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OVERLAP

Purpose: To determine the relative offsets in the zero levels of the observations to be merged. While the resulting zero level after the observations have been merged may still include an additive offset from true zero, it will be considerably more uniform across the field. OVERLAP determines the relative offsets between all adjacent pointings in their region of overlap, and then uses a single-value deconvolution algorithm to solve for the best-fit group of offsets.

File Input:
1) Fixed input - after the band and mask have been selected, the program reads in the appropriate cast count, background, exposure and mask for each observation.
2) Directory list - the file dir.dat must contain an ascii list of the path names to the directories containing the observation arrays to be cast. OVERLAP will use these path names to create the file names for the observations to be compared. For instance, if the band being analyzed is R5, the mask is mask_47.fits, and the directory (VMS) is [SNOWDEN.DATA.OBS1], the program will input the four files: mask: [SNOWDEN.DATA.OBS1]mask_47.fits, count: [SNOWDEN.DATA.OBS1]b_5.fits, background: [SNOWDEN.DATA.OBS1]back_5.fits, and exposure: [SNOWDEN.DATA.OBS1]expose_5.fits.

Interactive Input:
1) Central coordinates for analysis - central coordinates for the center of the projection which will be used for the analysis, in equatorial coordinates.
2) Scale factor for the analysis - the pixel size in arc minutes. Care should be taken that the pixel size is as small as possible yet include all observations in one pixel field.
3) Radius limit - the radius of the region in the observation, in arc minutes, to be analyzed (applied to the sky image rather than the detector image). If a point-source mask is used, generally the entire field can be used (default to 100'). If no point-source mask is used, a limit of 53' for PSPC data will again serve reasonably well (hopefully) to exclude the worst problems caused by the large point response function at large off-axis angles.
4) Band to be analyzed - the program will analyze only one band at a time. Enter 0 for HRI data.
5) Mask file name for masking of the data - the data will be masked using files with the input file name. The mask files must be in the same directories as the data files. If no additional masking is desired, enter ``none''.

Ascii Output:
1) File overlap_n.dat (overlap.dat for HRI data) contains the record of the individual overlap region offsets plus the final offset values.
2) File overlap_n.out (overlap.out for HRI data) contains in addition to the final results, the directories and central coordinates of the individual observations and is required as input to the program MERGE_OFF.