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 extentions 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!
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 discernable 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!
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:
We are happy to say that Jay soon replied, clarirfying 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:
13 mm Stratus:
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:
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!
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 an archive of his webpage.
There is quite a debate, in artistic astronomical sketching circles, about technique. Modernists love the repeatable, controllable perfection provided by graphical editing software. But, purists would aver that the only way to do a sketch is simply with drawing pencil on paper--period. One makes every possible effort to scale star magnitudes by the size of the circle of individual stars. However, eye and brain don't perceive star brightness -- and create a perception -- QUITE that way.
Starlight photons arrive at the outer periphery of Earth's atmosphere in parallel bundles; then the air shakes them up; and finally, when they pass through the aperture of the optical device, a diffraction pattern is created, and its complex character is affected by the size of aperture and other aspects of the optical system (see this highly technical article for more information.)
For novices: the gist of it is that a star's image is formed through your optics as an "Airy disk" with a strong, round, bright central core of light, surrounded by one or more 'diffraction rings'. The nature of your telescope will determine the diameter of the entire phenomenon, the complexity of the rings, and the relative diameter and intensity of the 'central peak'.) The telescope will inevitably distort the 'ideal' diffraction pattern, as no objective is perfect; and eyepieces will add more distortion. Most human eyes have some astigmatism, which may increase with large exit pupils (low magnifications); stars end up being jaggedy little 'messes', the dimmer ones being 'smaller, dimmer messes' than the brightest stars' 'large, bright messes'.
The question is this: how do you render what you see of the stars? Just with a pencil, trying for consistency as the magnitudes increase/decrease?
No artist is PERFECTLY consistent, drawing by hand. No one can do it as precisely and repeatably as, for instance, the planetarium program Megastar 5: the way star sizes/magnitudes are scaled in their onscreen plot is shown here. Some artists are not happy with their inconsistent hand-rendered star sizes, and use a small plastic template with tiny holes of varying diameters. Some ultra-purists just draw, with no aid (perhaps holding a very sharp pencil upright, and twirling it) and you get what you get--it's 'raw data'. Some 'clean it up' after the image is scanned into a computer file. Others, non-purists, prefer to 'paint' the stars over the ones they've drawn, having made up tiny black dots that are scaled in diameter according to magnitude. The idea, to retain accuracy, is to match your 'pasted on' perfect, round, clean stars exactly to the ones you have tried originally to draw. There are more variations on the theme, but you get the idea: does one use some form of graphical computer "help" -- or not. The present author is agnostic, and likes renderings which seem honest, consistent, and reasonably reproduce what the eye probably saw. We seem to believe that Jay's Horsehead has had some computer-assist as the star images are so round, so consistent. As far as we're concerned--that's fine! Timo Karhula is of the "old school". That's fine, too!
Star chart software, and monochrome survey photos, are of NO help whatsoever--because the magnitude ratings were done with filters, by photometric processes that do not match human eye color response. So, do your best, whatever method you use, to show what you perceived. Below a certain magnitude, the stars all pretty much look 'white' or 'gray' and you've lost the color perception to show their total spectral shifts from neutral white light; but your perception of their brightness is important to try to register. In a mechanical drawing with pencil, you can only make the star bigger/smaller in diameter, or increase/decrease the density of your markings. In a way, it's easier to be precise this way, than in trying to do it with graphics software: for you can't take raw data at the scope, and simultaneously 'edit' a graphic image with, say, Photoshop! That comes later, at home, after your observing session.
So, try (by whatever technique you can manage) to get your perception of the individual star diffraction patterns registered accurately and consistently with pencil and paper--for this is the only real data you'll have to work with, later, at the computer. And, if you depict nebulosity and its fading 'edge' with careful pencil strokes, or by the 'spray' tool in your graphics editor: that's fine either way, in our opinion: as long as you take care to try to preserve what you saw so that your 'data' are not really falsified into something beyong your eye's capacity. -- srw, 2012
by Stephen R. Waldee, amateur astronomer
Developer with Ron Wood of Eyepiece 2.0 Software Program
San Jose, California