Doughnut to Jellybean: C8 Collimation issue with D7200 DSLR

[NeonSpectres]

Currently having fun getting my D7200 DSLR setup for imaging with the C8. Visually (through an eyepiece) I can get stars round and pin-sharp, but with the following setups:

C8 > Baader Click-Lock 2" for Celestron SCT > Revelation Universal 2" T-Adapter > Nikon T-Ring > Nikon D7200

C8 > Celestron SCT T-Adapter 50mm > Nikon T-Ring > Nikon D7200

... as I steadily move towards focus keeping the (round) doughnut in place all is well until it suddenly turns into this (Jellybean) and then shoots off in a diagonal direction!:

Is more considered collimation required or is there something amiss in the hardware?

If I use the following then all is good (so possibly focal distance is playing a part here?):

C8 > Baader Click-Lock 2" Visual Back for Celestron SCT > Baader 2" ClickLock Mirror Diagonal > Revelation Universal 2" T-Adapter > Nikon T-Ring > Nikon D7200

Many thanks 😁 

[vlaiv]

52 minutes ago, NeonSpectres said:

Is more considered collimation required or is there something amiss in the hardware?

I think your collimation is fine.

SCTs focus by moving primary mirror. There is long screw that serves as the lead screw for mirror assembly.

My guess is that this screw is somehow damaged or out of alignment on one part of its run.

When using it visually - focus position due to optical path is such that mirror separation does not get primary mirror assembly to the section of lead screw that is bent or otherwise damaged.

If you put less items in optical path - primary needs to move further to focus at that position and you hit the rough spot.

Here is diagram of SCT focusing assembly:

It is either threaded rod that is damaged on one section or sleeve / baffle tube contact.

Simplest solution is to figure out how much optical path does not create a problem and allows you to focus your camera and then use extension in optical path.

Other option is to open the scope and figure out if you can solve mechanical issue.

[dweller25]

Have you added a finder or something else to the OTA - if so check with a torch from the front of the OTA to see if any screws are contacting the mirror when you focus.

Edited March 24, 2022 by dweller25

[NeonSpectres]

3 hours ago, dweller25 said:

Have you added a finder or something else to the OTA - if so check with a torch from the front of the OTA to see if any screws are contacting the mirror when you focus.

This indeed appeared to be the issue, although I'll be able to confirm this evening! I did notice last night that as I turned the knob to the right that is did get a bit treacly (if that even is a word) and I have been playing around with my finder setup recently.

Will see if I have some other, shorter screws to replace these two, otherwise I will just leave them sticking out a bit.

Thankyou both for your prompt feedback which'll get me back imaging again! 😁👍