations from target published specs to actual measurements may add up, worst case, to a situation in which your eyepiece field of view is not quite what you'd expect it to be, from mere predictive calculations (we have even found that some well-known star chart software that will purport to show you true field on the screen, for the scope/EP combo that you input, are rather inaccurate.) That's why we have added many explanatory caveats, and 'approximate' signs, to our Free "Eyepiece" Calculation software program. It predicts ideal performance based on the specs you feed in, but according to the old adage garbage in=garbage out, if the numbers are wrong, the output is wrong!
Calculate Your Eyepiece Fields; then Star-Test Your Eyepieces to Confirm
Therefore, the wisest course -- to be able to prepare accurate drawings -- is first to calculate your eyepiece fields, and then to confirm the apparent fields by star drift-testing. The drift test is discussed by the author here; it's time consuming and (to be frank) even we haven't done this with every one we use; only those that over the years we suspect might not be accurately defined by sellers' specs, as many oculars of very high repute, sold by scrupulously honest makers, are quite precisely defined, though outliers can occasionally be found. Yet, even some small variations of FL and AF (focal length and apparent field) might be obtained in lab-quality tests. No matter what those precise figures may be, they do not take into consideration the exact optical performance of your telescope.
1. Write up your own reference scope/EP chart, starting with the headings of each scope and its instrumental focal length and focal ratio.
3. Search for informative reviews of your eyepieces -- by very experienced observers, one hopes! (As an example, we believe that this review by David Knisely is a very good one.) See if any users have complained about inaccurate or exaggerated AF specs in the eyepieces you use; if you find criticisms, star drift test them. (For instance, we did this to determine a published mfr. AF error of about 16% in one series of oculars we happen to own.) And, you should do this anyway for at least a few of your eyepieces, using reputable ones with trustworthy specs, to find out if your scope's published FL and F-Ratio specs are reasonably correct. Testing those telescope specs, directly, may likely require quite specialized methods (even involving partial dismantling of Newtonian types, for example) so we suggest this only to observers who are experienced telescope makers, assemblers, and collimators.)
Your Sketches Needn't Be "Pretty Pictures" -- Start Out With The Intent of "Taking Data"
You do keep an observing log, right?
If so, get in the habit of sketching your perceptions of your most difficult, ambiguous, or doubtful observations: so that you may check them later.
Forget about entering into a popularity contest or competing with other astro-sketchers: there will be plenty of time for that, later, when you have built up the skills!
The author, for example, has done a few sketches that he thinks aren't exactly awful (such as this one of Mars, or this careful panel of McNeil's nebula.) But, in his log book he merely takes the time to do a rudimentary sketch to try to record either a particular detail that seemed to be remarkable --
-- or a more careful account of a faint object, to check later and see if the eyepiece impressions were real, or were 'manufactured' data, a mistake:
In the above picture, we simply scanned the drawing in our logbook, making virtually no changes to it (other than to crop it from the handwritten text.) We 'bifurcate' our vision, using our dominant eye at the eyepiece (sans glasses); the other eye is used to read charts or the laptop screen, while then keeping the dominant eye under a black patch. The aging author uses a small magnifying glass in the field, to improve resolution of text under dim red light while employing his non-dominant eye.
That small hand-held magnifier we have to use helps us make the sketches, not the least by providing a handy way of drawing a circle. That is why, in our logbook drawings, the circle is incomplete -- we make it by tracing around the magnifying lens but are obstructed by the handle. To make sure we don't alter our sketches, we do not even bother to fix this after scanning them. We present them as-is: raw data that we then compare with star charts or astro images. To do that, we have to orient the pictures to agree with our sketch--same size of scale, same rotation, same orientation (i. e., sometimes they have to be reversed L-R if our scope optics have done that.) Then, we draw lines to indicate the items we find on the real picture -- field stars we've shown, for example -- so that we can correlate our sketch with "reality". In comparing our drawings from the last few years, to ones we made, say, in 2005, our errors -- with respect to star positions, and the sense of where a nebula or galaxy was placed -- are now much lower, after seven years of repetition of this method. During only that period of practice, this technique has improved our ability to detect objects, quantify their dimensions, relate them to the field stars, and correlate the entire overall field image details to accurate references. We're actually seeing, with reliability, FAINTER objects (at over 60 years of age) than we could at 50 or 40: primarily, we believe, because prior to 2005 we did not make systematic sketches during our observing.
The proof of our improved accuracy is this example of our sketch of Abell 64 -- full observing report here, with 11" scope, July 9, 2009 -- which we scanned and converted into green marks on a white background. We then did a transparent overlay of a DSS image, colored red, centering the photo of the nebula over the spot in the field where we drew it, so that we could see how our star positions matched with the survey photo. Believe me: our sketches from 2005 could not withstand this devastating test, but there is still lots of room for improvement. If the sketch is ultimately inaccurate with the respect to the precise positions of some of the stars, it is nevertheless very useful to confirm that we did see the nebula, and had the correct field:
And, after another five years of practice, the present author is getting even more accurate; click here for our eyepiece sketch of the Delphinus constellation wide field (53 arcminutes' diameter) including planetary nebula NGC 6905, made with a 4.7 inch refractor, with star placement compared to a DSS image (both pictures are reversed E/W). The sketch is completely unedited and 'raw', as found on the session logbook page. It's not quite yet "Jaakko Saloranta quality and accuracy"... but we're pleased with it (full article about the observation, covering also some issues of importance about making sketches, found here.)
Horsehead Sketch Snafus We've Noticed
The Horsehead nebula, being somewhat visually elusive, and hard to 'hold' with direct vision (often requiring intense scrutiny over time, using averted vision: getting mere GLIMPSES of it rather than substantial, unmistakable, unambiguous perceptual certainty), is especially hard to sketch.
One of the main problems is that the contrast differences between the darkness of the obscuring cloud (the Horsehead); the general eyepiece field illumination (the interstellar 'luminous ground'), and the faint hydrogen-beta radiation of IC-434 (visible to the dark adapted eye only as a faint gray) are very small. Beginners sometimes cannot sense them, so they miss the Horsehead--or even the much longer and wider nebula 'beyond' it, IC-434.
Furthermore: any attempt to then back away from the eyepiece, turn on the red light, change your eye's focus to the notebook page, and draw what you see, will tend to (a) wreck dark adaptation; and (b) diminish your precise 'memory' of what you've just seen.
Now, add to this the incorrect 'propaganda' of some amateur advice about the Horsehead (critiqued by the author in Horsehead Nonsense--and Sense): telling beginners that the Horsehead is sized WAY out of all realistic proportion. One piece of "advice" we found on a certain forum said that the Horsehead was 'larger than you think' and gave angular dimensions that were greater than those measured from modern deep exposures: nearly an order of magnitude bigger than most viewers will perceive with a scope of about 8" or smaller.
Whereas, expert deep sky observer and widely respected author Steve Coe says that it might often appear to be about the same size as the Ring Nebula (M-57), which is slightly oval, having a larger diameter of about 1.5 arcminutes. Often, all that the amateur observer may easily perceive, recall, and solidly quantify is the 'head' -- to Coe, maybe 1.5 minutes' diameter except the way the Horsehead appears in the biggest amateur scopes, or under simply
amazingly superb meteorological conditions.
