Collimation after cleaning primary mirror

[jdg600]

With a few days off over the yuletide period I decided it was time to clean the mirror of my 8-inch dob which had got to looking quite grubby. This is the first time I've ever performed this procedure but all went well and I reassembled and re-collimated the scope with no problems. This morning presented the first opportunity I've had to give the scope a try so my first job was to do a star-test. The star test looked good with nice round concentric diffraction patterns visible on both sides of focus but I noticed that on one side of the focused position three equally-spaced small depressions were evident in the outer ring of the pattern. I assume that these are due to the clips that hold the main mirror in place.

It's entirely possible that the diffraction patterns looked like this before I had the mirror out and I had just never noticed but I'd be grateful for any advice, is this normal or have I made some mistake in reassembling the scope.

John

[Scooot]

Hi John, sounds like the mirror clips to me, especially as you say it happens on one side of the focus, presumably when the focusser is racked out?

I don't think you've made a mistake putting it together.

[ronin]

It all sounds correct, the 3 clips would give rise to the pattern.

The one question is are they a bit too tight?

Not sure there are guideline to how tight they should be and everyone will have their own opinion. Used to be the problem on goto's years ago the manual said "Tighten the clutch" so people almost put 8ft bars on and bounced on them to tighten them up. Result was a broken goto.

[trazor]

Hmmm not too sure you should see the clips in a star test, see this.........

http://www.telescope-optics.net/diffraction_pattern_and_aberrations.htm

[JemC]

Hi Jdg600

Merry xmas to you to

collimated it no problem eh

I have been trying for a number of weeks now to collimate my 8-inch newt with no success  what procedure did you follow to get it done if you don't mind me asking

Regards