Always Follow a Cheshire Collimation with a Star T

The title pretty much says it all for this helpful hint.

Recently during two achromat ATM projects, after squaring up the focuser with a laser, I collimated the scopes using a well made Cheshire by "lining up the dots". According to this procedure, both scopes were "spot on" (pun INTENDED) in their collimation and this has been my experience in previous achromat projects.

However, out under a star test, both scopes displayed classic astigmatism at high power. One was quite mild and easily swallowed by the airy disk. The other was moderate and just barely fit into the airy disk at focus but would spill out during rougher seeing.

I did the usual drill of swapping out diagonals and eyepieces but the results were always the same. I was getting fustrated, then I recalled something Roger Ceragioli said in an e-mail exchange we had concerning an oiled triplet that I got from him. He basically said, do the Cheshire thing but don't trust it, always follow up with a high power star test. If you see astigmatism, adjust the lens collimation to see if it goes away.

So that's exactly what I did last night with both scopes.

BINGO!! It only took a simple tweak of one "push" screw on each scope and both were free of astigmatism and gave excellent star tests.

Rechecking with the Cheshire today showed the spots to be slightly offset from one another.

Lesson learned! Maybe one of the practicing opticians on this forum can give us an explanation of how the Cheshire is suppose to work and why it may give misleading results.

Jeff

Hi Jeff, I did the same thing when I check the collimation of my MCT's. I use the Cheshire to get it close then tweek collimation under a good star test or fake star 1st during daytime conditions then a real star at night, again as you stated it only takes a tiny amount to do the final tweek, very good info! I have used that on a IM M603,AT6M,Ioptron MC-152 and when I used to own a Celestron CR6R.
Jim

Good advice! There is always some small difference
between testing horizontally indoors and vertically
on stars -- elements shift, OTA bends, etc.
Also the Cheshire test assumes there is no
wedge in the glasses -- a reasonable assumption, but
not always true.

Lesson learned! Maybe one of the practicing opticians on this forum can give us an explanation of how the Cheshire is suppose to work and why it may give misleading results.

I think it probably relates to the lens elements behaving differently in relation to each other when pointed vertical as opposed to horizontal on a test bench.

I once had an FS152 that showed slightly different results when using a Cheshire/TAK Collimating Scope and the star test.

I think the difference is probably due to the weakest link in the system and that would be the accuracy of the alignment of the Cheshire in the focuser.

What amazes me is that anyone can see any amount of astgmatism in a decent lens close to the axis. If you look at the published spot diagrams that are available, astigmatism doesn't show until you are well off axis. If you look at any of the wide angle photos published, even the hi-res versions, there is little to no trace of astigmatism even at the edges. I certainly don't see astigmatism in any of the refractors I've cobbled together and these don't have sophisticated adjustments built in.

So what is it you folks can see that I don't?

dan