The 'neck' poses a different kind of perception/sketching challenge than the 'head', which is positioned, as seen from Earth, right over a bright rim of the nebula IC-434. The opacity of the 'neck' -- extending east of the 'head' with its 'snout' pointed north/northeast -- thins out considerably, over a dimmer part of the molecular cloud that blocks the 'bright' hydrogen nebula IC-434. Beginners who have not even seen the Horsehead in their scopes MUST, first, learn to judge properly the astronomical grid orientation of their eyepiece view of the sky. The picture below -- the generalized nebula isophote plot of the region supplied by Megastar 5 -- illustrates the "standard" orientation used in charts and professional images:
In your eye view, the orientation will nearly always be different, unless you have employed an orientation-correcting 'Amici' prism (not recommended for deep sky observing, for various reasons.) Therefore, in a typical Newtonian scope, the image is reversed L to R and may appear inverted N-S; a standard SCT or refractor, with star diagonal, usually merely reverses the image L to R.
As stated above, the opacity of the 'neck' varies considerably, and in the best eye-views (if not in deep multiband color exposures) it tends to seem to transition gently into the barely-luminous ground of the opaque, somewhat dark molecular cloud east of the 'bright' part of IC-434. Since this cloud is actually more luminous than the Horsehead, there is a slight contrast difference between the neck, and the background: allowing you to see at least some of the long diameter of the Horsehead. This "fadeup" of the darker neck into the slightly lighter opaque background can be rather rapid in views with small-aperture scopes, especially with any noticeable field light pollution.
A very large aperture scope, in a super-dark sky (using the h-beta filter at EXACTLY the right exit pupil for best contrast) may show a relatively longer, darker neck length before it is impossible to tell exactly where it transitions into the background. Even though a professional modern image can reveal perhaps 6 to 8 arcminutes of 'neck' length, nevertheless it is nearly impossible to see this much of it by live-eye, except under superb conditions in the largest scopes. Don't succumb to "suggestions" from images and feel that you must draw the neck as long as shown in a photo (especially with a small telescope); if so, highly experienced Horsehead visual observers simply will not TRUST your drawing to be honestly realistic. (One cannot say with certainty exactly 'how long it should look' as so many factors control the result; just do your best, with honesty: and show what you perceive.)
A Technical Digression: Why We Can Detect and See the Dim Horsehead By "Live Eye"
We may easily photograph the Horsehead (and even see it by live-eye with proper gear and technique) because it BLOCKS visual light from other phenomena that lie "behind" it from our perspective. As Drs. Marc Pound, Bo Reipurth, and John Bally explain in their definitive modern study (which, by the way, cites the present author's research of 1989, in collaboration with Dr. Martha Hazen of Harvard), "the Horsehead is seen predominantly in silhouette against the background plasma in IC 434, indicating that it is mostly illuminated on its back side." Some of IC-434 was vaguely spotted by William Herschel, though not described very definitively (he never saw the Horsehead part!); about a century later it started to show up on early Harvard photographic exposures, though only partially: merely the greenish wavelength radiation -- hydrogen-beta -- of mostly the HII region (plus a weak trace of the dimmer molecular cloud, heavily affected and somewhat obscured by light scatter on the exposure, from blinding Zeta Orionis: as seen here. The old glass negatives were sensitive only to the spectral range of green and blue light, and suffered from "halation" due to instrumental and glass plate substrate reflections.) The old 1895 first "Index Catalogue" cited Harvard's modern 'rediscovery' of the large nebula, but did not cite at all Mrs. Williamina Fleming's 1888 first-discovery of the Horsehead.
Though William Pickering of the HCO, in consultation with Mrs. Fleming (who discovered and measured the Horsehead on Harvard plate B2312), speculated immediately that the interesting "bay" or "notch" was a dark occluding substance, most subsequent Horsehead researchers and photographers, for some years, believed it might be merely a 'hole' or a sudden jog in the bright nebula: merely an "absence" of glow. Finally, in 1913 Edward Barnard looked at it visually with the Yerkes 40" telescope, and decided he could discern a definite 'presence' or substance that merely blocked or dimmed the light coming from beyond it: "the spot is certainly not clear sky, for the field was dull, apparently indicating the presence of some material substance at this point. To me the observation would confirm the supposition of an obscuring medium"... and he has been entirely vindicated by modern spectrographic analysis. The Horsehead is now known to consist of polycyclic aromatic hydrocarbons (PAH molecules) which, as Dr. Alexander Tielens of NASA-Ames Research Center wrote to this author in 1990, might be described as a "soot-like" material; it's perhaps not unlike cigarette smoke. According to Tielens, "IC-434 is a huge cloud of nebulously, an H-II region exhibiting the hydrogen-alpha line of radiation, which is ionized by Sigma Orionis... The Horsehead region is part of a giant molecular cloud in front of part of IC-434 and has the appearance of a tongue of gas and dust falling back and folding over." (see full quote in this Horsehead Project article.)
So, the Horsehead is now known to be independent of IC-434, revealing that Mrs. Fleming discovered something altogether different from that 'bright' nebula: pity that this was not fully understood at the time. If the original HCO photo had been capable of registering a wider spectral bandwidth -- and if a longer exposure could have been used; halation effects prevented it -- perhaps something palpable of the molecular cloud could have been detected, apart from the HII region. But it wasn't definitely registered on film until 1920 (by John C. Duncan, with the 100" Mt. Wilson telescope, seen here)--and then, not in its full glory as revealed now 'in living color' with almost three-dimensional depth, by Marco Burali, Tiziano Capecchi, and Marco Mancini of Osservatorio MTM.
It would take a few subsequent decades following Duncan's superb monochrome photo for "true color" multiband images to be developed; the iconic composite photograph by Dr. David Malin was one of the first widely-distributed pictures that helped stimulate so much interest in the object in the last couple of decades. This showed the vivid H-alpha light of the HII region of IC-434 (turned to a slightly off-red/pinkish color by the weaker greenish H-beta wavelengths) along with the duller, 'dusty' gray-brown of the adjacent opaque cloud: with the Horsehead registered in fabulous detail, enhanced by unsharp-masking techniques.
When you study that Malin image, it is readily apparent that the earliest monochrome photos from 1888 to 1920 (chronicled in other articles on this Horsehead Project website) registered very little -- sometimes nothing at all -- of the opaque molecular cloud: indeed, Isaac Roberts' underexposed 1900 photo makes IC-434 look like merely a very narrow, vague streamer, with only the 'top head part' of the Horsehead registered, in h-beta light. This picture is, in fact, just about what one MIGHT be able to see by eye with, say, a 10-12 inch aperture telescope, under 'just less than the best' conditions, in our experience (in fact, it corresponds to a 'visible simulation' once posted to an astronomy forum by an observer who used a 12.5 inch Dobsonian and wide-angle eyepiece to view the Horsehead, and then modified a photo to try to represent his experience.)
