The Spectrometer FITS keywords for the primary HDU conforms to the definition for common FITS headers as described in GBT Software Project Note 4.0 ``Device and Log FITS Files for the GBT.'' Only the keywords FFT (already described), BANK and QUADRANT are added to identify the bank and set of quadrants used to generate the data. Note that the Spectrometer unlike other GBT backends produces concurrently separate files for each BANK rather than collating them into one file or forcing the selection of only one bank. The FITS file HDU provides a summary of the file contents.
The primary header keywords are as follows:
SIMPLE = T / file does conform to FITS standard BITPIX = 8 / number of bits per data pixel NAXIS = 0 / number of data axes EXTEND = T / FITS dataset may contain extensions ORIGIN = 'NRAO Green Bank' / INSTRUME= 'Spectrometer' / device or program of origin GBTMCVER= '3.4.5 ' / telescope control software release FITSVER = '1.2 ' / FITS definition version for this device DATEBLD = 'Wed Jun 26 20:16:27 GMT 2002' / time program was linked SIMULATE= 0 / Is the instrument in simulate mode? DATE-OBS= '2002-06-26T20:25:42' / Manager parameter startTime TIMESYS = 'UTC ' / time scale specification for DATE-OBS TELESCOP= 'NRAO_GBT' / Green Bank Telescope (Robert C. Byrd 100m) OBJECT = 'test ' / Manager parameter source PROJID = 'spec1.2 ' / Manager parameter projectId OBSID = 'test ' / Manager parameter scanId SCAN = 1 / Manager parameter scanNumber FFT = 0 / Have the data been Fourier transformed? BANK = 'A ' / set of quadrants identifier QUADRANT= '1 ' / Quadrants contributing to Bank NLAGS = 8192 / number of lags in correlation function MODE = '1N2-0A-50-9' / Spectrometer mode NYQUIST = 0 / Twice Nyquist Sampling Flag SELFTEST= 0 / Is the spectrometer in self-test mode? END
There are seven non-standard GBT keywords. For the keyword FFT, false indicates that the DATA table contains lags and true indicates Fourier transformed data. The keyword BANK identifies which of four possible banks (A, B, C or D) the file represents and QUADRANT lists which of the four quadrants were used. MODE is a string used to describe the basic setup of a set of quadrants used for engineering purposes. NYQUIST indicates whether the setup will generate twice Nyquist sampling or not.
Note that the SIMULATE keyword indicates that the Spectrometer Manager is running in simulate mode (most likely on a machine other than the VME sparc board), and any data stored is generated by the software itself. The mode of observation where the system is actually reading out the spectrometer hardware, but the hardware is in self-test mode is described by the keyword SELFTEST.
As for all GBT scan data FITS files, the DATE-OBS keyword takes the value of the Manager parameter startTime. This is the scheduled start time for the scan as a whole. The actual time at which an integration is initiated is recorded in the DATA table.