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This documentation is woefully incomplete. Complete documentation on the contents of the FITS file produced by sdfits and how each value is constructed from the raw FITS files is being working on. That will replace this information when it is available. In the meantime, direct all questions on sdfits to BobGarwood and PaulMarganian.

The FITS file format generated by sdfits contains a primary header without an image, and one or more SINGLE DISH binary extensions.

Each SINGLE DISH extension has a header and a table. The header contains keywords that represent constant information which would otherwise be too repetitive as a column in the table. The table contains the bulk of the information.

A SINGLE DISH extension is structured such that SDFITS CORE keywords/columns appear first, followed by the DATA column and descriptive axes, then by SDFITS SHARED keywords/columns, and finally by GB-specific keywords/columns. See the AIPS++ SDFITS website for a detailed explanation of the CORE, SHARED, and DATA column/descriptive axes sections.

The adoption of multiple SINGLE DISH extensions is to facilitate scans having different numbers of spectral channels. When a scan is encountered that has a different number of spectral channels than the previous scan, sdfits closes the current extension and starts a new one. This eliminates wasted "padding" space.

The format of the SDFITS file is explained in more detail below.

Standard FITS Keywords/Columns

Field	Notes	Keyword or Column	Primary HDU	Binary Table HDU
DATE	Creation date and time of the SDFITS file.	K
ORIGIN	The origin of the observation ("NRAO Green Bank").	K
INSTRUME	The creator of the FITS file ("sdfits")	K

SDFITS CORE Keywords/Columns

Field	Notes	Keyword or Column	Primary HDU	Binary Table HDU
OBJECT		C
TELESCOP		K
BANDWID		C
DATE-OBS	FITS y2k date and time with 0.01s accuracy.	C
EXPOSURE		C
TSYS		C

SDFITS DATA Column and Descriptive Axes

Field	Notes	Keyword or Column	Primary HDU	Binary Table HDU
DATA		C
TDIM6		C
CTYPE1	Frequency-like (Hz)	C
CRVAL1	Not adjusted for reference frame; always FREQ-OBS.	C
CRPIX1		C
CDELT1		C
CTYPE2	Longitude-like (deg)	C
CRVAL2		C
CTYPE3	Latitude-like (deg)	C
CRVAL3		C
CTYPE4	STOKES (enumeration)	K
CRVAL4		C

SDFITS SHARED Keywords/Columns

Field	Notes	Keyword or Column	Primary HDU	Binary Table HDU
OBSERVER		C
OBSID		C
PROJID		K
SCAN		C
OBSMODE		C
FRONTEND		C
BACKEND		K
TCAL		C
VELDEF	Not WCS compliant; neither is SDFITS.	C
VFRAME		C
RVSYS		C
OBSFREQ		C
LST		C
AZIMUTH		C
ELEVATIO		C
SITELONG		K
SITELAT		K
SITEELEV		K
RESTFREQ		C

GBT-Specific Keywords/Columns

Field	Notes	Keyword or Column	Primary HDU	Binary Table HDU
FITSVER	GBT SDFITS Version (currently "1.2").	K
SAMPLER	For example, "A1" or "A1xA3".	C
FEED	Feed number (signal).	C
SRFEED	Reference feed number.	C
BEAMXOFF	Beam XEL offset.	C
BEAMEOFF	Beam EL offset.	C
SIDEBAND	'U'pper or 'L'ower.	C
PROCSEQN	Scan sequence number.	C
PROCSIZE	Number of scans in procedure.	C
LASTON	Last "on" scan for position switching.	C
LASTOFF	Last "off" scan for position switching.	C
VELOCITY	Line velocity in rest frame.	C
FOFFREF1	The ref phase offset such as in frequency-switched data. Only present in spectral line data. CRVAL1 + FOFFREF1 = center sky frequency for the ref phase.	C
SIG	Boolean. "T" indicates "signal", "F" indicates "reference"	C
CAL	Boolean. "T" indicates "cal-on", "F" indicates "cal-off"	C