Re: [APML] more new pics

[Aplanatic@aol.com]

Colt,

> I've got the same thing showing up on one of my scts. It didn't
>  behave this way in the past, but my diagnosis is astigmatism.
>  Same star test data as you've described, and based on the star test
>  patterns in the star test book(the name escapes me at the moment)
>  it sure looks like astigmatism.
>  I've been led to believe that this probably isn't the case for a
>  sct.

You're right, it would be unusual for a standard SCT to exhibit much 
astigmatism unless the optics are pinched or the corrector plate is 
not up to snuff.  Since a standard SCT uses two spherical mirrors, 
mis-collimation should produce on-axis coma and not astigmatism, 
to first order, anyway.  And it sounds like you have adjusted the 
collimation, so it's probably not that.  Have you taken the scope 
apart or removed the corrector for any reason?  I have never owned 
an SCT, but they are notorious for having "tube currents" (actually, 
inhomogeneous regions of air at different densities) that take quite 
a while to dissipate, so a long cool-down period may be necessary.

How bad is the problem?  Low power, very high power?

>  So than it must be something else. I've also got the usual coma
>  in all corners but in addition I've got a minor exaggeration on one side
>  that appears to be a camera focal plane axis error.
>  I've messed with my collimation as best as possible after a 3 to 4
>  hour cool down under my wonderful magnitude 3 skies and this
>  tremendous 6 arc second seeing.

In L.A. it's often hard to find Polaris, so I can relate.

>  I don't think I have astigmatism even though it looks like it.

Why do you say that?  If the star is elongated on one side of focus 
and the elongation rotates 90 degrees on the other side of focus, 
then you have on-axis astigmatism.  However, if your eyes are 
astigmatic, as mine are, you should use high power and wear your 
glasses/contacts during the test.

>  Please let me know how you correct your problem, and if anyone has
>  any comments on what looks like astigmatism during star tests in sct's,
>  I'd sure like to hear it.

I have looked through quite a few SCTs, and sneak in a star test 
when I can.  The most common problem is residual spherical 
aberration, "tube currents," and coma from misalignment.  Coma 
looks like an asymmetrical image, often with an obvious flare, and 
it doesn't rotate as you focus in and out.  Tube currents can mimic 
astigmatism.  Using a knife edge at focus is a good way to observe 
the tube seeing and every other defect known to man.  You might 
want to give it a try.

Dave Rowe.

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