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Data format

The observer takes home a copy of the data tapes, that has been produced in FITS format. The original D-tape is archived at RGO. The header information on the data is kept on-line on a Vax computer, and can be accessed using the La Palma Data Archive (E.J. Zuidewijk, 1989, Technical Note 69) by the owner and, after one year, by anyone interested in the data.

Prior to 16/1/1989, the data from the CCD was copied into tape as produced: unsigned 16 bit. FITS data conventions are unsigned 8 bit; signed 16 bit and signed 32 bit. STARLINK FITS routines therefore considered the data as supported by the conventions which is fine for count values in the range 0 to ()=32767. For output in the range () to () the routines would produce negative numbers in the range to . This data looks as if it is saturated. The simplest recipe is to search for negative numbers, and add to them . This is done by simple routines available in STARLINK.

Changes have been introduced to handle CCD data at the telescope in a consistent and correct manner. After the 16/1/1989, CCD images created on the INT are labelled internally on disc as U2 (unsigned 16 bit) and FITS tapes now conform exactly to the international FITS standard.