ISSA Explanatory Supplement
I. Introduction
F. The IRAS Zodiacal History File
The same resampled and rephased time-ordered data described in § I.C were used to produce the IRAS Zodiacal History File (ZOHF) Version 3.0, released in December 1988, and all subsequent versions. The ZOHF is a time-ordered record of the entire IRAS survey in which all detectors in each band of the survey array have been added together over eight-second intervals to produce a 0.5°× 0.5° beam. An important difference between the ZOHF and the ISSA images is that the ZOHF retains the zodiacal dust emission as observed during the IRAS survey. For additional information on the ZOHF refer to Appendix H.
Version 3.1 of the standard ZOHF was released in May 1990 and corrected a single error found in Version 3.0. The problem affected the intensities of a very small number of ZOHF samples in Version 3.0: none at 12 µm, one at 25 µm, one at 60 µm and 382 (0.03%) at 100 µm. The affected samples were lowered 23% on the average, with a maximum decrease of 45%. The description and analyses presented in the IRAS circular accompanying Version 3.0 and in Appendix H of this Supplement are not changed by this correction.
The ZOHF is also available with the zodiacal emission removed. This product was made by subtracting the zodiacal emission as predicted by the J. Good model (Appendix G). The Zodiacal Emission Removed ZOHF is available from IPAC by special request.
In response to requests by IPAC General Investigators, two additional versions of the ZOHF were produced and released by IPAC. A version of the ZOHF was generated giving each pixel the maximum in-scan resolution of 2' while maintaining the 0.5° resolution cross-scan. This product is known as the 2' In-scan ZOHF and was produced for the purpose of studying the zodiacal dust bands near the ecliptic plane. The Bright Point Source Removed (BPSR) ZOHF was produced in response to a user request and was generated by removing flux contribution due to bright point sources and associated tails. Point sources were identified using the IRAS Point Source Catalog. Detector samples within a 10' radius of the known source and along the source tail were removed from the scan data prior to computing an eight-second intensity average. Under certain conditions this algorithm produced a nonphysical increase in a 0.5°× 0.5° pixel brightness compared to the brightness in the ZOHF Version 3.0. This can occur in areas where sources are fainter than surrounding structure. Due to this discrepancy, IPAC recommends that special care be taken when using the BPSR ZOHF.
All released ZOHF products as well as the ISSA are available on permanent media through:
Coordinated Request and User Support Office (CRUSO) NASA/GSFC Code 633.4 Greenbelt, MD 20771.[NOTE: All released IRAS data products are available from IRSA.]