Re: Photographic Size of a Star

"Chris L Peterson" <clp@xxxxxxxxxxxxxxxxxx> wrote in message
news:jm2pm2hfjbb8qt3qrllikea7b5bf04pkp9@xxxxxxxxxx
[snip]
Because telescopes _don't_ produce pinpoint images. Diffraction,
tracking errors, optical aberrations, and atmospheric effects all act to
increase the size of the stellar profile. Even with perfect optics under
perfect conditions, a circular aperture produces an image described by a
Bessel function (the Airy disk and surrounding rings). As the star gets
brighter, you'll see farther out on the central disk (the star image
will appear larger), and eventually you'll see more and more rings.

But the Airy disk and the diffraction rings are visible with high
magnifications relative to aperture, aren't they? For example, on my Tasco,
I'd need to go at least up as far up as 140x to start discerning the Bessel
profile of very bright stars.

I seem to remember that higher apertures need higher magnifications to show
Airy disks and rings, at least visibly.

Why do we get such a profile even with low magnifications relative to aperture
or with photos where the notion of magnification doesn't even apply, such as
by prime focus imaging? For example, the least magnification needed to see
diffraction rings on a 20-inch scope must be quite high. I don't know HOW
high, but usually photographers take photos using prime focus. And the star
images STILL saturate circularly.

If memory serves right, Schmidt cameras don't even have any magnification (I
may be wrong here). They STILL produce disks for star images though.

Perhaps even with photos at prime focus, the Bessel profile is visible through
the extreme sensitivity of the scope/camera and would otherwise not show
visibly?

Thanks,

[snip]

Chris L Peterson
--
Ioannis
-------
The best way to predict reality, is to know exactly what you DON'T want.

.

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