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One of the biggest problems for asteroid photometrists is the asteroid moving across
field stars. When this happens, the data from the images must be rejected. This means
losing a substantial amount of data and, in those months where asteroids tend to be in or
near the Milky Way, can bring an asteroid photometry program nearly to a halt.
StarBGone! can help recover a substantial number of those lost data by subtracting out
the interfering star as each image is measured. What's more, more than one star can be
defined so that a entire night of images can be measured in one pass.
How it Works
StarBGone! uses a "reference" star and the "subtraction" stars on a
reference image to determine the ratio of intensity between the reference star and each
As each image is measured, the reference star is found on that image and its intensity
measured. A small segment of the image around the reference star is copied and the pixels
in that subframe are scaled so that the intensity of the modified star matches the
intensity of a given subtraction star. That subframe is then matched to the location of
the subtraction star to sub-pixel accuracy and subtracted from the main image's in-memory
buffer. The subtraction is repeated for each subtraction star.
After all subtraction stars have been processed, the resulting in-memory image is then
measured to find the magnitudes of the comparison stars and target. The
original image is not permanently modified.
Not A Cure-All
How well StarBGone! works will depend on many factors. Most important is that the
reference star be brighter than any of the subtraction stars. The more so the better.
However, if the reference star, a subtraction star, or the combined asteroid/subtraction
star reaches the non-linear portion of your CCD camera's response, then StarBGone! will
not work well, if at all. This is because the reference star cannot accurately be scaled
and so residual pixels, either too dark or light, will be left behind.
StarBGone! in Action
The plots below show examples of how well StarBGone! can work.
||StarBGone! was developed by making reference to a
number of sources:
Alard, C., Lupton, Robert H., A Method for Optimal Image Subtraction,
1998, ApJ 503, 325-331.
Gary, Bruce L., Healy, David, Image Subtraction Procedure for Observing
Faint Asteroids, 2006, Minor Planet Bulletin 33, 16-18. Available for download at http://www.MinorPlanetObserver.com/mpb/default.htm
Harris, A.W., private communications, 2003, 2005, 2006
Menke, John, Asteroid Photometry: A Tricky Business (StarZap), 2005,
Proceedings for the 24th Annual Conference of the Society for Astronomical Sciences, ed.
Warner B.D. et al, Society for Astronomical Sciences.
See "StarZap" link at http://www.menkescientific.com/images.htm
Pravec, P., private communications, 2005, 2006