Does mirror cooling cause this?

[Atmos]

Last night I set up my 12” solid tube dob to do some visual under dark skies while I was imaging. It has been months since I last collimated it, months since it was last used. The mirror clips had been loosened to remove the initial astigmatism that the scope had.

It had 1-1.5 hours to sit and cool a bit but the nights were cooling faster than the mirror could handle without a fan anyway. Collimation looked good but what I did notice when approaching focus is that the star would start to flatten out and bean shape a bit as it approached focus.
Is this caused by a cooling mirror?

I tried it on stars at different elevations but with the same result. Heavily out of focus I could see that the mirror still had some ways to go before the mirror was anywhere near ambient.

[xelasnave]

I dont know if this works but I would have my scope(12") sit upright in the belief that any movement when cooling would be somewhat evenly distributed.
Alex

[clive milne]

Colin,
If the blank is homogeneous and properly annealed then cooling should not result in astigmatism, but you may see a slight change in its correction for spherical aberration.
Typically, however, the disruption to the final image will be overwhelmed by the effect of the convective plume at the boundary layer.

The assumption here is that the primary cell or secondary support mechanism (rtv?) do not impart stress on the optics as they cool.

Easy way to test is to rotate the primary in the cell and see what happens to the orientation of the astigmatism. If it also rotates, then I would bet London to a brick that the mirror blank has internal stress (for which there is no practicable cure). If it doesn't rotate, then the problem is with the secondary, or by virtue of the manner in which it is attached.

~c

[Atmos]

At this stage I don't think it is astigmatism, I'm pretty sure that was being caused by the mirror clips (pinching the mirror). The in focus stars appear nice, at least as nice as they could while the mirror was still cooling. They were round little fluff balls :lol:

I looked at Saturn and I could see the Cassini Division quite cleanly but it was lacking the detail/colour on Saturn itself. This was what I was expecting due to the cooling mirror.

[clive milne]

Ok,
astigmatism will look circular at focus.

It will look bean shaped outside of focus.
The long axis of the bean will flip 90 degrees inside of focus.

Typically, if the mirror clips are stressing the mirror the distortion will be trefoil.

~c

[Atmos]

Before loosening the mirror clips the stars looked more like triangles, this I assumed was astigmatism (mirror stress).
What you’re saying though is that the astigmatism is noticeable in our of focus, I’ll have to do some more testing when I’m at my dark site and have a few hours... and a few screw drivers Hehe

Thanks Clive.

[codemonkey]

I'm certainly no expert but I'd be suspicious of:

1. Collimation
2. Edge support
3. Something intruding into the light path... I've seen misshapen stars and then realised the dome was partially obscuring the OTA... derp

I've recently been looking into some optics issues and, from what I've read, a cooling mirror can cause over correction... here (https://www.youtube.com/watch?v=tAD-2r6gW9A) is a good video which shows a star test on a known badly corrected mirror. Doesn't sound like your problem though.

Good luck Colin... optical issues drive me nuts. Hopefully yours is just collimation.

[clive milne]

Correct, trefoil astigmatism.

But be careful with the terminology.
Not all instances of mirror stress result in astigmatism.
Not all astigmatism is the result of mirror stress.

^^^ These are two very different statements.



You may want to re-read that sentence, I'm finding it difficult to parse.

If you are asking whether (classical) astigmatism is most obvious just either side of focus?
then yes, that is correct.

It will look like this:

http://slideplayer.com/slide/9437250/29/images/14/Through-focus+spot+diagram+for+astigmatism. jpg



You're welcome.

~c

[codemonkey]

btw... here's one of the issues I'm currently seeing with my scope. Three frames, one outside of focus, one roughly at focus and one inside of focus. I believe this to be astigmatism.

[clive milne]

Correct.
Try rotating the primary mirror 90 degrees.
If the astigmatism in the image plane also rotates, then you have a defective primary.

~c