Re: "A method for optimal image subtraction" on TASS WWW site

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

On Tue, 13 Jan 1998 11:05:42 -0800, Chris Albertson <chris@topdog.pas1.logicon.com> wrote:

*>I have not yet finished reading the paper but so far it appears 
*>that the technique may let us do what Tom had suggested ages ago:
*>Directly compare the images from three offset cameras to see if
*>anything changed/moved.  It seems to cover the case where the PSFs
*>of the two (or more) images or unequal.  Conceptually the technique
*>seems intuitive (once someone else first thinks of it) but the
*>math is "nontrivial" and then their are the details.  A one minute
*>compute time is attractive too.

I believe I've also "called out" simple image comparisons before for
TASS data. I'm sure the paper reference offers a more robust method.

As I said, I've done it on my own camera with simple subtraction. You get
some negative pixels when a source is subtracted from the background,
so in my method you should be thoughtful about how you treat this.
Again, with the Mark III if you have different bandpasses, you will have
the problem of sources that are particularly red or not-red (the data
base may offer some statistics on this). If you have the same passbands
you still have the problem of shifts in
seeing or image alignment. One can "equalize" the images to compensate
for some of this. And again, even brute-force subtraction may yield
the DETECTION of moving sources, if not a proper *measurement* of them;
but the *confidence level* of those detections depends on the above-mentioned
false detections. The benefit of doing additional "equalization" is the
possibility of using successive day's images from

Anyone interested can take raw data images as available and try
simple sequential subtraction or the "equalized" methods as presumably
suggested by the quoted paper, and see what they come up with. We'd
see what the apparent magnitude gain, and the uncertainties and losses,
particularly the "false detections". And before any drivers are
rewritten.

Herb Johnson

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