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(fwd) [vsnet-chat 6074] Re: Request for AIP4WIN and other CCD photometry software
Date: Fri, 28 Feb 2003 09:00:31 -0500
From: Stupendous Man <richmond@a188-l009.rit.edu>
Kato-san wrote:
> With the
> present-day automatic pointing of the telescope, one can reasonably
> get images over 100 objects within a single night ....
...
> I thereby request to authors of these programs to implement a function
> to make photometry of many targets (i.e. many variable stars) within the
> same night, without bothering to issue many independent commands or
> time-consuming eye examination of individual images.
Kato-san provided two possible modes of operation, both of which
involve differential photometry: one starts with a catalog of known
stellar magnitudes in some field, and then measures other stars
relative to these known ones; and the other starts with a target
star, and picks comparison stars (of unknown magnitude) against
which to compare it repeatedly.
The TASS Mark IV reduction pipeline
http://spiff.rit.edu/tass/pipeline/pipeline.html
combines these two approaches. It follows a sequence of steps which include
a) measure the instrumental magnitudes and (x, y) positions
of all stars in every image
b) use an astrometric catalog (Tycho-2) to determine the (RA, Dec)
positions of all stars
c) identify stars of known magnitude by matching to a photometric
catalog (again, Tycho-2, but this works much less well
for photometry than for astrometry; alas, it is one of
the few catalogs which covers the whole sky)
d) transform the instrumental magnitudes to standard magnitudes
for all stars in all fields
The result is a list of positions and magnitudes for all the stars
detected on all the frames. No human intervention is required.
The procedure is far from perfect, however. It depends upon having
images with a wide enough field that there are 20-50 astrometric and
photometric reference stars in each one. I have found that _automatic_
procedures require many stars to yield a fair solution; a human may
be able to calibrate images with 6-10 reference stars, but my software
cannot do so reliably :-(
The Tycho-2 catalog is not a very precise photometric reference.
However, it does provide many stars in every one of our images (which
are 4x4 degrees on a side). Observers at clear, dark sites may be
able to perform traditional, all-sky photometry: take some images of
fields with photometric reference stars, and other fields of target
stars; then make a photometric solution for the night to transfer
the calibration to the target stars. The pipeline contains a software
package which can handle this job. However, the skies at our sites
are often partly clear, on and off throughout a night, so that we
are forced to perform differential photometry within each image
instead (using the same software package, but with different options).
I suspect that many other observers suffer from the same problem.
Those with small fields of view may have a difficult time finding
stars of known magnitude within each field.
The pipeline is broken up into different pieces to handle the different
stages. Perhaps some observers might use one or two of the pieces
which fit their needs. Reading through the documentation might give
other observers ideas for their own work. I warn everyone that my
software is not designed for inexpert use -- and I don't mean 'inexpert'
in a perjorative sense! -- and is far, far less easy to use than
AIP4WIN or other professional packages.
Michael Richmond
Return to the Powerful Daisaku
vsnet-adm@kusastro.kyoto-u.ac.jp
