IRAS Explanatory Supplement
X. The Formats of the IRAS Catalogs and Atlases
Appendix X.4 Zodiacal Observation History File (ZOHF) Format

Database Organization:

The ZOHF is organized into 572 files, with two consecutive file marks after the last file on each tape. There is one SOP per file starting with SOP 29, i.e. file 1 corresponds to SOP 29, and file 572 to SOP 600. Within a file, each 80-byte logical record contains the timing, pointing and infrared data for a single eight-second average. Each logical record is identified with an OBS, but OBSs are not othervise separated from one another within a SOP ffie. SOPs for which no survey data exist contain one dummy record which has a valid SOP number but zeros in all other fields. The ZOHF is available only as an ulabelled ASCII formatted magnetic tape. The logical record length is 80 bytes with a physical blcck size of 200 logical records or 16000 bytes. The approximate size of the data base is 96 Mbytes. All of the fields contain data. There are no header or trailer records.

1. UTCS is elapsed time in seconds since 0 hours UT, 1 Jan 1981

2. Elongation is the angle between the line of sight and the Sun. Inclination is the angle between the eclipatc plane and the plane containing the Earth, Sun, and observation direction (i.e., the azimuth angle about the Earth-Sun axis). Elongation was fixed in each OBS, and inclination increased at a constant rate. These angles are related to geocentric ecliptic coordinates by the following expressions:
   cos(ELONG) = cos(X) cos(X - X)

   sin(INCL) = sin(X)/sin(ELONG)

   where (X) is the ecliptic longitude of the Sun.

3. Conversion from in-band brightness to brightness density assumes a source with an energy distribution which is flat in flux per unit octave as explained in Section VI.C.