Re: Increasing primary and secondary mirror quality

Chris L Peterson wrote:
Also, a real piece of junk may be able to split close doubles very
nicely.

Ehh, it's conceivable, but it's not at all likely. True, when you look
at close doubles, you're plumbing the rightmost extremity of the MTF,
but your eye still uses a non-trivial segment at the end. If your
mirror's rough, that's going to degrade everything, and the effective
endpoint of the MTF is going to stop well short of the theoretical zero
point.

It's possible that a scope could be good at splitting doubles and yet
not be good for planetary observation--consider a highly obstructed
scope--but that speaks largely to the eye-brain's ability to interpret
simple sources (such as binaries), even when they are convolved with
spectacular diffraction patterns. I suspect the experienced observer
could tell what was going on. After all, the PSF contains everything
you need for the MTF.

That isn't a reliable single test for quality. If I were using
doubles for evaluation, I'd compare the ability to split them in the
center of the field to further out. Lots of bad optics will perform well
along the optical axis.

I honestly don't see how. I mean, what are your options here for
off-axis aberrations? Astigmatism: OK, a problem that is either
inherent in the design or a result of a poorly supported mirror. Coma:
Also inherent in the design. Distortion: Annoying, but doesn't affect
contrast and again inherent in the design. Field curvature: Only
requires refocusing, and also inherent in the design. In short, the
aberrations that cause the optics to perform poorly off-axis *only* are
mostly caused by design or mechanics. A bad optical surface would cause
problems on-axis as well as off-axis.

Consider that we do star tests *in the center of field*. If off-axis
performance were a measure of optical *quality* (as opposed to a
property of the design), wouldn't we ask people to also do star tests in
the edge of field? We don't, because then the optical aberrations we
care about are mixed up with things like coma and astigmatism, which we
don't want. We want to be able to separate the causes of image
breakdown as finely as possible.

--
Brian Tung <brian@xxxxxxx>
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html
.

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