HELP - SCT Extremely out of Collimation.

[GalaxyHunter]

I have acquired a second hand 8” SCT.  The scope was given to me by a friend who picked it up at a car boot sale of all places, and just wasn’t getting along with it (he gave up and bought an 8” Newt).  The crux of the problem is that the scope is so far out of collimation that the shadow of the secondary is nowhere to be seen.  I can guess where the shadow is  by virtue of what I can see as a series of connected bright arcs which I can only describe as a stylised impression of a flying swallow.  I have collimated my own SCT on numerous occasions, but have always had a recognisable frame of reference, being the classic out of focus collimation ‘dohnut’, with this scope I really don’t know where to start.  Can anyone recommend how best to recover this situation?

GalaxyHunter

Edited March 21, 2021 by GalaxyHunter

[michael8554]

A basic visual check of the alignment of the mirrors :

https://www.cloudynights.com/topic/623603-collimation-of-an-sct-from-scratch-a-photo-guide/

Michael

[Cosmic Geoff]

I'm not sure if this will help you, but I acquired a second-hand SCT whih was badly out of collimation.  So far as I recollect, I first took off the star diagonal and looked through the hole, adjusting the secondary mirror screws till what I could see looked concentric.  Then I aimed at a star and adjusted the screws carefully till the out of focus star image changed from badminton shuttle to donut.  Adjusting the screws changes the telescope aim relative to the finder.

[Stu]

I once completely messed up the collimation on a C925, so the secondary was at a crazy angle. I blame the fact that one of the collimation bolts was rounded off so was not turning, unknown to me at the time. One could just as easily put it down to my incompetence 🤣🤣.

I ended up removing the secondary completely as it was a Fastar compatible one which allowed this easily, and then adjusted it visually so the secondary was level with the base plate, refitted it and then it was close enough to be able to adjust.

Can you post a picture of your unit so we can see what it looks like?

[rcwinter]

There is a device called a Duncan mask that may help with that, you can even make one yourself and follow the tutorial. 

Mind you, you do not know what position the set screws are at, so it may be wise to check in case you go to far and a screw detaches 

I use a set of bobs knobs which allow finger adjustment rather than a screwdriver, but it’s what you prefer 

if you were local I would do it for you but you may not be close by 

kind regards

Richard

Southampton

[PeterCPC]

When I cleaned the corrector plate recently, I had to collimate. As a first step, put the scope level, step back about 1m or so at look down the tube. You should see a series on concentric circles. If not adjust the screws/ knobs until you get them all lined up and equally spaced. When I did this I the did a star collimation but it was so good that no further adjustments were required.

[GalaxyHunter]

6 hours ago, Stu said:

I once completely messed up the collimation on a C925, so the secondary was at a crazy angle. I blame the fact that one of the collimation bolts was rounded off so was not turning, unknown to me at the time. One could just as easily put it down to my incompetence 🤣🤣.

I ended up removing the secondary completely as it was a Fastar compatible one which allowed this easily, and then adjusted it visually so the secondary was level with the base plate, refitted it and then it was close enough to be able to adjust.

Can you post a picture of your unit so we can see what it looks like?

Stu, thanks for that, the scope is an EdgeHD, I just did as you suggested, and true enough the glass was skewed off at a crazy angle.  Now with bobs knobs all tight and with the glass visually flat, I have returned the secondary to the scope and had a look at an improvised artificial star (sun reflection off electricity insulator about a k way), and whilst there is now a recognisable image, a vast improvement on what was previously just a mass of mush, I still can’t distinguish the shadow of the secondary.  I’m going to wait for night and see if a genuine star sheds a little more light on the issue.

GalaxyHunter

Edited March 21, 2021 by GalaxyHunter

[dweller25]

6 hours ago, PeterCPC said:

When I cleaned the corrector plate recently, I had to collimate. As a first step, put the scope level, step back about 1m or so at look down the tube. You should see a series on concentric circles. If not adjust the screws/ knobs until you get them all lined up and equally spaced. When I did this I the did a star collimation but it was so good that no further adjustments were required.

This is a good procedure and will get you very close, you may possibly need to do the final tweak on a star.

[Stu]

44 minutes ago, dweller25 said:

This is a good procedure and will get you very close, you may possibly need to do the final tweak on a star.

I think you are supposed to go back about a focal length or so, so probably more like 3m to make it easier.

[AKB]

I found this, from the BAA, particularly useful, and straight-forward...

Kitchen table collimation of a Cassegrain-type reflector

 

Edited March 21, 2021 by AKB