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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 subtraction star.

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.

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A514_NoSS_Full.GIF (11540 bytes)

A514_WSS_Full.GIF (9227 bytes)

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