2.1. FITS

All galactic and extragalactic astronomical data are stored and distributed in the Flexible Image Transport System (FITS). As the standard, all astronomical reduction and analysis packages need to be able to read and write FITSfiles. The details of the FITS convention can be found in "Implementation of the Flexible Image Transport System (FITS)" published by NASA's Office of Standards and Technology and in Wells et al. 1981, Grøsbol et al. 1988, Harten et al. 1988, and Ponz et al. 1994. These and other documents can be downloaded from ftp://nssdc.gsfc.nasa.gov/pub/fits.

FITS format data consists of

• an ASCII header
• binary data

The header contains all the information about the observation and the file in a sequence of keywords. These keywords are written in a fix 80-column format of the form:

2.1.1 HST data: group format extension

HST observations use an extension of the FITS convention, the so called group format extension. This means that a group of data is stored within the same filename. For example, all four images from each chip of the WFPC2 camera are stored with the same name; each one constitutes a group. Spectra obtained with the FOS and GHRS might have hundreds of groups.

These FITS extension data needs a special way to be written to tape; ie., the group information needs to be stored and recovered upon reading. The tapes distributed by the HDA will then have

• a file with nine alphanumericcharacters ending with _cvt and an extension
• a binary table (with the .tab extension)

Data downloaded directly from the HDA consists of just one file withthe .fit extension.

HST datasets in the archive have filenames composed of nine alphanumeric characters. The first one identifies the instrument used to obtained the data. The convention is shown in Table 1. The other characters are related to the proposal number under which the data were obtained, the order in which they executed and the way they were transmitted to the ground.