Re: Photographic Size of a Star

On Tue, 28 Nov 2006 21:55:16 +0200, "Ioannis" <morpheus@xxxxxxxxxxxx>
wrote:

But the Airy disk and the diffraction rings are visible with high
magnifications relative to aperture, aren't they?

You need to distinguish between visible at the eyepiece and visible on
an image. Unless the image time is very short- less than a few seconds
under good seeing, and perhaps tens of milliseconds under average
seeing- the PSF ends up being a convolution of the various aberrating
effects, mainly atmospheric motion. When you watch a star at high
magnification, you normally see the Airy pattern dancing around, both in
position and shape. Imagine what you get if you make a long exposure of
that- it isn't going to look like a central disk and outer rings. It is
going to look like a Gaussian.


Why do we get such a profile even with low magnifications relative to aperture
or with photos where the notion of magnification doesn't even apply, such as
by prime focus imaging?

While "magnification" is largely meaningless when imaging, it has an
analog called image scale. Unless you are grossly undersampled (that is,
even bright stars will fit on a single pixel) you will always see the
stellar profile with a diameter that is some function of brightness.


Perhaps even with photos at prime focus, the Bessel profile is visible through
the extreme sensitivity of the scope/camera and would otherwise not show
visibly?

Certainly, with a short exposure. When I collimate my scope I use a
video camera, and the diffraction pattern is very obvious on the image.
But never with a long exposure.

_________________________________________________

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