Dark-adapted ("scotopic") low light level human vision happens to be most sensitive very near the wavelength of the h-beta Balmer nebular line of atomic hydrogen at 486.1 nanometers: see this useful response graph for the Astronomik UHC filter, with the nebular lines plotted on the x-axis, superimposed over a curve representing dark-adapted vision's wavelength response. We see the 'bright' part of IC-434's HII region, and traces of the opaque molecular cloud, predominantly in the h-beta wavelength (greenish light); virtually none of the much stronger h-alpha red radiation is picked up from low level (i. e., telescopic eye view) diffuse nebula light (though it may be seen when registered as point-sources: individual stars.) Since only the luminance-sensitive rod cells of the retina can detect h-beta wavelength hue at incredibly low intensity levels -- the more numerous color-sensitive cones lacking this capability -- the brain interprets the rod stimulae as representing only luminance, on a gray-scale from near-black to white. This is, of course, why most persons see very little to no color whatsoever when looking at diffuse nebulae with an amateur scope.
Ergo, when the eye/brain system works on the weak stimulae from the eyepiece field view of the Horsehead region, human observers register nearly only lights and darks and the continuum in between. Since the Horsehead cloud seems VERY, very dark with 'small' (i. e., amateur sized) scopes, success in being able to see it depends primarily on maximizing contrast. Most of the time one has to use averted vision, which shifts the retina slightly off-center to the most critical concentration of rods; but this tends to diminish DETAIL. It is only a very rare viewing circumstance, with ANY scope, that permits a very clear, unambiguous straight-on, non-averted vision of the Horsehead, combining both good contrast and detail. We suspect that the drawings we've selected, below, were done under such excellent conditions.
Reiterating: the original old catalogue reference to IC-434 (which in modern decades after the development of sensitive spectroscopy and color photography became known as an HII region: a "red hydrogen nebula"), really represents TWO nebulae: the 'diaphanous part' that glows in strong H-alpha light (accompanied by weaker H-beta which the dark-adapted eye can see); and an adjacent, denser and darker molecular cloud on the eastern side that partially blocks the relatively vivid glowing part. The former is almost "easy" to see in small telescopes -- even binoculars -- in ultra light-pollution-free rural skies; the latter is very hard to perceive unless conditions are superb, with larger aperture, more efficient gear. The Horsehead 'part' is actually quite distinct when the dim molecular cloud is even slightly detectable: though most visual observers know nearly nothing about its existence and don't attempt to discern and describe it. This author has seen this very faint 'luminous ground' relatively few times (maybe once out of five or six B33 viewing sessions in clear California air), much less often than the discrete and more obvious Horsehead itself (which both the present author, and Cyprian observer Rolandos Constantinides, have even spotted, with supreme effort, in 3-inch aperture scopes.)
Horsehead's Apparent Dimensions Have Changed As Technology Has Advanced
The less-dense part of the Horsehead neck is easily seen in its entirety in a very deep multiband exposure (such as the one by Richard Crisp, shown below in this article.) But, even monochrome images made in the visual spectrum very close to the peak frequency at which the dark-adapted eye is most sensitive (early photographs made in 1888-1920, registering mostly the hydrogen-beta nebular line in the greenish color region) often do not show the 'neck' in full scale or detail, or with the darkened density of the 'head'.
For instance, here is a thumbnail image (scaled to fit on this page: for the original full-sized one, click here) showing how the 'neck' has changed its apparent detected diameter over the years, as technology improved. The Roberts photo of 1900 (registering IC-434 only via its h-beta line of 486.1 nm) has about half the discernible length traced out in the Richard Crisp composited wideband image, illustrating how the apparent size of the Horsehead would change over the decades, depending on the optical spectrum that could be recorded. The dark adapted eye discerns only the 486.1 nm greenish h-beta nebular line of IC-434, NOT any of the stronger red h-alpha at 656.28 nm; ergo it will seem to perceive a Horsehead -- best case, in a huge scope -- with dimensions like the 'shorter' versions in the very old photographs with "actinic" sensitive emulsion that registered virtually no red, compared to the modern multiband digital image. So, when you read in Wikipedia that the Horsehead's diameter is "6 x 8 arcminutes", or claims on the Net of a '10 minute' diameter, do not believe that this is exactly what you will see in your scope when using the h-beta filter. And of course without that filter you may not be able to see much of a trace of it at all on most occasions.
Do not be biased by experience with 'official' measurements and by modern color images into 'believing' you are seeing a 'long neck'... if you really aren't. Some sketches we've seen -- using scopes as small as 6" aperture -- show a 'neck' that is as proportionally long and dense as in an exceptional professional quality modern color image: maybe 3 or more times the angular diameter of the 'head': based on a quarter-century visual study of the object in many telescopes, we claim this is an impossible feat, imagination at work -- an exaggerated perception, or possibly a mistaken impression of something at the very threshold of detection, "aided" by knowledge of the object's appearance in photos.
Furthermore, there is a tendency for the eye/brain combination to have something of an exaggerated sense of 'novel views'; and the Horsehead is certainly "novel" for most amateur astronomers (if not the author, who has made a specialty of observing it since the mid-1980s, and who has many dozens of observations -- perhaps by now more than a hundred -- under his belt. In the winter of 1989/90, for instance, he viewed it no fewer than thirty times. He's looked at it from three to perhaps ten times every season from 2005 through early spring 2012, so far. It's no longer novel to this writer; but unless you have spent years viewing it, it will be novel to you!)
In fact, some years back this observer began to realize that he all too often committed a systematic error: drawing objects -- that had not been viewed before many times -- consistently TOO LARGE. Here, for example, is our 2009 logbook report and sketch of planetary nebula NGC-7027, which we sent to our friend Jaakko Saloranta with apologies for having drawn dimensions that were too large, compared to an unprocessed DSS monochrome image. Jaakko nevertheless replied, "I really love your NGC 7027 sketch. It looks just like a peanut and just like the planetary appears in the eye piece!" Despite having, apparently, registered the character of the nebula in accordance to one talented and reliable observer's experience, we're still miffed that the scaling was incorrect, and afterward began to use greater care in relating the proportions of such objects, with respect to field stars and overall eyepiece FOV.
So, when we later found in 2014 another amateur's drawing of planetary nebula NGC-6905 that seemed to us to be registered nearly THREE TIMES its actual angular dimension, relative to the eyepiece field of view, we undertook an extensive investigation to sort out the various aspects of this alleged 'distortion', finally being satisfied that we'd found the possible source of an illusion: see here. There must have been, between 2009 and today, some general improvements in our accuracy, for several years after we posted the rather embarrassingly over-large NGC-7027 sketch, a noted Utah amateur observer/artist expressed his compliments, saying that our articles and drawings had helped him to increase his accuracy of perception and rendering!
