Astigmatism - Did i cause it?

[irishbloke]

Hi all,

Just recently I replaced my primary mirror donut with a Catseye Hotspot (which I think is great). While doing this I cleaned the mirror too. So I had the primary out of it's cell and in the bath!

Well I replaced the clips and have it all back together. Last time I had it out though I noticed that with my star tests (not that high mag as I've a dob and a 7mm is my best eyepiece x171) were a bit oval horozontally on one side of focus and vertically on the other.

Looking at Astro Baby's guide: Astro Babys Guide to Collimation and the star test images I think what I saw was the astigmatism.

My question is then "have I introduced the astigmatism when I replaced the mirror in the cell?" Might I have tightened one of the clips too much? I noticed the clips are tight as i put them in. Should I put them in in a certain order, sort of 1, 3, 5, 2, 4, 6.

I have since had a play with collimation and think I have it better. I adjusted the secondary down the tube a bit.

Thanks for any thoughts

Brian

[James]

Mirror clips shouldn't be tight at all and in fact should just sit over the edge of the mirror - I think the phrase is that you should be able to slide a piece of paper between the mirror and the clip... if the clips are too tight then yes, you may get astigmatism.

James

[irishbloke]

James,

I thought they may be tight on the sides rather than the face. The clips are rubber and there seemed no way I was getting paper between clips and side of mirror.

My mirror hadn't shown this before. It's actually a good mirror. So how do you do you replace your mirrors when you clean your mirrors?

Brian

[Doc]

I don't remove the mirror from the cell when I wash mine.

[John]

Before you removed the mirror I think you should mark it's position in relation to the secondary so it goes back in orientated the same way - or at least thats what I do whenever I've removed mine. I have put an index mark on the tube, the mirror cell and the edge of the mirror to help with this.

You could try rotating the primary mirror 45 degrees and see if the "issue" moves with the mirror, or even goes away.

NB: This may be out of date advice from a time when mirrors could have a slight "wedge" to them - this may well not be an issue these days with modern mass production techniques. Shows how old I am !.