Star images

In article ,
Jeff Root wrote:
Why do brighter stars appear larger than dimmer stars in
photographs and CCD images, even when made in Space with
very good instruments?

I think the issue is light scattering on the CCD chip and within the
emulsion of the film itself; certainly the dots are much bigger than
the Airy disks of the diffraction patterns would be, and even much
bigger than seeing. There's a cross-hair effect from diffraction by
the bars holding the secondary mirror, in a fair number of images.

Possibly it's also scattering from surface imperfections on the
secondary and tertiary mirrors.

If you look at the Palomar Sky Survey plate with Sirius on it, you
see all sorts of extra spurious images, from light reflecting off
the photographic plate, back through the optics of the telescope,
and back again to the plate.

Sorry, this is basically an "I don't know" response; and scattering
from mirrors wouldn't explain the non-point-like stars you see in
photographs taken through good refracting telescopes.

Tom

Thomas Womack wrote:
In article ,
Jeff Root wrote:
Why do brighter stars appear larger than dimmer stars in
photographs and CCD images, even when made in Space with
very good instruments?

Any optical system and detector has a point-spread function (PSF),
describing its image of a perfectly unresolved source, which is
of finite extent. The intensity of this response naturally scales
with the source brightness. A display of such an image involves
choosing a mappig between data numbers and output (screen intensity,
for example) sliced in a particular way (be that linear, logarithmic, or
whatever). This cuts across the PSF of a bright source at a relatively
lower level than for a faint source, hence farther out in the response
function and at a larger radius. The only exceptions to this I've
seen are specialized systems in which the pixel size is very large
compared to the PSF core (easier to do numerically than physically).
The eye itself is rather better at conveying a field full of point sources
of different brightness than almost any image display I know of.

The PSF may in general have multiple contributors. These include
- diffraction from the telescope optics
- atmospheric turbulence, from the ground
- scattering in atmosphere and telescope
- sampling and crosstalk in the detector
- tracking errors
- defocus


Bill Keel

In article ,
William C. Keel wrote:

The eye itself is rather better at conveying a field full of point
sources of different brightness than almost any image display I know
of.

That's certainly true; I'd seen lots of photos of globular clusters, but
looking through a 36" reflector at Omega Centauri was something else!

Tom

In article ,
"William C. Keel" writes:
Any optical system and detector has a point-spread function (PSF),
describing its image of a perfectly unresolved source, which is
of finite extent.

The PSF may in general have multiple contributors. These include
- diffraction from the telescope optics
- atmospheric turbulence, from the ground
- scattering in atmosphere and telescope
- sampling and crosstalk in the detector
- tracking errors
- defocus

Bill has explained very well; I just have a few additional comments.

1. That last item, defocus, can be generalized to include other
optical aberrations, as we know from HST.

2. At large distances from the image center, I believe scattering
from imperfections or dust on the optical surfaces is probably the
biggest contributor. This might depend on what optical system is in
use.

3. Many years ago, John Kormendy took plates of bright stars with the
Palomar 48-inch Schmidt telescope. If memory serves, he was able to
detect scattered light 6 degrees (!) from the image center.

--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
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JR
From: (Jeff Root)
JR
Subject: Star images
JR
Date: 8 Sep 2003 06:29:12 -0700
JR
Organization: http://groups.google.com/
JR
JR Why do brighter stars appear larger than dimmer stars in
JR photographs and CCD images, even when made in Space with
JR very good instruments? (As evidenced by the roundness of
JR the star images.) What can be done to minimize the effect?
JR
JR -- Jeff, in Minneapolis

Here's to the notion of 'stellar magnitude'! Brighter stars are
bigger!!

---
þ RoseReader 2.52á P005004

"William C. Keel" wrote:

The PSF may in general have multiple contributors. These include
- diffraction from the telescope optics
- atmospheric turbulence, from the ground
- scattering in atmosphere and telescope
- sampling and crosstalk in the detector
- tracking errors
- defocus

That makes me feel a little better.

Mostly for my own amusement, I've written software that draws the night
sky. I've put a couple of examples on my website,

http://mywebpages.comcast.net/erniew.../starplot.html

The sizes of the star dots it draws is just a user-defined fudge factor
that scales with magnitude. I periodically think that I should revisit
this to make it more rigorous, but maybe that's not even worth trying.
I certainly don't have the background to model this list of effects from
first principles.

- Ernie http://mywebpages.comcast.net/erniew