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

Chris L Peterson wrote:

FWHM is simply a measured value that describes the
size of a star on an image. It is the same for every star in the image,

regardless of how bright or dim that star is (assuming you are above
the
noise, and below saturation). The blur of the stellar image is a
convolution of multiple sources: tracking error, telescope resolution,
seeing, etc. Seeing is usually the most significant component when
imaging longer than a few seconds.

FWHM is an excellent indicator of seeing, and is probably the metric
most commonly used by imagers. I'm not sure what you mean by the term
"blur size", but for me (and most people, I think) it is essentially
synonymous with FWHM.

Blur size as an actual image of a star, as opposed to the FWHM, which
is
determined by a radius at which the measured photon count drops to half
of
that at the center (peak intensity). Strictly talking, FWHM can't be
identical
for all stars, since it is wavelength dependent (in both, diffraction
and turbulence),
and also a subject to the wavelength sensitivity of detector. FWHM at
some mid-wavelength
can serve as an averaged FWHM for the sensitivity range, but it is
still far from
being even an approximation of actual blur sizes. If it would be, it
would imply that all stars,
regardless of their inherent brightness and dominant emission, are
imaged in a
similar size, which evidently is not the case.

That is why I said that the FWHM term seems inappropriate: it is not
directly
related to the actual imaged stellar sizes, nor the smallest resolvable
details on
extended objects. In that, it is fairly similar to the "basic" FWHM
(Lamba/D) as a
conventional limit to diffraction resolution - a rule with many
exceptions. However,
in the same manner, it can be useful as a general resolution indicator.

Vlad

.

Relevant Pages
Loading