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

On 24 Apr 2005 18:16:04 -0700, "nick" <vladis.2@xxxxxxxx> wrote:


>Don't know why you are refusing to acknowledge the obvious. The PSF
>doesn't
>determine actual resolution in imaging anymore than it does in visual
>observing.
>What does is the actual size of recorded image, if allowed by the
>contrast level.

You clearly don't understand the most basic elements of optics or of
imaging.

>The PSF's FWHM is not an "accurate measure of the PSF"

In astronomical imaging, it is almost always true that the PSF if
approximately Gaussian. The FWHM is used to characterize that Gaussian.
The FWHM is universally used by astronomers to assess the resolution of
their images.


>what it says it is:
>a width of the PSF at its half-maximum level. The PSF extends many Airy
>discs
>beyond that, asymptotically approaching zero.

Yes, it does (although in reality the diameter is determined by the
noise floor).


> The FWHM is accepted as an
>approximate limit to point source resolution

Yes. Very widely accepted as a good measure of resolution. I've never
heard the term "point source resolution" used, however.


> but actual resolution
>is determined by - what else? - the actual size of images recorded,
>which is in
>turn determined by the properties of object source, aperture, detector,
>and
>a number of other factors, seeing usually being one of them. A point
>source image
>is always some kind of blur, wheteher it results from diffraction
>alone, or combination
>with other imaging factors. "Blur" may not sound as "scientific" as
>FWHM, but has the
>advantage of being a direct representative of image size.

"Blur" tells you nothing about resolution because it doesn't have a
concrete meaning. What does have a concrete meaning- and tells you
everything about the resolution- is the PSF. And the PSF can be
reasonably quantified by the FWHM of a stellar profile. It doesn't
matter what the components of the PSF are- it is a measure of the
performance of the entire system, which is usually all you care about.

I'm curious to know how you use "blur" to determine resolution. I know
exactly how I (and everyone else) does it by measuring the FWHM. A
single measurement immediately provides the maximum possible resolution
of features in the image. How do you make this determination from the
"blur"? How do you even measure the diameter of the "blur".

_________________________________________________

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

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