Posts Made By: Chris Nisbet

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.

I live in a northern suburb of Minneapolis area and I too, finally got out to image the comet and M45 last night. Sky seemed hazy but you have to take what you get around here. Observatory temp was +6F. Used an 8" Epoch/Celestron Schmidt on an Ultima 8 mount for imaging. Got the comet with it's 4 deg tail and M45 the same frame (after two tries). Apparently, these drives do not like this type of cold. Drive made a god-awful noise then quit after the last exposure. But you're right, NO BUGS!!!

Great building. It looks like you use an inverted angle iron and wheel for the roll off. What kind of wheel did you use and where did you get it?

Thanks for the quick responses. I think I would like to stick with SBIG but I would be open to any suggestions. As far as budget (should have put that in my original post), that will depend what I can get for equipment I am going to sell to finance this camera. (Two familiar constraints on new equipment - money and the wife.) I'm going to sell a 5.5" and an 8" (Epoch) Schmidt camera and an ST-4.
So depending on what that brings, that will be the budget.

Thanks for all the advice. I should have mentioned that I have a non-internet (internet always seem to screw up my computer) 2004 Dell Dimension 8400 (3.2G/P4 1.5G RAM, 128MB graphics card) inside. Seems like if I use a laptop outside to aquire the raw image, I could bring the file insde and process it on the Dell. That way I could use an inexpensive (used) laptop for scope control and image aquisition. Am I right?

Regarding the comment on starting out with a SCT, I always do things the hard way. I have had the C8 and C11 for a long time so I had to decide where to spend the money. I decided to get a good mount and camera first. I figure I can always get another OTA to put on this mount if I want to.

Don't forget the most important body part to keep warm - your head! I use a wool cap and a hood (either from the parka or a hooded sweatshirt) and that keeps me warm down to about 10 deg F. Below 10 and the full head gear comes out in addition to the wool hat. Long underwear, sweat pants and ski pants with wool socks and Sorels and I'm good to about -10 F. Colder than that the equipment wonders why it has to work.

Chris
Cedar MN

Here is a picture of the field setup (even though this is in my living room).

That is just downright COOL!


Chris
Cedar MN

I have a 1992 C11 that I use for imaging and I can attest to value of a CF tube. Here in MN the temp can and does drop rapidly over the course of the night. With the original aluminum tube, it almost always required refocusing after about one to two hours. I decided to replace the tube with one of Franks's (Public Missiles) CF tubes. What a difference! In the year and a half since I have had it, I have never once needed to refocus during an imaging run (now perfectionists may disagree but it is tough to tell the difference in star size from the first image to one taken three hours later). I usually image at f/12 (3370mm) with this setup. My two cents worth.


Chris
Cedar, MN

Thanks for all your responses. Here is a shot of a flat. I think the optical center of my imaging train is not centered on the chip. Is that correct?