So, if indeed our hypothesis that unexpected or novel sights tend to create exaggerated impressions, perhaps it is understandable that some of the sketches we've seen on the net are not particularly realistic.
|Measurement of Pictures:
Here's the simple method we used: (1) Using a graphics editing program, import the picture. If it has been done in 'realistic' manner (white stars, black background) then make a negative; and enhance the 'darks' a bit above normal. (2) Measure the eyepiece view sketch pixel diameter. Set up a ratio based on the claimed eyepiece field of view by the sketcher: i. e. X no. of pixels equals Y number of arcminutes. (3) Measure the width of the B33 'head' and 'neck' in pixels. (4) Since the size in total pixel width is theoretically equivalent to the claimed true field of the drawing, it is now only necessary to use simple math to convert the pixel widths measured of 'neck' and 'head' to arcminutes' angular diameter. (5) Measure the pixel distance of the field stars opposite each other, if any, closest to the edge. (6) Try to see if you can triangulate those stars, from the ones drawn, using the plot generated by an accurate planetarium program (such as Megastar 5) at the same scale. (7) See if the separation of those 'edge stars' are close to the claimed true field. If there are no stars opposite each other near the edge, then try the widest pair of stars drawn in the sketch, and match with Megastar 5. Then, see how the angular diameters measured from the picture compare with the careful, realistic plot of Megastar 5. (8) If you would insist on even higher accuracy, use the Skyview website and do the X,Y measurement on a chosen deep sky survey image.
Here are some of the problems we've noticed, using the method of analysis described above:
Eyepiece FOV/Horshead scale are out of whack: Some sketchers clearly do not know the true field they are trying to sketch, most likely because they haven't calculated (or preferably drift-measured it) for each eyepiece. It is sensible to reason this way: if a deep color image of the Horsehead (often captured in frequencies the eye cannot see, which are then shifted into the visual spectrum) shows a 'neck' diameter to be, say, at most 8 arcminutes -- and your true field is 80 arcminutes, then the 'neck' should be about one-tenth of the field, right? But, no: that's not what we actually see! Amateurs, viewing the dark nebula by eye in almost any practical size scope, are rarely able to see the full extent of the 'neck' as depicted, at right, in a very deep multiband color composite exposure by Richard Crisp (original here.) Nor can they discern the fainter extensions of the 'head' (though a general impression of a 'chess piece horse' shape, minus filamentary detail, is visible in larger scopes.)
In one embarrassing example we found on the net, the 'neck' was sketched at a size that measured approximately 1/7th of a claimed field of nearly 1.5 degree! This calculates to a diameter of nearly 12 arcminutes: IMPOSSIBLE. (Remember, that professional catalogue 8 arcminute dimension' is not what one can see by eye, probably not even with a monster scope! So, then: how can one possibly use a scope even smaller than 8" diameter to see a 'neck' that is 12 minutes long? Yet this sketch is published on the Net, and hasn't drawn any complaints that we've seen so far.) We won't go into the precise figures of our calculations to avoid sparing embarrassment to the poster of that drawing; but our Horsehead Project testers used a scope of that same aperture, and so has the author in recent years. Best case, the 'neck' might appear 1.5 to 2 arcminutes long, not 12: so there is arguably an order of magnitude error here.
We've also used almost exactly the same claimed magnification for this published sketch, which was quite low (shall we say, vaguely, that it was claimed to be about 3 to 4 times the magnification typically used by binocular observers): in our numerous observations, stretching back to the 1980s, this same claimed low power would yield a rather 'tiny' Horsehead, not the 'big' distinct one, proportional to the entire field shown in this erroneous sketch.
Sometimes the entire field has significant geometrical distortion. This can be a problem with certain eyepiece types, in fast scopes. Low focal ratio simple Newtonians have lots of coma, and a curved focal plane. Unless one uses an eyepiece (and/or a coma corrector) to flatten the field, the drawing may have geometrical distortion, causing the angular diameters of objects at various places in the field to be out of kilter. One such drawing we saw, and analyzed, had enormously curved field and barrel distortion, with stars the edge not in proportion to the ones near the center, and a Horsehead and immediate 'close by' field stars at distinctly different relative placements, being wider spaced than objects further from the central axis; indeed, it took a long time to match the stars to judge the claimed true field, and we were never completely convinced. And, it turned out that the gear being used was particularly cheap and rudimentary. We were disturbed by the dimensions of the Horsehead versus the measured displacement of identified stars at the edge-of-field; worse, the artist/observer was very well known (and should have realized this, so that he could -- if possible -- take steps to get more geometrically linear eyepieces that would produce a more accurate total field display in his scope, since he obviously cares deeply about artistic sketching, and makes every effort to distribute his creations to the amateur community.)
One supposes that an objection to our critique might be made: that, if we're right in the diagnosis, "the sketch is realistic, even though the Horsehead is out of proportion, and the stars aren't correctly distanced--after all, that's what the artist saw!" Our answer is: this is systematic error, for all drawings made the same way will have an instrumental error that can be avoided, improving the realism. So: get better geometric accuracy in your eye views, and your drawings, if done faithfully, will be more realistic! Otherwise, the audience for the drawings will possibly never see things about the same way, as their gear probably won't have exactly the same distortions.
And, conversely: certain "high end" eyepieces are intended to clarify perfectly the star points near the edge of a curved focal plane, calculated from expected performance of certain f/ratio scopes generally in use by advanced, well-heeled observers, the market demographic for such products. While the stars may truly be 'pin-point' across the field, it is a fact (which may be corroborated by viewing test grid charts) that one's view now has another type of distortion: it has acquired curvilinear pincushion distortion, the center of the field seeming -- in comparison to the stars at the edge -- to be compressed. Such oculars give 'beautiful' stellar views; but not necessarily geometrically accurate ones (as the author happened to observe only the day before we wrote this particular article, in a 10" f/4.7 Newtonian, the same model he personally owns, with a $400+ eyepiece.)
Some sketches have stars placed unconvincingly. The author is not very accurate, admittedly. His precise angles, triangulated from the field stars drawn and the nebulae or galaxies placed therein, are not always exactly in agreement with survey photos; the author simply does not follow the careful method, and lacks the uncanny geometric skills of, say, Jaakko Saloranta, the master-sketcher and observer from Finland. We intuit that there are two factors that limit the present author's abilities: (1) reluctance to follow 'method' in the field, which has been pretty well defined by the experts; the author is more interested in the observation and the success or failure of it, than in the ultimate character of documentation; and (2) sheer inexperience in sketching. We've made perhaps 100-200 eyepiece sketches, from the crudest to the best ; Jaakko has done many thousands: see the photo he took, below, of his sketch cards -- as of about 2009 -- the two stacks referenced in size by a small book of matches.
Our best sketches do not come close to Jaakko's, though we can see evidence of gradual improvement. But, we feel we've been successful in achieving the actual observation (not the sketch quality) if it's possible to triangulate significant agreements in field stars and other phenomena drawn, with a real plot or picture, even if all the angular relationships are not perfect -- as demonstrated by our sketch of NGC-6185, a bit too tall to show on this page.
In truth, not many amateurs are indeed capable of such an extraordinary feat as the sketch of open cluster IC-4756 that Jaakko Saloranta accomplished, below:
Often we find that Jaakko's drawings of such objects make more 'sense' of them than Palomar Observatory Sky Survey photographs (which have stars to about magnitude 18-19, which simply cannot be seen visually in perhaps 99% of amateurs' scopes, and burned-in bright stars that turn into artifact-riddled blobs.) Please consult the Deep Sky Archive 'search' page, by entering the name Saloranta into the dialogue box for 'observer' and you will be able to pull up any of over 800 of his reports and drawings; unfortunately, Jaakko's extensive websites are no longer online.
