Re: How does projection magnification in astrophotos change the Airy disk size?

On 25 Apr 2005 20:44:22 -0700, "nick" <vladis.2@xxxxxxxx> wrote:

>Not clear what is "normal" (or abnormal?) distribution. Aside the more
>or less typical formalistic description, the PSF in imaging usually can
>be
>approximated by Gaussian functions due to disappearance of ring
>structure
>(Gaussian functions do not form successive minimas and maximas,
>and purely approximate the bright rings structure).

The reason the PSF approximates a Gaussian is because if you convolve
the Bessel function that describes the diffraction (and includes ring
structure) with the statistical position shifts caused by tracking
errors and seeing, the resulting function assumes a shape that is very
nearly Gaussian. In the absence of these shifts, you would not use a
Gaussian to describe the PSF.


>Limiting "resolution" to that of a pair of point objects images, for
>the sake of
>simplicity (very much needed here, indeed), resolution can be described
>as producing an image with such a pair fully or partially resolved.
>Therefore,
>it is not directly determined by the PSF, rather by the property of the
>detector

It has nothing to do with the detector, unless you are undersampled. We
are talking here about _measuring_ the actual PSF (by looking at a
stellar profile) in order to _calculate_ the maximum possible resolution
of the image. This has nothing at all to do with any actual features in
the image. That is, there may actually be nothing in the image (aside
from stars) that actually contains high frequency components. The
measurement just tells you the best resolution you can have. You can't
judge this visually. If you have a pair of close stars, just changing
the black and white points can determine whether they _appear_ as
separate or joined. But if you have measured the PSF, you know right
away the degree to which the pair is resolved.



>The FWHM is determined by the PSF, which is in turn
>directly dependant on the wavelength in its limiting (diffraction)
>form.

When imaging, the PSF is almost unrelated to any diffraction effects.
The PSF is primarily the result of seeing, resulting in a FWHM several
times larger than the Airy disk.


>I'd love to see an example of an image where the closest resolvable
>stars and the FWHM of the stars in the image are unrelated.
>
>Who seeks, will find.

I'll take that to mean you can't provide one.

I'm glad you've found a system of assessing resolution that you are
happy with. Personally, I'll stick with the system used by every other
imager in the world.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
.

Relevant Pages

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