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Chapter 2
HST File Formats

In This Chapter...

 Historical Perspective / 2-2
FITS File Format / 2-3
GEIS File Format / 2-10

STScI automatically processes and calibrates all the data received from HST. The suite of software programs that performs this processing-part of a system known as OPUS-is frequently called the pipeline, and its purpose is to provide data to observers and to the HST Data Archive in a form suitable for most scientific analyses. Pipeline processing assembles data received from HST into datasets, calibrates the data according to standard procedures described in the instrument sections of this handbook, and stores both calibrated and uncalibrated datasets in the Archive.

Before the 1997 servicing mission, HST data files passed through pipeline processing in Generic Edited Information Set (GEIS) format and then were stored in the Archive in Flexible Image Transport System (FITS) format, a machine-independent format that simplifies the distribution of data to many different platforms. Users of FOC, FOS, FGS, GHRS, HSP, WF/PC-1, and WFPC2 data currently need to convert FITS data files received from the Archive back into GEIS format before they can reduce them further. Recent developments have made the FITS format much more useful for data processing, and it is now the standard format for pipeline processing and subsequent reduction of STIS and NICMOS data. Observers using these new instruments should not convert their files to GEIS format.

This chapter describes these two HST file formats, first giving some historical perspective on the reasons why they were selected, then explaining the FITS and GEIS formats in more detail. STIS and NICMOS observers should pay particular attention to the section on FITS files, which shows how to identify and access the contents of these files and covers some important conventions regarding header keywords. Veteran observers with the other instruments will find little new in the section on GEIS files, but newcomers to the older HST instruments should consult the material on data groups and conversion from FITS to GEIS before proceeding to Chapter 3.


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Copyright © 1997, Association of Universities for Research in Astronomy. All rights reserved. Last updated: 11/13/97 16:24:32