Super C8 OTA Slips out of position
Posted 21 November 2011 - 10:18 AM
The real problem is that the tube slips out of its rear, im not sure what its called, round fixture when its horizontal. I noticed this when I was trying to collimate it. If i point to zenith the tube slides slides back flush against the back/ bottom.
When i move it horizointal you can see the tube slip about 1/2" so in essence the tube angles down 2 or 3 degrees more than it should be.
What should I check? Do I need to take the whole tube assembly apart? I cant tighten the screwa around the tube any more than i already have done without stripping them.
This is my first SCT and I really am flying in the dark here. Any help would be great!
Posted 21 November 2011 - 10:52 AM
If that's not the issue, then try to explain in more detail or take a picture and show us what you're referring to.
Posted 21 November 2011 - 11:11 AM
Posted 21 November 2011 - 11:33 AM
Can you take a pic? - There should be screws that go through the rear cell and hold the tube in place - Look down the tube (from the corrector end) and see if these screws are visible with nuts on them - My feeling is these are missing - If so then the OTA along with the corrector plate may be out of alignment - Anyway let us know and hope this helps some
Same skies, older eyes
Posted 21 November 2011 - 11:59 AM
Posted 21 November 2011 - 12:30 PM
Look down the tube (from the corrector end) and see if these screws are visible with nuts on them - My feeling is these are missing
Rick I can see the screws but i don't see nuts on any of them. just standard allen head type screws
Posted 21 November 2011 - 01:58 PM
Posted 21 November 2011 - 02:19 PM
Here are some pics i took the other night. When the scope is placed horizontal it will slip out from under the top of the rear cell by the finder scope. i tightened everything. I even removed the finderscope and replaced the screws in that but the tube still slips.
It's very unclear what is slipping. Is the rear cell threatening to come off the scope? Is the finder falling off? Is the visual back loose? Is the scope slipping in declination?
Posted 21 November 2011 - 03:16 PM
When the scope tube is level, horizontal with the floor on the fork, the top of the tube begins to slip out of the rear cell.
If i push up on the side of front of the tube, correct plate part, the tube slips back flush into the rear cell. If i release my hand from the tube it slips back out of the rear cell. Not all the way but at least 1/2 inch. It's like there should be a screw there preventing this from happening but all the screws around the rear cell are tight, none missing.
I hope that helps a little...
Posted 21 November 2011 - 03:51 PM
Posted 21 November 2011 - 10:08 PM
Still a bit unclear on what is slipping - FYI the finderscope mounting has nothing to do with holding the OTA - There should be screws on the perimeter of the rear cell - Can you take some side pictures (left and right) - Also, try to take the side pictures with the tube in its most forward and rear movement -
That should help clarify what you are describing
Same skies, older eyes
Posted 21 November 2011 - 10:15 PM
Posted 22 November 2011 - 02:20 AM
If I am understanding you correctly is it possible that the screw holes on the OTA/Tube have become elongated and is allowing the OTA/Tube to slip out of the rear cell. Are you sure the screws around the circumference of of the Cell are tight and the nuts inside are not turning and are tight?
It really sounds to me like the holes in the OTA have become elongated. Is the tube only slipping out from the top of the cell or is it slipping out all the way around the edge of the cell?
Posted 22 November 2011 - 07:53 AM
Posted 22 November 2011 - 08:16 AM
Okay watched the video - No doubt the perimeter screws are not holding the OTA in place - The movement does not appear to be 1/2" but quite a bit less which again indicates the screws are the problem.
As far as removing the corrector plate - You will have to do this to fix this problem, but, before tackling that be sure to read up on exactly how to do it - there is plenty of info available on the correct procedure - Once you have removed the corrector be careful about the main mirror - In order to get the nuts back on the ota perimeter screws you may have to remove the main mirror -
Keep us up-to-date on your progress and BE CAREFUL!
