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Re: Coma, work to date



On Tue, 28 Dec 1999 14:00:13 -0800, Chris Albertson <calbertson@logicon.com> wrote:
*>> 
*>> While you are looking it over, is there anything in IRAF that offers
*>> measurements of coma? We need some numbers and common terminology to
*>> discuss coma in a consistent and useful fashion. It would also be
*>> another encouragment for me to use it.
*>
*>This is a good question.
*>
*>Maybe the ratio of the sum of the counts inside a 2 x PSF
*>aperture over the counts inside a 1 x PSF aperture.

I take this and your other comments to say that IRAF does not offer
an explicit method for measuring coma. I understand that one could
come up with an ad-hoc method using IRAF in a programmable way. I
don't care for ad-hoc methods, for this reason: why should Glenn
Gombert (just to pick a name) use Herb Johnson's "coma method" that
seems to work? And will that method work in the future, as you suggest
below..

*>The problem is that this would also measure trailing, poor focus
*>and bad seeing.  Maybe you take the above and normalize it so
*>that the result is always 1.0 for a central star. The result would
*>be a "blurryness ratio" that is relative to the image center.
*>I think if you plotted "blurryness ratio" v. distance from center
*>you see a steep curve for TASS Mk IV images.  We could talk about
*>steepness or range of the curve as a comma indicator.

So that is the trouble with qualitative, or even one-valued, methods.
Let me be plain, apparently my comments have not been clear. Then I'll
mention what has been done, and ask "what's next?"

Coma is a fixed function of the optics. It is therefore repeatable
and to some extent consistent (one hopes). It is not desirable. So,
*IF* it is desirable to compensate for coma, we need a method to characterize
it sufficiently so that an algorithm - a program - can be constructed
to do so. Waving a hand and making general statements about coma does
not directly lead to either measuring it or compensating for it.
It informs us of a problem, but not of a solution.

*>Didn't Michael do something like this already?  He looked at
*>the effect of aperture size across an image.  The above is
*>about the same thing.

TN59 I believe says that with a square aperture of various sizes, you will
not accomodate comatic stars to various degrees. I think Glenn Gombert
has or will report as much, he said he'd look at that. Both Glenn and
Richmond have or will explore apertures up to 11 X 11 pixels. Also: myself and
Bennett have looked at some Mark IV (CD-ROM #5) images and said that
apertures of 10 or 11 pixels would be CONDITIONALLY effective - I said
7 by 10 for the center of the image worked, the elongation was due
to tracking error. (Tracking error has apparently been reduced in
subsequent Mark IV images from Tom.) There is other work by Gutzwiller
that I have not looked at due to time, holiday and illness.

But the "state of the art" to date is: a bigger fixed, square aperture
will measure some but not all of the comatic stars on Mark IV images.
There are some reports that quantify what may be lost, and which offer
some rough analysis of how that loss varies from the center of the
image. Also, there are some preliminary uses of "star" with fixed apertures
vs. PSF's that provide some overall statistics across the entire image
as to RELATIVE effectiveness of fixed apertures - relative to a maximal
aperture that is, NOT to a count of all stars on the image, with one exception.

Andrew Bennett's report "cd5sat.html" on his Web site about saturation
is especially informative in that he has used both "star"
for automated work on dim objects and MANUAL methods for the bright objects.
Bright objects exhibit the effects of coma as well as requiring large apertures.
I am myself not comfortable with his conculsions about saturated objects,
but the research itself is informative and offers some numbers. His
methods for bright stars - he says "saturated" but they are also comatic -
is to fit the aperture to the star image. I've asked him to be more
explicit about his methods and he concurred that was appropriate.
He has now listed his bright object on his study image for others to consider.

Incidently, NONE of these studies have suggested the impact of coma on
astrometrics - position measurements. A 10 X 10 aperture of 7.4 arc second
pixels means an UNCORRECTED position error of 74 arc seconds. Yet one
can "simply" find the brightest pixel in the aperture - will "star" do that,
and is that sufficient to locate the star-object?

So we now have a number of studies that suggest the impact of coma, but
with few indications about how to deal with it other than "bigger aperture".
I am no expert on coma or optical design. I fell into coma discussions
as part of my prior work on Mark IV darks and saturation - the latter
of which I'd like to complete. If this is only "Herb Johnson's coma
crusade" I will drop it. I suggest my colleages look at the accumulated
work and decide what they want to do about it, or want someone to do
about it, and ask explicitly for help. Richmond has cataloged the Mark IV
work on the TASS Web site under "Mark IV": my colleagues should make
sure that page has referenced their most current work.

Restated, if a fixed photometric aperture
of whatever size is an acceptable method - presumably combined with
some cutoff for "saturation" to avoid very bright stars, and some cutoff
of the image corners or edges to avoid very comatic star images - then further
discussion of coma is moot except to validate and refine that particular method.
This question is simply up to the Mark IV programmers and end users to resolve,
the key word is "acceptable".

Herb Johnson

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