| Around 1994 I started looking for a fast and small telescope with a large field of fiew. Celestron's Comet Catcher (Schmidt-Newton) appeared as a nice choice, since it had a 140mm aperture at 500mm focal length (f/3.6). However, its production was stopped many years ago, so I had to look for a used one. When it was still in production one could find them regularily on the second-hand market, mainly because it was designed for photographic use only, thus being more or less unsuitable for visual observations. When I got interested in it basically all exemplars seemed to have found a happy owner, since they were no longer advertised in the magazines. It took me two years to dig up three people in Germany who were willing to sell their CC. The first one declined after thinking over it again, the second one had a very old one which looked horrible, and the third one, finally, was the one I was looking for:
Visual usability of the CC
With a 22mm wide angle eyepiece the CC provides one with a 3 degree field of view. And THAT was an improvement compared to the C8. I remember that in the very first night I started at M 52 in Cassiopeia, and then just scanned along the milky way until I suddenly ended up at M 42, only looking through the telescope. I thought that happened by chance and decided to move "back", which brought me to M 52 again.
An UHC filter in the 22mm eyepiece unveiled many of the nebulas in the Milky Way, some of them appeared exactly as outlined in the Uranometria 2000.0 sky atlas. All this was great, but a disadvantage was the very difficult optical adjustment of the CC. In general, telescopes with fast focal rations are very sensible once it comes to adjustment, and the CC with f/3.6 is a very fast instrument. Besides, one has to to remove the corrector plate before one can adjust the three setting screws of the secondary mirror, which makes a collimation at night, while looking at a star, impossible. However, with a laser collimator I could achive reasonable stellar images. The Schmidt corrector plate in front removes about 75% of the comatic abberations in this system, with the remaining 25% very well visible. In order to remove 100% of the coma one would have to place the Schmidt plate in a much larger distance to the primary mirror, which would make the OTA (optical tube assembly) much longer and heavier, and thus more expensive. The longer tube then requires a sturdier mount, and so on. So Celestron voted for the light-weighted, compact compromise, as did by the way Meade with their Schmidt-Newton series.
Photographic usability of the CC
Due to the tiny back focus I didn't use the CC photographically in the beginning, since there was no way to squeeze an off-axis guider between the OTA and the camera. Maybe the Lumicon Newtonian Easy Guider could have done the job, but I never gave it a try. Only when I got the G11 mount I started imaging with the CC, since I could mount it parallel to the C11, using the latter one as a guide scope. With this 20 kgs load the G11 was at its photographical limit (according to my taste), but still working fine. Unfortunately this setup quickly turned out to be insufficient, since I always got more or less trailed stars in the images. This was very unsatisfactory, since I could see that the CC was indeed a very powerful photographic instrument with 35mm film cameras. Residual comatic abberations wouldn't show up on low resolution Scotch Chrome 400 or Kodak E200 slide film. However, these trailed stars spoiled all images I made. In the beginning I thought it was the mechanical conncetion to the mount, or the tube rings, or the non-parallelity between the CC and the C11 OTAs, and that this would get me those shifts. In the end I learned that the culprit was just the "mirror flop" in the C11. Since the primary mirror in Schmidt-Cassegrain telescopes is used to focus the telescope and is thus not fixed, it changes slightly its position when the telescope tracks, due to gravity. One can very well fix this problem with some effort, and it is extensively described in the internet (see Roth Ritter's homepage for example). However, I was too lazy doing this modification, and the CC started a long hibernation period in its box until I came up with a CCD camera (see below).
The CC is very difficult to focus with a CCD camera. Although it perfectly keeps the opical adjustment while focusing (the secondary is moved along the optical axis for focusing), the fine resolution and the f/3.6 ratio of the CC require the focus to be reproducible within about 0.02 mm. The focuser of the CC, however, allows only 0.1 mm. With the tiny focus knob and the not very satisfactory mechanism it was not easy to achieve good focus with a 1 kg heavy CCD camera attached to it. I think that I never achieved a perfect focus with this instrument, but I mostly managed to be reasonably close. The advantage of the short tube and focus tolerance I finally accepted for myself was, that even during large temperature drops I did not have to refocus the scope. The short focal length made guiding easy, and the huge field of view with the ST8E (90' x 60') allowed shooting also extended objects, which is really fun with a H-alpha filter in case of galactic nebulae.
Images with the CC and a ST8E
Images taken with the CometCatcher can be found in the gallery.
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