C11 Mirror Shift Fix

I'm going to just cut to the chase on the mirror support mod I did to my 2002 (I think based on the mirror markings) factory carbon fiber tubed Celestron 11. The mirror support "issue" has been well documented on other posts. Here is the short story of the "why" of the mod.:

I bought the scope, used, in June of 2011. Instrument took wonderful images when I got it but over time I started to notice star images on one edge of the frame were definitely not right. What changed equipment wise? In 2011, I did not have a remote setup. I was at the scope getting everything ready to image. After getting the focus close by me turning the focus knob, I used the Hotech focus lock device on the focuser to "lock" down the mirror. Living in MN, we get the bitter cold and in summer at dusk, we become part of the food chain for bugs. Enter the remote observatory setup (I would rather donate blood to Red Cross than the mosquitoes). I installed a Robofocus for course focusing so I really did not have to leave the house to get everything set for imaging. That's when I started to notice some image degradation but not all the time.

Experimenting revealed I could collimate with the scope pointing SE and if I took a pic in that position, the image would turn out reasonably well. I have CCDAP5 set to refocus with FocusMax4 every 60 minutes (with an OPTEC TCF-S) and over the course of a night it might move 500 steps. Meridian flips would definitely throw off collimation and images taken after the flip were not the best. Using CCD Inspector on the images confirmed something was off.

So after fighting with this for the past couple years, I decided enough is enough. I know the optics are capable of taking excellent images so I was convinced it was not the optics. And I was not afraid to take things apart. The Celestron SCT's are not that complicated. Reading on forums what other people have done to cure this got me thinking about what might be the best "fix" for me. I am amazed at the ingenuity of contributors to AM and other forums. Seems like it all comes down to the clearance of the mirror support on the support tube attached to the rear cell. So how to take up the clearance? I liked the "fix" by a CN'er that put nylon screws into the mirror support tube to take up the clearance. That "fix" had the nylon screws on the mirror support tube in front of the first surface of the mirror. That was going to be my fix with one twist. I know the mirror assembly is very heavy on the mirror end. So in addition to the screws in the front (I put 4) I put two nylon screws behind the mirror. After tightening the screws, I put a dab of black automotive RTV to secure those two screws since I could not get a nut on those screws due to clearance.

Picture is worth a thousand words. Mirror assembled to rear cell. Screws just finger tight.

Next post I will show the results.

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Now for the results. Collimated everything using Hotech and under the stars. I was a little disappointed that my focus shift didn't improve much upon initial testing. But now it was time to image. I picked targets on either side of the meridian and in different directions and did an imaging run. The morning arrived and I opened up the images with some (or a lot of) apprehension. I was pleasantly surprised. No, I was elated! No difference in image quality no matter where the scope was pointed (confirmed with CCD Inspector). I check the FM4 logs to see where the Optec ended up. To my surprise, the Optec did not have to move that much over the course of the night (at first I thought focus had failed and returned to an initial setting but it had not!). I still have focus shift (target moves in the FOV when focusing up to a point) but it would appear that the primary mirror does not shift around on the tube.

Last night I took 18 subs at 10 min each exposure and the Optec only moved 47 steps over the course of the night.

Below is a shot of CCD Inspector. My stars are noticeably "tighter" as was my guide stars.

This mod does not require any special tools just ordinary tools. A drill, a SHARP bit (buy a new one), a tap, nylon screws and a deep breath before starting. BTW, drilling through the rear of the cell took a little time but it was all aluminum. Did not have to center punch with a sharp bit. I would recommend someone hold a vacuum near when drilling to pick up the chips.

I have taken close to a 100 subs since this mod and all are consistent in quality. So far, so good. Time will tell.



Chris N
Cedar, MN

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