One Particular Forum Keeps Stressing Incorrectly "Horsehead is LARGER THAN YOU THINK"
I have noticed that on one widely-read forum, a leading contributor started a 'meme' that the Horsehead nebula is "larger than you think", asserting that it is about 10 arcminutes in diameter. That is NOT correct, though it has been taken up (by 'argument from authority'?) by other less experienced contributors and repeated many times. I believe that this has helped cause a misperception among observers, and finally found what seems to me to be concrete evidence.
An amateur has posted that, indeed, the Horsehead seemed larger than he expected--and I wonder: is the expectation based on careful triangulation from proper charts and pictures, or merely from 'forum advice'? -- for his posted drawing shows the Horsehead as being at least TWICE the real N/S diameter.
Here is a fair-use excerpt from a very small part of his sketch, which has been greatly modified by me (by inverting it to black-on-white, and scaling and rotating it to match Ryan Wood's photograph.) I demonstrate the actual narrow diameter by putting a cut-out of the 'real' Horsehead next to the region of the dark nebula that the viewer sketched:
While the observer got some of the stars in proper orientation, others (particularly the faint ones to the NW of the 'snout') are not quite right. Yes: the Horsehead was seen; but it was drawn (in a zone that I have colored yellow) that is probably a bit more than twice the N/S diameter of the neck; he does not discriminate between the 'upper head and snout' and the eastern neck, the longer part. The position matching is not nearly as good as done in the excellent examples I show below. I can only ask: is the 'super wide' Horsehead (which I certainly have NEVER perceived, in 25 years of studying it in a large variety of scopes!) the product of SUGGESTION...and furthermore, absolutely incorrect suggestion?
Furthermore: the 'snout' is hard to see, clearly, in the smaller aperture scopes. It is not quite as densely dark as the 'top of the head' and part of the 'neck'; so it may fade into the brighter background. Larger scopes will show it more distinctly; in the very largest aperture instruments, under superb conditions, it may be seen as being at least partly detached, connected only by a small extension to the 'top of the head'. Here, the snout, head, and neck are all conflated into one overly large 'lump'.
But: since you are (in smaller scopes, at least) detecting the Horsehead as something that is merely 'darker than the surrounding area', using averted vision, it is extremely hard to be precise about boundaries, recall them accurately, and match them to BRIGHT objects in the same field. Congratulations to the observer for finding and SEEING the Horsehead; your drawing certainly proves that!
But, improvements in the sketch accuracy may be made by including as much information as possible in your drawing: this is why it is always valuable to sketch also as many field stars as possible, along with your targeted galaxy or nebula. Then you might be able to match those stars to a correctly oriented/rotated display on a computerized sky chart program, and to check the diameter of your perceived eyepiece true field; the separations of some distinctly registered field stars; and your perceived and drawn object diameters. For example: a very faint galaxy or nebula will NOT be seen, by eye via most amateur telescopes, as having the same dimensions as those given in profesional catalogues, derived by measuring deep photographic exposures: possibly in wavelengths of light that the dark-adapted eye might not detect. And you tend to lose outmost boundaries of faint glow fading into the overall sky background, or perhaps cannot quantify and judge them very well with averted vision.
And, if you happen to notice a couple of field stars that seem, to your eye, to be separated about the same apparent diameter that you perceive in your target object, be sure to note them so that you can measure their separation later, by the method described in the above paragraph: then you can get a corroboration of your judgment of object diameter, useful to know in case it might help demonstrate any sort of systematic error of diameter-perception.
Problem: How Do You Register "Darker than Dark"?
A major difficulty in being able to produced a "realistic" (white detail on black backgound) artistic rendering of the Horsehead is the problem of dynamic range: the continuum from utter pitch black [luminace value of 0] to full white [luminance value of 255, in the 8-bit system used by computer screen grayscale rendering.] When you do a 'negative' sketch, you must show the Horsehead by defining its OUTLINE: since you cannot make any mark on the paper, at all, for the dark nebula; you only show it by filling in, or shading, what is AROUND it. When you scan your drawing, and invert it to POSITIVE mode, then the Horsehead will show up as being a 'blank' spot; but your indication of 'glows' near it (IC-434) plus any further-distant region of the luminous ground, will be rather uneven and mottled, not the smooth way it was perceived by eye.
Some of the astro-sketchers who are willing to use post-processing by a graphics program have become very adept at adding shading: perhaps their original drawing had sharp pencil outlines of the major step-changes of light grading; they erase (or blend) these distinct areas into a smoothed, varying continuum (we feel that the Jay Eads rendering of the Horsehead, below, was probably processed this way.) Whereas, the "raw realism" of the sketches by Timo Karhula and Jaakko Saloranta are much simpler, more generalized. There is no attempt at an absolutely photorealistic luminance gradient, as this is almost impossible to render, accurately, with pencil on paper at an observing session.
Furthermore: in various apertures, the Horsehead appears quite different: in density and overall relative size, with the background light level affected hugely by the magnification/exit pupil employed with a given filter. In some views, the Horsehead may seem utterly 'darker than dark' against a very slightly illuminated background (which is brighter on the western side, due to the faintly seen h-beta light from IC-434.) Users of, say, a 5 mm or larger exit pupil with the h-beta filter will tend to see more of the background gradient than if they switched to higher magnification.
Unless you are of the 'cartographical school' of style, and prefer merely to render your raw sketch in "negative" mode with an absolute minimum of clean-up, you are likely to want to try to subtly emulate the finest nuances that you SAW. This almost certainly requires computer-manipulation; but that can be done honestly and minimally, trying at all times to evoke your "best recollection" of the original view. It's not 'science', nor is it 'objective'; but it can give reasonably correct 'impressions' of what one might see under similar circumstances.
To try for "darker than dark" in my own sketch of B33, using my 90-mm aperture f/10 Orion refractor (at a 5 mm exit pupil, achieved at 18x magnification by using a 25 mm Orion Ultrascopic eyepiece with 0.5x Knight-Owl focal reducer for an equivalent eyepiece FL of 50 mm), I first inverted the raw scan of my sketch to "positive" mode (white foreground on black background.) Then I selected just the circle representing my eyepiece field and changed the full-on black to a very dark gray: just about all of the 'luminous ground' that I could discern with this relatively low efficiency scope. I isolated the very small fuzzy patch where I had pencilled in my impression of the placement and dimension of the Horsehead, and darkened the region, smoothing and "fading up" the edges to the luminance level of the surrounding medium; then I smoothed my pencilled-in shading of what I could detect of IC-434 and turned that from 'fine scratchings' to 'smoothened glow'. The end result is not the raw data of the original, but it might (in some way) actually be a closer simulation of my actual eyepiece experience.
For the purposes of this illustration, I confess that I exercised some perhaps inexcusable "artistic license": for I 'fixed' the inaccuracies of the drawing, done at my observing table several feet from the telescope (which was on an equatorial mount, easy to recenter during the several trips from scope to table and back, while making the sketch.) I usually make drawings AWAY from the scope in this manner, employing a stand-magnifier lens and red light on a flex mount, produced in order merely to represent something approximate for checking later at home to verify if I had indeed seen an object; precise placement is not required for that purpose. But since I wanted to show, here, how "raw" work can be turned into "art", I figured that since I had seen the stars and Horsehead placement PROPERLY by eye -- but merely could not REMEMBER that precisely, a few moments later at the sketching table -- I could legitimately argue (um, rationalize) that slight corrections might be warranted. (I suspect that many other eyepiece "sketchers" use this technique; I virtually never do that and live with my inaccuracies for regular observing reports and blog posts.)
