Reformatting to FITS

To minimize the amount of code that needs to be written the FITS reformatter takes sortac as the starting point. In parallel with creating hypercube format data products, the output routines are being rewritten to write FITS files as well. These routines make heavy use of the FITSIO library written by Pence (1992). The status of the sortac modifications is that header information is now written to FITS files and we have created a parameter file interface (c.f. IRAF parameter files and the ASCA reformatter) which facilitates running in batch mode.

Science information will be stored in a binary table extension similar to that used with ASCA (c.f. Corbet et al. 1992). The precise FITS formats will be defined in close collaboration with a project to reformat the HEAO-1 data base to ensure maximum compatibility. It has not yet been decided whether to store house keeping information as separate files or as separate extensions within science files. It is expected that the FITS creation software will be run once on the entire Ginga data set. The resulting FITS files will then be made available to the community by the HEASARC at the Goddard Space Flight Center.

The Ginga LAC could be operated in several modes. These made trade-offs between time resolution and detector information (which affects the accuracy with which spectra can be measured). The FITS formats for the various modes will be similar, with the principal difference being the number of columns in the binary tables.

MPC1 mode:

mainly used for spectral studies of faint sources. Events are accumulated in sixteen separate spectra of 48 channels each. The sixteen spectra comprise the top and middle layers from each of the 8 detectors. The separation of top and middle layers improves the signal to noise ratio for weak sources and helps the background estimation.

MPC2 mode:

provides compression of data by a factor of eight enabling better time resolution. Combines top and middle layers from four detectors into one and gives two separate spectra. The combination of layers decreases the signal to noise ratio and background estimation is less precise.

MPC3 mode:

carries the process further by combining the 48 energy channels into twelve and grouping all eight detectors together, giving a further factor of eight compression.

PC mode:

used for timing studies; it by-passes the ADC to avoid dead time effects. Signals are divided into two energy bands by three discriminators (lower, middle and upper) and no other energy information is retained. In this way the dead time is reduced to 16.5 µs/event and time resolution down to 976.6 µs (1/1024) is obtained from two energy bands per detector group. The lower discriminator is the same as that used for the pulse height spectra, while the other two discriminators have two commandable levels.