GBT SDFITS Files

Convention

The single-dish FITS (SDFITS) convention is used for observer-facing GBT data. The GBT records raw data to FITS files, and then the SDFITS filler populates SDFITS files with information the observer needs. GBT SDFITS files contain 2 or more Header-Data Units (HDUs). Note that keys sometimes appear to be truncated because they can only have up to 8 letters.

HDU 0 (PRIMARY)

Header

Key	Value	Definition
DATE	UTC Timestamp	Date and time the file was created
ORIGIN	“NRAO Green Bank”	Origin of the observation
TELESCOP	“NRAO_GBT”	The telescope used for the observation
INSTRUME	“VEGAS”	The backend used for the observation
GBTMCVER	“sdfits ver#.##”	Version of SDFITS used to make this file (current: 1.22)
FITSVER	“#.#”	FITS definition version

HDU 1 (SINGLE DISH)

Header

Key	Value	Definition
XTENSION	“BINTABLE”	binary table extension
BITPIX	8	8-bit bytes
NAXIS	2	2-dimensional binary table
NAXIS1	integer	width of table in bytes
NAXIS2	integer	number of rows in table
PCOUNT	integer	size of special data area
GCOUNT	1	one data group (required keyword)
TFIELDS	integer	number of fields in each row
TELESCOP	“NRAO_GBT”	The telescope used for the observation
PROJID	string	project identifier
Backend	“VEGAS”	backend device
SITELONG	-7.983983E+01	longitude of intersection of the az/el axes
SITELAT	3.843312E+01	latitude of intersection of the az/el axes
SITEELEV	8.245510E+02	height of the intersection of az/el axes
EXTNAME	“SINGLE DISH”	The name of this binary table extension

Data

Key	Unit	Type	Definition
OBJECT		32A	name of source observed
BANDWID	Hz	1D	bandwidth
DATE-OBS		22A	date and time of observation start
DURATION	s	1D	total integration duration in seconds
EXPOSURE	s	1D	effective int time (excludes blanking) in seconds
TSYS	K	1D	system temperature in Kelvin
DATA		131072E	Actual data
TDIM7		16A	data dimensions of the array
TUNIT7		6A
CTYPE1	Hz	8A	first data axis is frequency-like
CRVAL1	Hz	1D
CRPIX1	index	1D
CDELT1	Hz	1D
CTYPE2		4A	second axis is longitude-like
CRVAL2	1D	deg
CTYPE3		4A	third axis is latitude-like
CRVAL3	deg	1D
CTYPE4			fourth axis is Stokes
CRVAL4		1I
OBSERVER		32A	name of observer(s)
OBSID		32A	observation description
SCAN		1J	scan number
OBSMODE		32A	observing mode
FRONTEND		16A	frontend device
TCAL	K	1E	calibration temperature
VELDEF		8A	velocity definition and frame
VFRAME	m/s	1D	radial velocity of the reference frame
RVSYS	m/s	1D	radial velocity, Vsource - Vtelescope
OBSFREQ	Hz	1D	observed center frequency
LST	s	1D	LST at midpoint of integration/scan
AZIMUTH	deg	1D	azimuth
ELEVATIO	deg	1D	elevation
TAMBIENT	K	1D	ambient temperature
PRESSURE	1D	mmHg	ambient pressure
HUMIDITY		1D	relative humidity
RESTFREQ	Hz	1D	rest frequency at band center
FREQRES	Hz	1D	frequency resolution
EQUINOX		1D	equinox of selected coordinate reference frame
RADESYS		8A	Equitorial coordinate system name
TRGTLONG	deg	1D	target longitude in coord. ref. frame
TRGTLAT	deg	1D	target latitude in coord. ref. frame
SAMPLER		12A	sampler description (e.g., “A1” or “A1xA3”)
FEED		1I	(signal) feed number
SRFEED		1I	reference feed number
FEEDXOFF	deg	1D	feed XEL offset
FEEDEOFF	deg	1D	feed EL offset
SUBREF_STATE		1I	subreflector state (1,0,-1) - 0=moving
SIDEBAND		1A	resulting sideband (‘U’pper or ‘L’ower)
PROCSEQN		1I	scan sequence number
PROCSIZE		1I	number of scans in procedure
PROCSCAN		16A	scan’s role in the procedure
PROCTYPE		16A	type of the procedure
LASTON		1J	last ‘on’ for position switching
LASTOFF		1J	last ‘off’ for position switching
TIMESTAMP	UTC	22A	date and time of scan start
QD_XEL	deg	1D	QuadrantDetector cross-elevation offset
QD_EL	deg	1D	QuadrantDetector elevation offset
QD_BAD		1I	QuadrantDetector flag: 0=good,1=bad
QD_METHOD		1A	Quad. Det. method A,B,C. Blank indicates none.
VELOCITY	m/s	1D	line velocity in rest frame
ZEROCHAN		1E	zero channel
DOPFREQ	Hz	1D	Doppler tracked frequency
ADCSAMPF		1D	VEGAS ADC sampler frequency
VSPDELT		1D	channel increment between adjacent VEGAS spurs
VSPRVAL		1D	VEGAS spur number at VSPRPIX
VSPRPIX		1D	channel number of VEGAS spur VSPRVAL
SIG		1A	signal is true, reference is false
CAL		1A	cal ON is true, cal OFF is false
CALTYPE		8A	LOW or HIGH, may eventually be other types
TWARM	K	1E	4mm RX ambient load temp (K)
TCOLD	K	1E	4mm RX cold load temp (K)
CALPOSITION		16A	4mm RX table position
IFNUM		1I	Spectral window (IF) number
PLNUM		1I	Polarization number
FDNUM		1I	Feed number

Index Files

Header

Key	Value	Definition
created	Day Month DD HH:MM:SS YYYY	Date file was created
last_modified	Day Month DD HH:MM:SS YYYY	Date file was last modified
version
observer
backend
tcal_rx_table
sprotect
created_by	“index_writer”	Method that created this file

Rows

Key	Value	Definition
INDEX
PROJECT
FILE EXT
ROW
SOURCE
PROCEDURE
OBSID
E2ESC
PROCS
SCAN
POL
PLNUM
IFNUM
FEED
FDNUM
INT
NUMCHN
SIG
CAL
SAMPLER
AZIMUTH
ELEVATION
LONGITUDE
LATITUDE
TRGTLONG
TRGTLAT
SUB
LST
CENTFREQ
RESTFREQ
VELOCITY
FREQINT
FREQRES
DATEOBS
TIMESTAMP
BANDWIDTH
EXPOSURE
TSYS
NSAVE
PROCSCAN
PROCTYPE
WCALPOS

Flag Files

Flag files indicate the data that should be ignored. For example, these flags can include the locations of VEGAS spurs. GBTIDL sometimes auto-masks data that is flagged in these files immediately upon start.

Header

Key	Value	Definition
created	Day Month DD HH:MM:SS YYYY	Date file was created
version	1.0	Version of ?
created_by	sdfits	Created by the SDFITS filler

Flags

Key	Value	Definition
RECNUM	integer or “*”
SCAN	integer or “*”	Scan number
INTNUM	integer or “*”	Integration number
PLNUM	integer or “*”	Polarization number
IFNUM	integer or “*”	Spectral window (IF) number
FDNUM	integer or “*”	Feed number
BCHAN	list of integers
ECHAN	list of integers
IDSTRING	“VEGAS_SPUR”	Type of flag

Other Resources

The full SDFITS documentation for GBO can be found here: The GBT SDFITS Project Wiki