As you will note, there is really no absolutely accurate dimension shown in the rendering of the Horsehead. I could only see it with some vagueness using averted vision, and lost the exact shape--so did not try to record it on paper; merely I sketched a "dark blob". The focal reducer employed enabled me to get an effective exit pupil, but had the problematic artifact of causing significant vignetting, which cut off the field illumination away from the center, reducing the visible extent of IC-434.
And... guess what? I probably drew the blob a bit too large! I don't consider this a "reference drawing"; it was done pretty quickly, just to have some sort of visual record that accompanied my report that claimed I could see the Horsehead in a 3.5 inch scope.
If I had used a larger aperture instrument, at higher magnification, I would probably be able to get better dimensions plus the shape and position angle, as I did back in 1989 when sketching it with a 7" Astro Physics high-end refractor, here; apologies for the shortcomings of my very primitive technique back then, and the poor condition of the only existing version, an old photocopy (not to mention a few star placement errors!)
For contrast, you may view my RAW SKETCH, uncorrected, in this factual, original blog report.
This ends my dalliance with artistic license and computerized editing and reprocessing; in future I'll stick to the raw data, in uncorrected cartographical mode!
Fine Example of Shape, Size, and Detail: Large-Scope Horsehead Sketch
The following drawing, done by a very reliable man who could perhaps be called the 'dean' of northern European deep sky observers, Timo Karhula, can be used to document some real-world results. With a sky naked-eye limiting magnitude of 7.2, Timo used a UHC filter, and magnification of 100x in a 17.5 inch aperture scope (yielding a FOV of 20 arcminutes):
Mr. Karhula reports: "This was my best and easiest observation of the famous Horsehead nebula. I could repeat this observation at least 10 times in a few minutes.The 'head's' shape was quite easily discernible with averted vision. I could also glimpse B33 without filters but it was more difficult. The border of IC434 S of B33 was almost invisible. The four * N and W of the Horsehead always help to locate this low-contrast object. The observation was done 55 minutes before culmination and from latitude 60 N. IC434 is the background 'bright' nebula to the W of B33."
According to our calculations of his drawing (comparing the field diameter pixel size relative to 20 arcminutes, to the pixel size of the Horsehead neck and head), we estimate that his perceived dimensions are: head=2.94'; neck=2.49'. That is with a 17.5 inch scope, in a VERY dark sky: observed by an expert with decades of experience.
We did some calculations of star separations from Timo's sketch. The star at N-top (GSC 4771:1167) and the one closest to the bottom (GSC 4711:896) are separated, according to Megastar 5, by 13.8 arcminutes. These stars are not at the very edge of the field, so factoring in an appropriate adjustment, we estimate that Timo's sketched true field, based on the position of those stars, seems to be about 18 arcminutes: a good result for the type of procedure involved -- entirely by 'eye-balling' the distances -- compared to making a precise measurement with a micrometer. The true angular diameter separation measurements of some of the other stars, derived from the Megastar plot, conform nicely to the way Timo has separated them.
The author's conclusions are: (1) Timo's sketch is very fine, and is quite realistic; (2) Timo's perceived size diameters of 'head' and 'neck' are very good: they are realistic (as opposed to the ludicrous 'advice' post we ridiculed in our article, in which the values are outlandish); (3) Timo's rendering agrees with the author's own perceptions of many viewings with his own 17.5 inch scope, and the 20 inch scope of a viewing partner.
Using a very large, efficient scope, Timo rather significantly exceeded "Steve Coe's rule" that the Horshead often appears to be about as big as the Ring Nebula in some scope views (rules were made to be broken, aren't they!) But, Timo did not come up with "unbelievable" dimensions that are as big as you would derive from an exceptionally deep professional observatory photo or digital image. He gives you a sensible, accurate 'eye view'--in that particular scope aperture and under the conditions he documents. We are impressed!
In fact, had Timo employed a Hydrogen-beta filter, he very likely might have perceived a greater distinction of the 'neck' across the glow of the background nebula IC-434; his sketch indicates (if that is what he intended to show) that the neck 'stops' at about the intersection of the 'bright' glow of IC-434 with the change of opacity in the N/S swath of the molecular cloud blocking 'clear sky'. With even a 10" scope, we've had 'deeper' views of more of the neck, extending it a bit further east by perhaps about an arcminute, when aided by the greatly enhanced contrast provided by the h-beta filter, compared to the UHC type: making the Horsehead seem optimally almost "blacker than black" against the background. Nevertheless, Timo's very distinct sensation of the 'chess-piece shape' of the 'head', using the UHC, suggests that his observing conditions were outstanding; in this author's experience with a similar aperture scope, such detail in the 'head' simply cannot be discerned at all without at least the UHC filter (preferably the h-beta type.)
A Superb Artistic Rendering: an Observation with a Ten-Inch Scope
And now to provide a second fine example, one that is especially 'close to the author's heart' as he too owns the same scope: an Orion (USA) XT-10, with 10" aperture and f/4.7 focal ratio.
The drawing was found by us recently when perusing the elegant and informative "Jay's Astronomical Observing Blog" written by Jay L. Eads, one of several very worthwhile blogs and folios he has set up to document his astronomical activities (for instance: he's written a very interesting and even somewhat provocative commentary on astronomical sketching, found here.) Jay's graphical skills rather vastly surpass this article's author (as do his web layout design tastes!); and he observes in very dark sites in Utah, comparable to our venues (the Santa Cruz mountains when deep fog settles over San Jose and the cities further south; and Lake San Antonio in very rural central coastal California.) So it's interesting to us to see how he's done, with the same telescope (the only difference being that Jay's is the "Intelliscope" GOTO model; ours is the 'manual' standard Dob configuration.)
We found a sketch in his report, "March 12, 2010 Pit n Pole Rush Valley , with his accounting of a session in which he observed, with the XT-10, the Horsehead plus galaxies NGC 2683 and NGC 2859, and the emission nebula NGC 2359, "Thor's Helmet". We confess that are regular practice -- when finding 'realistic' sketches (white stars, black background) is always to copy them to a graphics editor -- often Irfanview -- and then invert them to negative mode: which is probably what most astro-sketchers do, in reverse, when they create such files for public posting. With a white background and black markings on that, we find it's MUCH easier to spot faint shadings without ambiguity (and we even increase the density of the 'darks' if necessary.) Doing that to Jay's picture, we were delighted with it: as we have owned an XT-10 since 2005 and have made (according to a very cursory search of our logs) at least nine Horsehead observations with it. Jay simply agrees not merely with the author (who certainly doesn't claim infallibility!) but also, simply, agrees with sensible reality.
Here are some of the things we wrote to Jay, in asking permission to test-measure his picture for this article:
I am extremely impressed with the detail and proportional accuracy of your drawing of the Horsehead nebula. I have the same telescope and have looked at it repeatedly with that scope...
I calculated the pixel width, proportionally, of the entire field you show, and the dimensions of the HH--and they would indicate to me (if the true field [of the 13mm Stratus in that scope] is indeed about 44') that you perceived the B33 'head' as being about 2.6 arcminutes wide, and the 'neck' as being 1.9 arcminutes long.
