Trying out a used Celestron C11

In January 2014, I made it¬†another step closer to becoming one of those amateur astronomers who has x number of telescopes…that his wife knows about. Celestron marketed the C11 Ultima, a 280mm f/10 telescope, in the mid-1990s, and I picked one up second hand at Hands on Optics for a very reasonable price. And by the way, I recommend that anyone interested in a telescope or binoculars stop in and see Gary Hand, because you never know what you might find there. The telescope had been installed in an observatory, and as a result it was somewhat exposed to the elements for a number of years and it needed some work. I cleaned the exterior surface with a premium car polish, and it looks as good a new with the exception of some minor scratches. The V-style dovetail plate was loose because the attachment screws were too long, and therefore would not tighten. I was able to easily repair this. In the short term, I wanted access to a larger SCT reflector to gain experience with a longer focal length and bigger aperture instrument, and to learn how to collimate a reflector. I have been able to take it out just once, and I can see that I have a lot to learn. In the longer term, there may be some potential for more work on the the telescope. I doubt that the mirrors have ever been resurfaced, so that is a possibility. The multicoating on the corrector plate glass is in pretty bad shape. I hear that this is an expensive repair, so I might just have to live with it. And finally, I may install a cooling fan, which will help the telescope reach an equilibrium temperature with the outside air faster. The telescope weighs 22 lbs. Since this approaches the maximum capacity of my mount, I am already thinking about my next major purchase.

© James R. Johnson, 2014.

4 thoughts on “Trying out a used Celestron C11”

  1. The problem that caused me to state that I have a lot to learn in the original post was that all of the stars looked like miniature comets. I suspected that I was not collimating correctly, so I spoke to an expert. I learned that the that the problem is probably caused by the corrector plate (and the secondary mirror along with it) not being centered in front of the primary mirror. I decided that it would be a good idea to clean the primary mirror will I was working on the corrector plate. There were some pretty nasty looking spots “growing” on the mirror. I cleaned it gently, and the spots, which I was concerned might be defects in the reflective coating, came right off. I centered up the corrector plate and waited for the next clear evening.

  2. I was able to take the 11″ out tonight for the first time since I took it apart to clean the mirrors and attempt to recenter the secondary two weeks ago. The stars now look like stars instead of comets, so that seems to have done the trick. On Jupiter and the moon, the images held up under higher magnification. Orion nebula was the best I have ever seen at the eyepiece, even with the nearly full Moon nearby. Did not try any photography…saving that for another day. Ummm, another night. All in all, I am very pleased with my new acquisition.

  3. I took two really bad images of Jupiter last night. Even though the images are fuzzy and lack detail, they demonstrate that a fairly large image is possible. I can detect more banding and perhaps the Great Red Spot is redder in this image. Certainly know that dew was an issue, because it was a really wet evening, and I do not yet own a heater for this telescope. Seeing was not great, so that is a possible source of a problem. I do not yet own a focus mask for this telescope and good focus is difficult to accomplish under less than idea seeing conditions. I am hopeful that I can correct the problems that I mentioned and produce a better image.

  4. I bought a Bahtinov focusing mask for the C11, and worked with it a little last night. The stars seem tight and focused, but I think that Jupiter’s detail that I expect to capture is the real test. Jupiter is coming up on opposition early next year, so I’m going to shelve this project until Jupiter is above the treeline at a decent hour.

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