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

On 24 Apr 2005 06:44:06 -0700, "nick" <vladis.2@xxxxxxxx> wrote:

>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.

You are talking about blur size as the apparent (to the eye) diameter of
imaged stars? That is not useful at all for determining anything about
the size of features present in the image. This blur disk is really just
a visual phenomenon, related to the way our eyes work. It has no real
meaning in the underlying data. A star that appears huge on an image,
and one that appears as a tiny point, both have the same FWHM..


>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.

Well, lambda/D is pretty useless when imaging, as it is not generally a
significant component of the PSF. Within a narrow range, the FWHM value
will be the same for every star in an image. The value is useful because
it is a good descriptor for the PSF, which is precisely what determines
the maximum possible resolution in an image. The PSF of the system is
usually described in terms of the FWHM of an unsaturated stellar
profile. It is sufficient to measure the FWHM of a test image to know
what the smallest resolvable details will be, and to calculate the
optimum imaging scale.

_________________________________________________

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

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