Same skies, older eyes
Posted 22 November 2011 - 10:00 AM
Posted 23 November 2011 - 01:40 AM
Posted 06 March 2013 - 10:38 PM
Were you ever able to fix this? If so, could you give some details? I have the same problem with a C8 I just acquired.
Posted 07 March 2013 - 08:14 AM
You will also have to pull the Primary mirror out to get to the screws on the rear cell.
Posted 07 March 2013 - 09:16 AM
The holes for the tube are not precision drilled for tube alignment to the optical axis.
This means that if you don't take special precautions, the tube can be tilted with respect to the optical axis.
This is a very bad condition to have.
When you re-assemble, you can try measuring from the front edge of the rear cell to the rear edge of the front cell at several positions to ensure that the distance is as exact as you can possibly get it.
While you still may not get it perfect perfect, you have to get it close enough so that if you have to re-center the corrector, there is enough side-to side movement to accomplish this.
Taking the tube loose is necessary in this case, but getting it back into perfect alignment can be a bit of a fussy proposition.
And again, it doesn't have to be perfect perfect, but close enough so that if the corrector needs to be moved a bit to get your centering back, there will be room enough for it to move laterally.
Posted 07 March 2013 - 01:20 PM
Posted 07 March 2013 - 07:11 PM
It seems odd to me that Celestron only used two screws to secure the rear cell to the tube in this OTA. Especially given they are in the same plane as the forks. The way the tube is built, the weight of the front cell, corrector, secondary and the tube is supported by just the two screws. That seems like a high load for such a small area considering the material used for the tube, especially when the tube is horizontal or pointing down.
I'll probably decide if I'm going to add couple of extra holes, or just try with the dovetail bar and rail this weekend when I break the scope apart. I'm leaning towards adding at least a couple of screws along the other axis just in case I ever want to fork mount the tube again. I'm pretty comfortable with what's involved in taking the OTA apart and re-assembling it, but would appreciate any opinions or recommendations specifically on how remove the play between the rear cell and the rest of the OTA.
I'll try to remember to get some photos, and will update this post with how things go if anyone is interested.
Posted 11 March 2013 - 10:32 AM
1. Disassemble the OTA removing the corrector from the front cell, and removing the rear cell from the tube. There are tutorials on how to do this online, so I won't go into it in too much detail here. I was not able to remove the front cell from the tube, leading me to believe it is glued to the tube.
2. Sand the rear of the tube and the front of the rear cell to remove old adhesive and expose a clean metal surfaces to ensure a good bond.
3. Applied a thin ring of epoxy all the way around the rear of the tube and the front of the rear cell.
4. Re-assembled the tube and rear cell, and replaced the screws holding the rear cell to the tube before the epoxy cured. Clean up any epoxy drip or spills before the epoxy sets. I made sure to re-assemble the rear cell and the tube in the same orientation so as to not change the primary mirror/corrector orientation.
5. Checked to make sure the front and rear cells are square with each other. Once this was confirmed, I left it alone for a few hours to allow the epoxy to set/cure.
6. While the tube was curing, I checked the mirror cell for any play, and inspected the mirrors. The primary was pretty clean, so I just blew it clean with some compressed air.
7. Having everything apart, I decided to flock the tube as well.
8. I decided to clean the corrector and secondary mirror since they both had a fair amount of debris. I think this scope was usually stored facing down, allowing things to collect on the secondary and the corrector.
9. Applied silicone grease to the baffle tube and focuser threads hoping to smooth things up, and reduce mirror shift.
10. Installed Bob's Knobs on the secondary.
11. Re-assembled the rest of the tube, plugged the fork mounting thread with plastic plugs, and installed a dovetail bar on the OTA.
Everything looks OK at this point. I performed a rough collimation by visually centering the reflections when looking at the front of the tube. Quick daytime tests didn't reveal any issues, and at the very least, the focuser action is improved. I'll take it out tonight if the weather holds up, and see if it was worth the effort.