*This agrees exactly* with my perceptions with such a scope aperture. So I'd love to use your drawing, with these dimensions marked, on my existing webpage to show a very highly accurate sketch of the way this object appears, by eye, in such a field of view.
...The more I work on your remarkable B33 drawing, the better I like it! I now have done further measurements. The calculated true field of the XT10, with the 13 Stratus (I have both!) is about 44 arcminutes. You have drawn two stars that are separated exactly 41 arcminutes--and they are pretty near the edge of the field, each being perhaps 1-2' in from the field stop: agreeing perfectly with the angular separation as measured by Megastar 5... The fact that you placed those stars where you did, confirms the accuracy of scale of your drawing, and this is confirmed by the FOV of the 13 Stratus in your/my scope. The problem I've had in finding other reliable B33 drawings is that, often, the *stated* field and magnification will NOT agree with what's drawn!...
...I think that this contribution from you -- if you will authorize it -- will be a very valuable one, as part of my advice to sketchers to *make accurate drawings!*
It's particularly hard to draw a dark nebula like the Horsehead, realistically: because whenever there are very bright things in the same field, the eye has a hard time 'quantifying' the size and relative dimness of the dark spots. Then, if you use enough dim red light to see what you are sketching (on your white drawing paper) you immediately lose a magnitude or two of detection--and have to go by pure memory! This is why so many HH drawings I've found aren't very convincing when compared [spatially] to photos.
It's especially important to me to use your drawing: because I have the same 10" Orion f/4.7 scope (non Intelliscope model though) and have seen the HH with it so many times. I've never actually done a drawing with that scope--now, I won't have to, as you've captured it so well!...
Again: I offer my appreciation for your fine work, beautiful blog, and accomplishments!
We are happy to say that Jay soon replied, clarifying the caption of his drawing: he indicates eyepieces of 5 and 13 mm (Stratus) and a Barlow; but he explains: "July 14th, 2012. Steve, please feel free to use my image as you see fit... On this image I reviewed my notes and found that I indeed used the 13mm Stratus with the [h-beta] filter. The attempt with the 5mm Hyperion was a failure, as I could not make out anything due to the contrast. So this sketch is done using a Orion XT10 with a 13mm Stratus with the HB filter."
Using our own Eyepiece program, we have produced an inclusive chart of the calculated performance of all our oculars, current to July 2012, with the XT10, which you may obtain here as a rich-text file. We happen also to own both the 5 mm and the 13 mm Stratus oculars. The results we got were these:
5 mm Stratus:
Magnification: 240x; Estimated visual FOV: approx. 17 arcminutes; exit pupil: 1.1 mm;
13 mm Stratus:
Magnification: 92x; Estimated visual FOV: approx. 44 arcminutes; exit pupil: 2.8 mm.
Indeed: in looking over the findings of original designer of the first amateur visual Hydrogen-Beta eyepiece filter, Dr. Jack B. Marling, an exit pupil of 1.1 mm is out of range: too small, though 2.8 mm is very, very close to his 'minimal recomended' 3 mm.
Jack Marling Recommendations for Exit Pupils:
Courtesy of Dr. Jack B. Marling, Ph.D.
|Recommended Exit Pupil Range: Suburban sky
|Recommended Exit Pupil Range: Rural sky
So: no wonder Jay was, at first, unable to see the Horsehead, with the 5 mm Stratus scope PLUS the h-beta filter. At the exit pupil of 1.1 mm, everything except a few stars would 'go black'. This exit pupil might indeed work in a 20-inch scope, or larger; just not in an instrument with this particular light gathering area. Edward Barnard visually inspected the Horsehead at high magnification of 460x in the mighty Yerkes Observatory 40-inch scope in 1913, at an exit pupil of approximately 2 mm (according to calculations we performed back in 1990); he could definitely see the object: "the spot is certainly not clear sky, for the field was dull, apparently indicating the presence of some material substance at this point. To me the observation would confirm the supposition of an obscuring medium," he concluded. However, Barnard was not using any form of filter, so he did not have an 'artificially' darkened sky luminosity, nor a diminution of any visual wavelength that could pass through all that glass.
Jay's perception of the Horsehead with the 13 mm Stratus (exit pupil ~2.8 mm) and the h-beta filter is reasonable, practical, believable, achievable according to what our colleagues determined in our Horsehead Project tests.
Here is, by permission, our rendering of his original image, which we inverted (white background, black stars) and then marked, according to the method described above:
We have concluded that this is a great example of a very good and accurate rendering of a real-world visual perception of the Horsehead, in an exceptionally dark sky, using the h-beta filter and all other devices mentioned. We consider it to be, perhaps, something of a reference drawing. We have taken the liberty to clarify the label (incorporating Jay's specific documentation of eyepiece and filter) and -- since the original looks so very dark on our personal monitor -- a Samsung 2333T on Nvidia GeForce MX 420 video card, with 32 bit color at 1920x1080 resolution, we have also had the effrontery to tune his black level slightly so that the Horsehead is unmistakable, and the faint glow of IC-434 is well registered. And now, the normal realistic mode of seeing Jay's superb picture:
Thank you, Jay, for your excellent work! Everything seems 'right' and convincing to us: you have drawn stars that are as faint as about 14th magnitude; they are quite well positioned (and the negative version shows you have attempted to show that the stars are 'imperfect' in your eye view, to the extent that you have delicately replicated the light scatter seen in most Newtonian reflector telescope views of the stars, and at the same time you have very well registered the slightly irregular halo of light of NGC 2023); your Horsehead dimensions are comparable to what many accurate viewers seem to perceive; and you have registered about the sense of shape and dimension that can be distinguished in this aperture of instrument, best-case. Finally, in your exact technique in rendering the stars, the image seems to have a sort of depth compared to most 'simpler' hand drawn starfield sketches (more on that in the box, below.)
Depending on your exact monitor grayscale setting, you may have a somewhat more 'realistic' view (matching the extremely dim sense of pale gray above the dark background) if you see his original rendering, in his blog, against the webpage's overall black background: click for the entire blog entry, here. For instance, on our iPad with 'Retina' display, the glow of Jay's drawing IC-434 is just visible above the 'nothingness'; but on several of our PCs -- especially ones with older CRT monitors -- the overall-brighter rendering that we created by readjusting his picture, may show the difference between IC-434's luminance, and the darker Horsehead, with less ambiguity. Truly: no two systems look exactly alike, and the visibility of 'realistic' sketches also tends to change, psychologically, according to the brightness and color of the 'land' around them, when they are shown on a webpage (such as this one) with a fairly bright background. Since Jay mentions that he uses an iPad, perhaps such a comparable device will best replicate the experience he intends to convey. (One is reminded of the nature of 'high end' audio reproduction systems, versus the 'normal' so-called hi-fi gear most people have. The high-end audio experience can convey music with much more nuance and detail--but one pays a price for that. However, many music lovers still claim that 'they get the heart of the artist's musical experience' even from low-end playback, if they focus on purely musical elements -- phrasing, agogics, rhythmic accents, etc. -- even on equipment that doesn't have the purest 'tone'.)
