Re: Photographic Size of a Star


Chris L Peterson wrote:
On Tue, 28 Nov 2006 21:07:05 +0200, "Ioannis" <morpheus@xxxxxxxxxxxx>
wrote:

Chris,

On a related topic: do you or anyone else know WHY we have an increase of
apparent diameter on photo and digital plates for brighter stars in the first
place?

What exactly causes the circular saturation, i.e. the increase in diameter in
photos? Is it stray light, scintilation, or is it a property of the detector
to saturate sideways, always?

I know that overexposure in normal photos tends to saturate sideways, covering
the boundries with excess light, but why does it happen on point sources?

Assuming of course the scope is properly focused as to produce pin-point
images!

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.

In real life, the profile of a star is approximately Gaussian (because
of the factors listed above). That means it has an infinite diameter.
What you will actually see is the part of the profile that is above the
noise floor. Aim a scope at a very bright star and take a long exposure,
and the resulting star can be hundreds of pixels across.


Actually, in real life we can see the Airy profile and the diffraction
pattern extends to infinity, exactly as a gaussian. IOW, it is not a
matter of a specific PSF but rather that the PSF extends to infinity,
with infinitely decreasing intensity.

Andrea T.

.

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