SCT Collimation

[TonyGut]

Hi I have fitted 'Bobs Knobs" to my  Meade 8'' Lx90 and now I'm struggling with collimation.Apologies if this has been covered before i couldn't find it.

My set up is using an artificial star at the end of my garden, and using my DSLR  to view the image as i adjust.I like the fact i can magnify the image.I do still have the f6.3 focal reducer fitted as well.

The problem im having is if i defocus one way(say forward) i can adjust the doughnut so all is central.Then as a double check ,i   defocus the other way(rearward) and find that the doughnut is offset and appears to  require adjusting.But  if i adjust it it will be out the other way.Is it the way im setup? Am i adjusting incorrectly.?..Are you suppose to defocus in a particular way?i just don't know and hopefully someone on here will have experienced the same issue and can advise please.

Appreciate any help at all.Thanks

[Louis D]

Perhaps you're experiencing SCT mirror flop?  Try running the focus far in each direction and come back slowly to focus from each side to see if that helps.  That, and don't use the mirror to focus on either side of best focus once collimated.  Instead, pull your DSLR partway out of the focuser, lock it down, get best focus with the mirror focus knob, recollimate, then loosen the DSLR and slide it in and out on either side of best focus to see if the same anomaly recurs.  This would eliminate mirror flop as a cause.  If you still see the issue with this method, I have no idea what is going on.

Edited February 7, 2020 by Louis D

[Benjam]

This video may help you.
 

 

[TonyGut]

Thanks for the replies I'll check out the suggestions .Oops Just realised I typed STC instead of SCT...doh!

Edited February 8, 2020 by TonyGut

[Stu]

43 minutes ago, TonyGut said:

Oops Just realised I typed STC instead of SCT...doh!

Sorted

Definitely sounds like mirror flop to me. As Louis suggests, best to approach focus in one direction, likely pushing the mirror up the tube so that the mirror settles in the same way each time. Fitting a second focuser to the visual back may help eliminate this problem.

[Merlin66]

Tony,

Regularly, every few months run the focuser though full travel back and forth a couple of times.

This re-spreads the grease along the baffle and reduces any slop.

 

[TonyGut]

Thanks guys for your advice..all very interesting. 

Stu I did fail to mention that I do have a separate dual speed focuser attached to the rear of the scope.

I'll give  the suggestions a go and report back..

Thanks again 

[haitch]

25 minutes ago, TonyGut said:

Stu I did fail to mention that I do have a separate dual speed focuser attached to the rear of the scope.

Were you focusing with the Crayford at the time?

[TonyGut]

Yes I was 

[haitch]

2 hours ago, TonyGut said:

Yes I was 

Then none of the comments about mirror movement during focusing apply. You can get movement of the mirror as it tracks across the sky (mirror flop) but not of the mirror as you focus (mirror shift) with a Crayford.

Has your corrector ever been removed? If it has and it has been put back at the wrong rotation it can affect your ability to collimate. Another issue if the corrector has been removed is if the spacers around its edge don't get put back perfectly. I'm not sure the latter makes much difference in normal use but I know it does with a Hyperstar.

Bob's knob themselves can cause issues as you'll never tighten the thumbscrews sufficiently to really hold collimation. If you really want to lock down collimation replace the bolts with hex head bolts and use an allen key to make sure they are good and tight.