The DETECT command locates point sources in the current image by means of a sliding-cell method. The average background intensity is estimated using the method described previously. The position and intensity of each detected source are calculated in a box whose size maximizes the signal-to-noise ratio. If the source is not pointlike the estimated count rate is in general inaccurate and likely to be under-estimated. A good estimate of the intensity of extended sources can be obtained with command SOSTA.
Corrections for telemetry and instrumental dead times, for vignetting effects and for the fraction of the source counts that fall outside the box where the net counts are estimated are applied. Count rate errors include both statistical and systematic uncertainties added quadratically. To minimize the number of spurious sources detected the threshold used by DETECT is somewhat conservative. Consequently, some sources with intensity just above the image background can be missed. In order to allow DETECT to obtain a sufficiently good estimate of the background only images of size 128x128 pixels or larger should be used. Maximum accuracy is obtained running DETECT on full resolution images.
For crowded fields, or for fields with bright sources the qualifier /bright. This will make a weighted mean of each excess, and optimizing the psf. Consequently it can take much longer to run.
For busy images, e.g. ROSAT PSPC, it may be necessary to ``tweak" the detection parameters to avoid false detections. The /thr_scal and /source_box qualifiers are particularly useful. The /thr_scal scales the threshold for excesses to be detected. This can be increased by modest amounts for ROSAT PSPC deep fields e.g. 1.2 or 1.3. The /source_box qualifier determines the size of the box used, in detector coordinates. For the ROSAT PSPC the default is 80, and it can be increased to 160 to reduce confusion.