Re: [ATM] Re: Satisfying "Mililes-Lacroix" is neithernecessary norsufficient for "diffaction-limited" performance

[Michael Peck <mpeck1@ix.netcom.com>]

At 04:26 22-06-04, Nils Olof Carlin wrote:

>On another list have been shown some nice simulations of diffraction images
>for identical RMS errors but widely different transverse aberrations(RTA).
>My simplified view is that the RMS says how much of the light is within the
>Airy disk and how much in the rings, but the RTA says something about how
>wide the light is scattered in the rings. A small amount that is widely
>scattered may be objectionable in much the same sense that the spikes from
>the spider vanes are. One other consequence is that turbulence seems to
>affect a high RTA mirror by increasing the "spikiness".
>

Nils:

Sounds like a Steve Koehler project.

If I wanted to make a thoroughly modern case that slope errors matter I'd 
take a look at MTF curves for various types of aberrations. For example 
here <http://home.netcom.com/~mpeck1/astro/temppix/sa07_mtf.png> is an MTF 
plot comparing .07 waves RMS of low order vs. high order (9th) spherical 
aberration. The seemingly small differences on the left hand part of the 
graph could make all the difference if you're a critical planetary (or even 
deep sky) observer, where you are interested in details that are relatively 
large compared to an Airy disk size but of inherently low contrast.

Or, as you say, looking at the PSF 
<http://home.netcom.com/~mpeck1/astro/temppix/sa07_psf.png> the Airy disks 
for these two cases look about the same, but with high order spherical 
light is thrown further out into the wings of the PSF, mostly at the 
expense of the innermost diffraction ring.

But, this example barely satisfies Marechal's criterion. The differences 
get much more subtle if you cut the RMS error in half. Still, they might 
matter to a critical observer, and as you say other factors that degrade 
contrast (the atmosphere, secondary obstruction, spider vanes) may play a role.

The other issue I was musing about was a little more practical: namely if 
you follow Texereau's advice to the letter what are you really 
accomplishing? Could you accomplish the same thing by ignoring slope errors 
altogether and just setting your RMS goal a little lower. If you're using 
Sixtests as your data reduction tool you have no choice after all, since 
Jim says you should ignore slopes.

As Vlad says, that's really an empirical question. Too bad there's not much 
in the way of empirical data. I may have a data point or two to add in the 
future, but I think I might have to work up the energy to cut out a Couder 
mask.

Mike Peck


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Michael Peck
email mpeck1@ix.netcom.com
Wildlife photography page http://home.netcom.com/~mpeck1/index.html
Amateur telescope making http://home.netcom.com/~mpeck1/astro/astro.html 

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