HST Data Handbook for COS

2.1 HST Data Files

---

The STScI pipeline automatically processes and calibrates all the data received from HST and assembles them into a form suitable for most scientific analyses.

Data from instruments installed after the 1997 servicing mission (ACS, COS¹, NICMOS, STIS, and WFC3¹) are made available to observers as files in Multi-Extension FITS (MEF) format, which is directly readable by most PyRAF/IRAF/STSDAS tasks.

For first and second generation instruments (FGS, FOC, FOS, GHRS, HSP, WF/PC-1, and WFPC2), the data are in a format known as Generic Edited Information Set (GEIS). Byte ordering for binary data in GEIS files is machine-dependent. Therefore, for purposes of archiving and file transfer, GEIS data are converted to a modified FITS format known as "waiver FITS." It is necessary to convert waiver FITS files back to GEIS format for data processing and analysis using IRAF/STSDAS tasks.

As part of the reprocessing effort mentioned in Section 1.1.1, all WFPC2 data will be available² in either waiver FITS or MEF format. The user will be able to specify in which format the data should be retrieved from the HDA. STSDAS support for the analysis of WFPC2 MEF data files has recently been completed, therefore WFPC2 data, in either GEIS or MEF formats, can now be fully processed with STSDAS tasks.

For older instruments (FOC, FOS, FGS, GHRS, HSP, WF/PC-1, and WFPC2) data are provided in waiver FITS format, which needs to be converted back to GEIS format before processing. Note that WFPC2 data will soon be available in MEF format from the HDA. Newer instruments (ACS, COS, NICMOS, STIS, and WFC3) generate and store files in FITS format and should not be converted to GEIS.

Table 2.1: HST Instrument File Formats.

Instrument	Status	Format from HDA	Format for STSDAS Analysis
ACS	reactivated after SM4	MEF	MEF
COS	active after SM4	MEF	MEF
FGS	active	waiver FITS	GEIS
FOC	decommissioned SM3B	waiver FITS	GEIS
FOS	decommissioned SM2	waiver FITS	GEIS
GHRS	decommissioned SM2	waiver FITS	GEIS
HSP	decommissioned SM1	waiver FITS	GEIS
NICMOS	active	MEF	MEF
STIS	reactivated after SM4	MEF	MEF
WFC3	active after SM4	MEF	MEF
WFPC2	decommissioned during SM4	waiver FITS & MEF1	GEIS & MEF1
WFPC1	decommissioned SM1	waiver FITS	GEIS

1HDA now also stores WFPC2 data in MEF format.

This chapter describes these two HST file formats, GEIS and MEF, in more detail. ACS, COS, NICMOS, STIS, and WFC3 observers should pay particular attention to the section on MEF files, which shows how to identify and access the contents of these files and covers some important conventions regarding header keywords. Users of the heritage instruments should consult the material on data groups and conversion from waiver FITS to GEIS format found in Section 2.3.1 before proceeding to Chapter 3.

Throughout this document, references to FITS files will always mean HST Multi-Extension FITS format files. References to waiver FITS files will always be explicitly stated.

¹Available after installation on HST during Servicing Mission 4.
²As of January 2009, some of the WFPC2 data are available in MEF format. Reprocessing will continue well into 2009.