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I have made precise measurements on the back plane aperture and the sizes of several attachments to my 10" LX200. (same dimensions for the 12") I have also carefully viewed the image with a low power eyepiece to get an estimate of the actual amount of vignetting. The purpose is to get an estimate of the available image size and how it is limited by image quality and vignetting.
Here are the dimensions: The size of the aperture in the back plane is 57 mm. The normal adapter plate supplied has a thread for Schmidt adapters. The aperture in the Schmidt thread adapter is 38 mm. The large aperture on the rear of the tube is thus greatly constricted by the normal Schmidt adapter opening. This is because the Schmidt thread, while somewhat standard on the back of SCTs, was originally designed for small telescopes. Several manufacturers provide attachments for the larger telescopes which have a larger apertures. The JMI focuser can be obtained with an adapter that has a 51.2 mm aperture. They recommend this adapter when mounting their focuser on the larger SCT. Their focuser itself has an aperture of 50.6 mm. (basically 2 inches) The aperture on a 2 inch eyepiece is slightly smaller. Generally 48.5 mm. A 48 mm filter has a clear aperture of 43 to 44 mm. A "T" adapter tube (photographic style) has an aperture of 37 mm. The 0.63 focal reducer has a clear aperture of 41 mm.
So, what do all these figures mean? It is clear from the numbers that all so called 2 inch equipment attached to the rear of the SCT has its light path considerably constricted by the 38 mm Schmidt thread aperture. So there seems to be good reason to open up the back plate to a full 2 inches as many accessory manufacturers have done. Another question arises when doing 35 mm photography at prime focus. Is there any vignetting at the corners of the image? The 38 mm aperture of the "T" thread is quite marginal when one realizes that the diagonal of the 35 mm frame is 43 mm. In fact the "T" adapter was from the beginning recognized as marginal in size.
Incidental dimensions: The thread of the Schmidt connection is 50 mm diameter with a 1 thread per mm (1.00 pitch). The thread on the T adapter is 42 mm diameter with 1.333 threads per mm (0.75 pitch) The tread on a 48 mm filter is of course 48 mm diameter.
To judge vignetting one must take the following viewpoint. Think of looking from a point on the film plane toward the lens. Vignetting will not occur as long as the entire rear exit pupil of the lens is visible. Since any 35 mm camera lens is a very thick lens, the rear exit pupil is well toward the rear of the lens and often quite small (an inch or so). Thus even from the corner of the film plane, all of it is generally visible through the "T" mount aperture. If the rays that form the image on the film plane came from a great distance, the aperture would cast a shadow that cut off the corners of the image. The imaging rays from a telescope come from the secondary mirror through the center tube which forms the limiting stop. Because the rays come in very nearly parallel, the aperture in the "T" adapter is slightly too small and there will be a some significant vignetting of the corners of the 35 mm image.
For total freedom from vignetting, an adapter should be used which opens up the constrictions described above. Several manufacturers supply such adapters. Lumicon being one. I inspected the real image at the back of the telescope with an eyepiece just as one normally does when viewing but without the adapter plate. When inspecting the exit pupil it is clear that slight vignetting starts to take place with any deviation from the optical axis at all. This vignetting is very small. I estimated it to be a half stop to a whole stop at a distance of 25 mm from the optical axis. Moving to the very edge of the aperture, 28.5 mm from the optical axis, was unsatisfactory since the primary showed an image of the corrector plate. Allowing these rays into the imaging chamber would cause severe flare. It is very encouraging that a full 50 mm diameter of real image at the focal plane looks quite free of vignetting. (possibly one stop at the most) Some other significant optical aberrations seem to occur at about the same distance from the optical axis. But I would judge that a full 50 mm of real image is useful. We can be comfortable that this image easily covers a 35 mm frame if it is not restricted by adapter tubes that are too small. It also means that a full size focal reducer of 50 mm (0.63 reduction assumed) will give a 32 mm image which covers most of the 35 mm frame. The regular focal reducer with a 41 mm clear aperture only yields a 25 mm image circle which is not enough to cover the 35 mm frame. My experience is that with this reducer significant vignetting takes place on 35 mm film.
It makes sense to use larger diameter focal reducers for both 35 mm film and especially for 6 X 6 cm. size film. Lumicon makes such a focal reducer. Additionally, large rear plate adapters are made by JMI and Optec. Optec in fact makes a filter slider adapter which takes full aperture 2" filters. Additionally, when it is so clear that the real focal plane image is so generous in size, it makes good sense to use 2" eyepieces where the full real field of view of the telescope can be enjoyed. This is true mainly at lower powers of course. (Generally with 30 mm and longer focal length eyepieces which have a field stop nearly 48 mm in diameter.)
I have always been concerned about the connection of diagonals to the viewing back of the SCT telescopes because of the limited size of the tubes used in their construction. In the photograph below are shown two 2" Meade diagonals. One has the attachment tube with the ring for the Schmidt thread back plate. The plate is shown on the bottom right. The second diagonal has the attachment for insertion into a 2" tube. This is used with the JMI focuser for example. Both adapters are shown dismantled. That is, the diagonal is shown at the top, the tube and and ring below and in the case of the 2" tube a small adapter ring that is used to connect the 2" tube to the diagonal. The small ring is used so that both forms of the diagonal can use the same main diagonal body.
The reason for showing this detail is to show the internal diameters of the tubes. Both of the tubes for the diagonal are 41 mm clear internal opening. The tube shown at the right is a Tele Vue Schmidt to T-thread adjustable adapter tube. I show this tube because it has several interesting applications. It is intended to connect a camera with T-thread to the Schmidt thread on the back plate adapter of course. But it also has exactly the correct dimensions so that it will accept a 2" tube. The front part is shown below the complete adapter. Notice that it has a Schmidt thread on the front and a nice large set screw to hold any 2" tube.
Now consider the inside dimensions of the tubes. The opening on the Schmidt adapter plate has an opening of only 37 mm. This is one of the serious flaws in the use of the Schmidt thread. It was originally intended for small SCTs such as the 8" which itself has a limited opening in the back plate as described above. So when a 2" diagonal is attached to the back plate with a Schmidt adapter, the cone of illumination is limited by the 37 mm opening in the back plate. This is unfortunate with a larger telescope like the 10" or 12" SCT since they have a larger opening in the back which would take a larger adapter tube. This is why JMI and Optec provide back plate adapters with larger openings, to reduce vignetting.
Now the short part of the Tele Vue adapter could be used to take the diagonal with the 2" tube directly. This would enlarge (improve) the opening from 37 mm to 41 mm. Not much, but a difference of 20% in light passage and less vignetting. This might be helpful with the larger SCTs. Additionally, the use of the TeleVue tube provides a very nice way to adjust the angle of the diagonal for convenient viewing. Note also that the size of the field stop with both the 40 mm and 56 mm eyepieces is about 46 mm. Thus for these longer focal length eyepieces with the large field stops, the cone of illumination is definitely limited by the connecting tubes. Now the ideal situation would be to use the JMI back plate which has a full 50 mm opening and connect the diagonal, or better the eyepiece directly to the back of the telescope with an appropriate sized connecting tube. The Tele Vue tube is ideal for this application. If you use the JMI focuser you also have just this arrangement.
In summary, I suggest that the sizes of the diagonals and the connecting tubes
is a restricting factor and that a larger diagonal and connecting tube set would
be better when 2" long focus eyepieces are used. I do not at this moment
know of diagonals which are slightly larger with appropriate tubing which would
not vignette on the 2" eyepieces. Both the Meade and Tele Vue diagonals
are quite good however.
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