SuperCOSMOS IAM data pairing

22 August 1997

The program 'pairiam' will pair data (i.e. match up detected images by position) in standard SuperCOSMOS IAM data format files. Before running the pairing program, the input files MUST BE SORTED by declination, in increasing declination order. A sorting program ('sortiam') is provided.

The pairing program requires as input two separate IAM data files. It is also given a number of parameters to control the pairing process. As output, it produces four files: two are produced from each input file. For each input file, one output file contains images which were paired, and the other output file contains the images which were not paired. In the ‘paired’ output files, paired images have a one-to-one correspondence through the files, i.e. image number 42 in the ‘paired’ file created from the first input file has been paired with image number 42 in the ‘paired’ file created from the second input file.
The figure illustrates the process:

The method is as follows (see the source code for details): the input files are read through, normally a number of times. On each pass through the input a ‘closest match’ object is found based on the IAM RA and Dec positions, for each of the objects in the first data file. The range of the match is limited by input parameters for each pass. On the first pass, a short range should be used to pick out all the best matched objects; on later passes, the range is gradually increased (e.g. to cope with stars having substantial proper motions). However, objects previously paired are not considered in subsequent passes. After all the passes, the four output files are written.

A monitoring or log file is written so that pairing progress through the passes can be examined.

The program runs standalone in the unix environment; it is best run as a background process with the control parameters in a file, and the log data written to a file, e.g.

pair < pairdata >pairlog &

An example of the control parameters file is given, together with an example of the log file for a run.

The sorting program takes as input one SuperCOSMOS IAM data file and produces another, with the records sorted into increasing declination order, as output. The Dec parameter in the IAM file is used for the sort. As with the pairing program, this is run standalone under unix; the input and output filenames can be given in response to prompts or again put in a file and read on stdin.

Note that you may need to change the record length' value in the binary file 'open' statements depending upon which computer architecture you are using (SUN or DEC Alpha). Currently, these are set to '128', appropriate for SUN workstations. However, if you are working on a DEC Alpha, then you MUST set this to '32'.