A Horsehead Viewing Experiment
We've made a suggestion to Jay, for his future investigations and drawings of the Horsehead. The author has very occasionally -- on exceptionally fine nights -- had an uncanny experience of being able to discern the Horsehead cloud as an actual 'presence' -- an obstructing, dark body -- rather than merely as a dull, darker spot interrupting IC-434's faint glow. Edward Barnard had that experience back in 1913 when he examined it visually with the mighty Yerkes 40-inch refractor. To do this, one must set the exit pupil/magnification so that there is, even when using the h-beta filter, some overall field illumination visible.
The h-beta filter may often darken everything except just a few stars in the field: if so, the power is too high. That's what Jay experienced when he tried the 5 mm eyepiece (at 240x) and saw no trace of the Horsehead. In changing to the 13 mm (92x) he now found that it was visible; and that's when he made his drawing. But, lowering the magnification even more, on some nights of rare perfection, may bring about the revelation of the shading of the Horsehead's dull cloud. It changes from a 'very evenly dark bland spot' that is the same dullness as the dark field away from IC-434 (using an exit pupil that is on the smaller side of Marling's recommended range), to 'the one dark region in the ENTIRE field of view' that is just pitch blackness, which has 'depth' because it rapidly fades around the edges. This is a bit hard to verbalize in a few words; do the experiment by changing powers, using the entire "Marling range" of exit pupils, and see if you can have the experience; the author has enjoyed this with scopes ranging in aperture from 7 inches to 20 inches, so one doesn't necessarily get this heightened sense only in monster equipment.
Your optimal view, then, will be the one in which there is some faint level of field luminance even at the outer edge of your eyepiece field, the h-beta filter now not darkening everything too much: at some magnification you may be able to see the Horsehead as that 'vague body' that is truly black, blacker than black, having almost a dimensional depth, compared to the general faint luminous ground. In fact, one speculates that this kind of view might be possible only when there is minimal upper atmospheric sky glow: so it's a relatively rare experience. That suggests that you should be advised to check the Horsehead often, during Orion's season.
Let's find out if someone, in future, can have that same visual experience... and then reproduce it in a drawing!
Most Detail Sketched In Horsehead Field So Far?
On the "Deepsky Forum" website, the very well known optics technology expert, telescope maker, and observer Mel Bartels (of Cottage Grove, Oregon) began a discussion thread on February 24th, 2015: "The Horsehead area with a 13 inch f/3 telescope under superb transparency""; and he reports that with his unusual custom 13 inch f/3 reflector "with h-beta filter, the nebulosity just went on and on and on - never seen close to this extent. I spent half of my observing night tracing out all the extensions." The image itself, and his post, contain no copyright statements, so I am taking the liberty of reproducing it (in reduced size and NEGATIVE mode for clarity) here on this page; the original sketch he uploaded is here.
This drawing is probably the highest-detailed human eye 'live sketch' I have seen! From this version of his sketch, I derived the following information:
• The orientation of the drawing (not stated) is approximately S-up, W-right.
• The field of view, as stated, is 1.8 degrees. I made several attempts to match stars and measure this field diameter, using Megastar 5; my best try was a field diameter of somewhat more than 1.7 degrees, using stars on the computer chart that were not quite on the exact field-stop of the drawing; so Mel's perceived eyepiece TFOV is, in my estimation, CORRECT.
• 1.8 degrees = 108 arcminutes = 6480 arcseconds. Therefore, since the field width measures 512 pixels, the scale = 12.65" per pixel.
• The Horsehead itself measures: 29 pixels length for 'neck'; 19 pixels for width of 'snout' and top of 'neck'.
• Therefore, in this rendering, Mel's perceived dimensions for the Horsehead calculate to approximately 6.11 arcminutes (neck); 4 arcminutes (snout). These dimensions are, in my experience, very accurate with respect to an optimal eye-view in a 13" aperture scope. (It's very likely, in my experience, that viewed under poorer conditions, with with smaller apertures, the 'snout' detail will smear into a smaller dark 'lump' connected to a somewhat shorter 'neck'.)
Mel also shows the "Tank Tracks" or "Flame" nebula (NGC-2024) in the lower right quadrant, with good detail of its light and dark regions, and in the upper right he makes special note of what he describes as "dark ovals" in the bright nebulosity of IC-434. I have only seen these once , just barely: in the 22-inch f/13.4 classical Cassegrain "Tauchmann" reflector at Lick Observatory, using an Orion UltraBlock filter and 40mm TeleVue eyepiece (a magnification of about 187x, and exit pupil of 3 mm: lowest power I could achieve with the instrument), back in late 1988. Furthermore, I have NEVER BEFORE seen a drawing of these so-called 'ovals'!
An examination of POSS-1 blue and red plates in small rectangle boundaried on the north at (J2000) 05 42 00.4, -02 55 44.9 and south at 05 42 00.43, -03 15 24.7 reveals that there are dense 'nodules' in the opaque hydrogen cloud of IC-434. They might be overlooked or undersampled in a low-contrast image, particularly a low-res one done by an amateur wide-field small-aperture instrument; but they show up clearly in the high-res and deep exposure of the old Palomar plates taken by the 48-inch Schmidt camera. I cropped this region of Mel's drawing and compare it to highly enhanced negative and positive crops of these plates, downloaded from Skyview. The accuracy of Mel's drawing is admirable, as you can verify; and his perception of these 'ovals' (as he terms them) is uniquely distinct and remarkable!
Click on above image for larger one with better detail
Congratulations to Mel Bartels for his extraordinary contribution! Read all about his 13" f/3 scope here; and among the fascinating optics and observing articles on his general Amateur Telescope Making page, I should draw special attention to "Observing Dark Nebulae", with links to many sketches of Barnard objects and Lynds dark nebulae. (I would also like to express my thanks to Mel for his assistance provided in my 1997 study and usenet post about viewing Leo 1, and his permission granted back then to use his pioneering CCD image, which you will find at the top of my article about the galaxy, derived from an original sci.astro.amateur post. Mel is one of the very few ATM's -- amateur telescope makers -- who is also as deeply skilled and involved in the highest level of advanced celestial observing; he's not merely an optics or mechanics expert. As such, he reminds one of...none other than William Herschel!)
Small-Scope Horsehead Sketch
Jaakko Saloranta had a long-time aspiration to see the Horsehead nebula, which he had listed as one of his own personal challenges. Finally he was able to do it at La Palma, Spain, in the Canary Islands: one of the world's great deep sky observing sites, at an altitude of 2390 meters. He provides a short report and two sketches, using a 4.7 inch aperture Sky-Watcher achromat refractor of f/5, at 38x and 80x. The conditions were so good (with naked eye stellar limiting magnitude measured by him at 7.5) that he was able to render the Horsehead with a UHC type filter (a feat that I have not been able to match from my site in California, with a virtually identical scope; I've always needed the h-beta filter in such a small aperture instrument.) Click here for his current online Horsehead reports, and in case this, for some reason, disappears in future, I shall also give the link to my own archived version.
As you may see, Jaakko uses NO computer 'spray tool' or gradient-shading effects; these are remarkable raw sketches done in the field, and shown EXACTLY the way he originally produced them with pencil and paper at the observing session.