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'.
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.
STATUS = 'RAW'
At the time of file creation all FITS files generated by the Lick
Observatory Data Acquisition System (DAS) contain the FITS
SHUTTER = 'OPEN'
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.
LICK = 'FITS'
Before 1984 January 19 the row and column information in Lick FITS files
was swapped. The contents of NAXIS, CRVAL, and CDELT
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.
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.