There is no lens in this telescope.
The light travels the length of the tube undisturbed
to be reflected back to a focus near the top of the tube.
The glass of the mirror need not be optical glass since
its purpose is only to precisely support the aluminum reflecting coating. If you looked at the image with an eyepiece your head would block the incoming light. A small diagonal mirror is placed before the focus to reflect the light out the side of the tube for viewing. The telescope primary mirror must be polished very precisely to form the perfect image. It has been shown that persons of limited means are able to produce the precise mirror and the telescope. A telescope with a 12 inch primary mirror will gather about 2000 times more light than the unaided eye. Magnification is usually adjusted from 80X to more than 250X. A mid range of perhaps 140X is more usual unless atmospheric conditions are exceptional. Lower magnification is often used to view dust clouds in space. Dust clouds may be the remains of a nova or a place to look for new star formation. Using our telescope faint objects such as planetary nebula, globular clusters and galaxies become visible. Faint gray appearing stars take on colors of blue, yellow or red. The polar cap of mars, the moons of Jupiter, and the rings of Saturn are easily seen. Viewing the moon as the earth's rotation sweeps the field of view across the lunar craters reminds one of pictures taken from the Apollo Command Module while in orbit at the moon.
Mirror Design Overview
Don't bother to make a mirror smaller than 6 inches. You would be better off buying 7 x 50 binoculars. People will tell you "Start by making a six inch first". We think this is bad advice. Most people will only make one mirror so why not get the most results for the work? Lets compare a 6 inch and a 12 inch. Most of the extra effort of the 12 inch is involved in lifting and grinding the heavier glass. People pay health clubs for the opportunity work out so depending on your physical abilities this may be nothing. The 12 inch would be capable of twice the magnification were it not for the effect of atmospheric turbulence. The resolution of the 12 inch will be noticeably better than the 6 inch in all but the worst seeing conditions. Then there is the big plus of the 12 inch - four times as much light. Sixth magnitude stars that appear as faint gray points will become red, yellow and blue stars and nebula that are only barely detectable begin to have definite shape. In the 50's few amateurs made mirrors larger than 6 inch. In the 60's the 16 inch porthole mirror proved to be possible but most people were still making 8 inchers. In the 90's we have seen lots of 10 inchers. In the 2000's we see a 20 inch now and then plus many older scopes being restored.
Focal length is the distance from the mirror to the point where a stars light comes together at a point. Focal length is half way between the point at the center of a circle and the mirror. The mirror being just outside the circle so that the mirror face is on the circle. The 'Radius of Curvature' is the radius of this circle. F-number is the focal length divided by the mirror diameter. If you had a 10 foot focal length and a 12 inch mirror the telescope would be F10. Since you look in at the top of the telescope F10 is too long. For reasons that will be explained later mirrors shorter than F4 are only usable for photography. Almost all reflecting telescopes that you will look through are within the range F4 to F8. Here are some numbers for radius of curvature. For six inch mirrors the radius of curvature could be 90 inches (F7.5). For a 12 inch mirror this distance could be 150 inches (F6.25). Since the rest of the telescope will be built later these numbers for mirror radius need not be exact.
Mirror grinding kits are available from various sources. The only things you need that are not commonly available are the mirror blank, the pitch for the pitch lap and the various grades of carborundum. The carborundum can be bought at a lapidary shop.
If getting by as cheap as possible is the objective people have made mirrors out of scrap glass. You will spend a lot of time on this mirror so use Pyrex or a good porthole. The mirror blank should be at least 3/4 inch thick for mirrors up to 8 inch. For mirrors up to 16 inch use glass at least 1 1/4 inch thick. Mirrors thicker than 1/6 their diameter are unnecessarily thick and heavy.
An important part of the Dobson style of mirror making is to avoid time consuming details that might keep people from finishing their telescopes. Mr. Dobson has told me that a team of telescope makers have made a telescope in a day. If you are not in a telescope making class there are many parts and tools to locate - this will add to the time. In order to determine the required time more precisely I decided to build an 8 inch telescope from scratch and record the times. My style of working is to spend Saturday afternoons on the project. I would expect to spend at least six months of Saturday afternoons on the project. Doing the same project a second time with all the materials and tools collected would shorten the job dramatically.
Below are links to the 8 inch telescope project.
Making a full sized grinding tool. ( may be optional )
Making the pitch lap and polishing out.
Building the telescope tube and base.
Some jobs here require the use of power saws and grinders.
See the caution.
Coating the mirror, preparing the telescope and installing the mirror.
These plans may be scaled without
modification for a telescope of 12.5 inch aperture.
Large ready made telescopes cost
much more than small ready made telescopes. So if you want the most telescope
per hour spent building then build a large telescope.
Foot Note {2000 times}
To calculate the gain in image brightness use the following formula. The pupil of our eye; 6 millimeters divided by 25.4 mm per inch = .236 inch diameter or an area of .043 square inch. The diameter of the primary mirror; 12.5 inch or an area of 122.7 square inch. Subtract the shadow of the diagonal mirror = 116.7. Times the reflectivity of the primary; .9 = 105.03 Times the reflectivity of the secondary = 94.53. Times the transmission of the eyepiece, say .9 = 85.07.
85.07 divided by .043 = 1978 times more light.