Ximage

Source Detection Overview

If details of the mission are known, XIMAGE has the ability to locate sources in an image. The process is composed of three stages. The first, performed with the BACKGROUND command, estimates the background by dividing the image into equal boxes and rejecting those not complying to certain statistical criteria. The average of the remaining boxes is the background value. The second stage, performed with the EXCESS command, uses a sliding-cell method to find areas in excess of the background threshold. The third and final stage, performed by the SEARCH command, merges the excess cells into source boxes, using the PSF and vignetting to estimate source significance and statistics.

A related command, DETECT, is functionally equivalent to executing BACKGROUND/OPTIMIZE, EXCESS, and SEARCH in succession. It is not recommended to mix usage of DETECT with BACKGROUND, EXCESS, and SEARCH. If fine control of the source detection process is required, use BACKGROUND, EXCESS, and SEARCH. If not, DETECT is a convenient shortcut.

Background

In order to allow a sufficiently good estimate of the background, only images of size 128x128 pixels or larger should be used. Maximum accuracy is obtained by using full resolution images.

See the background example for information on tweaking the calculation to get a good background estimate. If the background is overestimated, sources may be lost. If it is underestimated, the probability that there will be false detections increases. It is important that the background calculation be as accurate as possible.

Calculated background boxes will be output to a text file if the OUTFILE qualifier is used. The command BACKGROUND/INFILE=[filename] restores the calculated background.

Excess

The EXCESS command uses a fairly conservative threshold when finding excesses to minimize the number of spurious sources detected. The threshold value is the maximum number of counts such that the probability that they could occur in a Poission distribution with the background as its average is less than 0.1/(Area of image/Area of source box). Consequently, this threshold may be adjusted with the SOURCE_BOX_SIZ qualifier, or more directly with the THR_SCALING qualifier. The value for THR_SCALING is multiplied by the calculated threshold to obtain a new threshold value. If a chat level of at least 15 is set, the final threshold value will be printed. For example, an increased THR_SCALING (e.g. 1.2, 1.3) can be used to reduce problems with fluctuations from extended emissions.

For crowded fields, or for fields with bright sources the BRIGHT qualifier is useful. It makes weighted mean for each excess, and optimizes the psf. Consequently it can take much longer to run, however the improved results for those situations may be worth it.

The excesses found are written to a text file with the OUTFILE qualifier. The command EXCESS/INFILE=[filename] restores the excess data to memory.

Search

Corrections to the net counts are applied if the proper calibration information are available for that instrument. The corrections applied are dead times, vignetting and psf (the fraction of the source counts that fall outside the box where the net counts are estimated). Count rate errors include both statistical and systematic uncertainties added quadratically.

In the final stages of source determination, there are two rejection criteria which could inadvertently throwing away actual sources. The signal to noise ratio of the candidate source must be greater than 2 by default, however, this value may be adjusted with the SNR_THRESHOLD qualifier. The probability that the candidate source is a background fluctuation must be greater than 1.0E-4, however, this value may be adjusted with the PROB_LIMIT qualifier.

The sources found are output to a file, which is named for the input data file ending in `.det`. This data also remains in memory until a new image is loaded for use by REMOVE/DETECT, which removes the detected sources and replaces them with a random background level, or SOSTA/DETECT, which calculates statistics for each detected source. To save the calculated sources in memory to a file, use the OUTFILE qualifier. They may be restored for later executions of REMOVE/DETECT or SOSTA/DETECT by using the command SEARCH/INFILE=[filename].


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Last modified: Wednesday, 29-Mar-2006 14:36:20 EST