Re: New Variable Star Discovery Technique

[hjohnson@pluto.njcc.com (Herbert R Johnson)]

On Sun, 11 Jan 1998 16:56:09 -0500, Glenn Gombert <gleng@infinet.com> wrote:
*>Hello,
*>
*>        Emmanual Bertin (who wrote SExtractor) forwarded a very interesting
*>URL that describes a new method by his college Christophe Alard:"
*>http://xxx.lanl.gov/abs/astro-ph/9712287."   That has been used to discover
*>new variable stars in the OGLE II Survey looking at the Galactic Bulge. It
*>looks like a very interesting technique to apply to TASS Images. Shown below
*>is a summary from the above URL:
*>
*>  "We present a new method designed for optimal subtraction of two images
*>with different seeing.... We demonstrate that it is possible to derive an optimal kernel
*>solution from a simple least square analysis using all the pixels of both
*>images, and also show that it is possible to fit the differential background
*>variation at the same time. We also show that PSF variations can also be
*>easily handled by the method. 

My read of this summary suggests they are subtracting successive images to
obtain additional information. I published an article about a method to
simply subtract successive images from each other in order to obtain faint
information about objects moving in the field of view. Successive images
with the imager stable in the field of view will have very close
characteristics by definition, limited by variations in seeing and any
instrument variations. In short, constant conditions cancel out.
I published my results in "Observatory Techniques", a small press quartely
on amateur astronomy, a few years ago: but the above is the gist of the
matter. The technique is useful and painless as I described it. FOr
images obtained at substantially different periods of time, or to more
properly determine error bars, one would have to perform some kind
of analysis on the initial images, but roughly speaking one can apply the
"root n" rule for uncorrelated addition of "noise" sources.

However, since Mark III cameras use different passbands, subtracting between
cameras would at best be a DETECTOR of objects rather than a proper measure
of them. THe more elaborate method suggested by Alard's comments might work
with same-camera images over successive days, to the limit of alignment and
seeing variations.

As this is of interest to me, I'd appreciate comments.

Herb Johnson



  **** ------------------------------------------------------ ****

Herbert R. Johnson                      voice/FAX 609-771-1503 day/nite
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