Nonstandard Elements of Lick Observatory FITS Files

During the history of FITS usage at Lick Observatory there have been a number of instances where the Lick FITS files do not conform with the FITS standard. This is an attempt to document the inconsistencies for the sake of posterity.

For practical purpoeses Lick Observatory has generated FITS images via three different mechanisms. The Lick Observatory Data Acquisition Programs run on Mt. Hamilton and produce original FITS files based on data from the telescopes. The VISTA data reduction program has existed in many incarnations, but these are adequately divided into `Mountain Vista' and `Downtown Vista'.

Disk file usage problems specific to VISTA

Up through version 3 Downtown Vista was a VAX/VMS-based program. Speed of disk I/O was held in higher esteem than standard conformance, and networked exchange of information between heterogeneous systems was uncommon. Floating point and integer images were written to disk in a VMS-specific format. Floating point and integer arrays were written using different styles of I/O, and the floating point images were stored as native VAX D-floats. In both cases the bytestream for these files contains VMS RMS information and they cannot easily be decoded by anything other than Downtown Vista. It is likely that VMS BACKUP and ANSI-formatted tapes still exist with such images.

Up through version 3 Downtown Vista misused the FITS BSCALE and BZERO keywords when writing integer images to disk. The sense in which BSCALE and BZERO were misused was as follows:
integer_array_value = BZERO + BSCALE * physical_value
Since these disk files can effectively only be read by Downtown Vista, the only confusion that is likely to result is if someone tries to reconstruct the FITS data in the absence of the FITS code.

Downtown Vista version 4 became available in 1988 December. At that time Downtown Vista began to use the BSCALE and BZERO keywords in conformance with the FITS standard. It also adopted the FITS floating point agreement and began using BITPIX = -32 IEEE floats. Disk files generated by Downtown Vista 4.0 or later may be presumed to conform to all existing FITS standards.

Mountain Vista did not do I/O of floating point images until 1988 April or later. Mountain Vista abolished the use of the DISK keyword (below) as of 1990 January 31. At the same time Mountain Vista adopted the FITS floating point agreement and began using BITPIX = -32 IEEE floats. Disk files generated by Mountain Vista since that time may be presumed to conform to all existing FITS standards. It is not known whether there was an interval when disk files written by Mountain Vista misused the BZERO and BSCALE keywords.

Because the tape I/O code was entirely disjoint from the disk I/O code it appears that Lick FITS tapes have been conformant with the FITS standard -- at least since 1984. However it may be wise to mistrust the BZERO and BSCALE information in Lick FITS tapes written prior to 1984.

Keywords specific to Lick

At the time of file creation all FITS files generated by the Lick Observatory Data Acquisition System (DAS) contain the FITS keyword/value pair
STATUS = 'RAW'
The Lick FITS readers use the existence of this keyword as an indication that the file was written at Lick. Some of the VISTA subprograms replace the value with other strings indicating the type of processing that has been done.

At the time of file creation all FITS files generated by the Lick Observatory Data Acquisition System (DAS) contain the FITS keyword/value pair
SHUTTER = 'OPEN'
or
SHUTTER = 'CLOSED'
The Lick FITS readers use the existence of this keyword as an indication that the file was written at Lick.

Files written to disk by VISTA as integers in the machine specific formats contain the keyword/value pair
DISK = 'INTS'
Files written to disk by VISTA as integers which conform to the FITS standard contain the keyword/value pair
DISK = 'FITS'
There are several other values of this flag, all of which are used by VISTA to distinguish old-style disk files from conforming FITS files. The keyword is usually not written to floating point images or to tapes, but it may occur in images with unusual conversion history.

FITS usage problems common to all Lick files

Before 1983 December 15 the (integer) data arrays of all FITS tapes written at Lick were byte/word swapped in the sense of Digital Equipment Corporation PDP, LSI, and VAX computers. Files written after this date contain the FITS keyword/value pair
LICK = 'FITS'
to indicate that the byte order of the data arrays conforms to the FITS standard.

Before 1984 January 19 the row and column information in Lick FITS files was swapped. The contents of NAXIS[12], CRVAL[12], and CDELT[12] need to be interchanged for proper interpretation. Files written after this date contain the FITS keyword/value pair
LICK = 'FITS2'
to indicate that the row and column information conforms to the FITS standard.

Keywords used inconsistently with emerging FITS WCS practice

The Lick DAS has used the CDELTn keywords to indicate the binning performed at the time of CCD readout. This is inconsistent with the FITS WCS Draft Standard. As of this date the Lick DAS continues this practice.

The Lick DAS has used the CRVALn keywords to indicate the pixel at the origin of a readout window. This coordinate is based on the unbinned CCD (I think). The practice in the Lick Observatory CCD Development Laboratory has always been to denote the origin pixel as (0,0). This conflicts with the numbering of FITS pixels and the WCS draft. As of this date the Lick DAS continues this practice.


Steve Allen <sla@ucolick.org>

Originally published: 1996-04-23

$Date: 2003/07/28 18:33:34 $