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

Chris L Peterson wrote:

>Would you care to explain to the uninitiated what is it that makes the

>PSF "approximately Gaussian" in astronomical imaging?


Sure. In a perfect system, with no tracking errors or seeing effects,
you would have a PSF described by a Bessel function. Now if you take
that and convolve it with the tracking error, which ideally shows a
normal distribution from the center, and with the seeing, which also
follows a normal distribution, you end up with the resulting function
closely approximating a Gaussian.

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 PSF merely describes intensity distribution of a point source"?
What do you think resolution is?

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
(will neglect other imaging factos, for the sake of simplicity), which
ultimately
determines actual size(s) of the recorded images.

The FWHM is nearly constant over an
image. It certainly does not vary with a star's brightness or color.

The FWHM is determined by the PSF, which is in turn
directly dependant on the wavelength in its limiting (diffraction)
form.
On the other hand, seeing error effects shorter wavelength more
(in proportion to Lambda^1.2), so that the two wavelength-based effects
partially offset. The FWHM still vary somewhat with the dominant
emission.
I never said it varies with the brightness (wouldn't hurt if you'd read
my posts
carefully). The brightness is determinant of the actual point object
image
size as recorded by a detector. Therefore, it is also a main
determinant of the
actual limiting (point object) resolution.

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.

> How do you even measure the diameter of the "blur".

>Measuring tape... ')



Yes, I kind of figured it was something like that.

Keep on figuring... ')

Vlad

.

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