Primary Header Keyword Definitions

The LO1 FITS files will contain all of the GBT standard primary header keywords (as described in GBT/SN/004), with their standard meanings. In addition, the following LO1 specific keywords will be supplied:

LSTSTART

The apparent local sidereal time corresponding to the instant in (UTC) time specified by the DATE-OBS keyword. LSTSTART time unit is sidereal seconds since local LST midnight.

SITELAT, SITELONG, SITEELEV

Currently, these keyword values are the location of the intersection of the azimuth and elevation axes in the NAD 83 reference system. These values were provided by the GBT metrology group. The current values are:

Longitude: tex2html_wrap_inline$79^&cir#circ;50^'23^''.406$ W ( $79^{\circ} \hspace{-0.37em}.\hspace{0.02em}839835$ )
Latitude: tex2html_wrap_inline$38^&cir#circ;25^'59^''.236$ N ( $38^{\circ} \hspace{-0.37em}.\hspace{0.02em}433121$ )
Height: 824.551 m

In the future, we may modify these to be ITRF values; in any event the values may change as we refine our knowledge of the position of the telescope.

SITESYS, SITETYPE

These keywords specify the coordinate system (datum) in use for SITELAT, SITELONG, SITEELEV, and whether this is a geodetic or geocentric system. The current values are `NAD83', `GEODETIC'.

RADESYS, EQUINOX

The RADESYS standard FITS keyword specifies the coordinate reference frame for the RA and DEC axis columns. These are described in detail in WCS paper II. For the LO1, RADESYS will normally always be 'FK5', with an EQUINOX of 2000.0.

DELTAUTC

This specifies the UT1 - UTC offset, in use at the beginning of the scan.

IERSPMX, IERSPMY

These keywords specify the values of polar motion corrections in use at the beginning of the scan.

COSVMODE

This value indicates the state of the cosVMode parameter.

IFFREQ

This value indicates the desired IF center frequency.

LOOFFSET

This value indicates the value of $LO1_{offset}$ in equation (5) of LO1DesignNotes. When non-zero, his value represents a receiver which has an additional mixing stage, which acts as a constant offset in terms of LO frequency. This information is included for engineering purposes only; when non-zero this term will be included in the IF Manager sky frequency formula calculations.

LOMULT

This value indicates the LO1 multiplier factor in use. When set to a value other than 1.0, this value represents a frequency multiplier stage between LO and mixer. This information is included for engineering purposes only; when non-unity this term will be included in the IF Manager sky frequency formula calculations.

SIDEBAND

This value indicates the LO1 sideband setting. The possible values are ``Upper'', ``Lower'', or ``Test Tone''.

RESTFRQ

This is the requested rest frequency, in the rest frame of the object under observation.

REQDPTOL

This is the requested Doppler tracking tolerance in Hz.

POWERLVL

This denotes in dBm, the LO1 power level setting at the start of the observation.

POSOFFST

This denotes the state (T/F) of the usePrimaryOffsets parameter. When true, the primaryOffsets parameter is included in the calculation of antenna look direction.

An example primary header is shown below:

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= 'LO1 Control Unit'   / device or program of origin
GBTMCVER= '3.3.0   '           / telescope control software release
FITSVER = '1.1     '           / FITS definition version for this device
DATEBLD = 'Thu Jun 21 18:46:25 GMT 2001' / time program was linked
SIMULATE=                    0 / Is the instrument in simulate mode
DATE-OBS= '2001-11-01T07:06:43' / Manager parameter startTime
TIMESYS = 'UTC     '           / time scale specification for DATE-OBS 
TELESCOP= 'NRAO_GBT'           / Green Bank Telescope (Robert C. Byrd 100m)
OBJECT  = '3C123   '           / Manager parameter source
PROJID  = 'tigerTeam_06'       / Manager parameter projectId
OBSID   = 'map     '           / Manager parameter scanId
SCAN    =                   63 / Manager parameter scanNumber
LSTSTART= 1.13114117570554E+04 / Start Time in seconds since local sidereal midn
SITELAT =        3.8433121E+01 / North latitude of intersection of az/el axes
SITELONG=        7.9839835E+01 / West longitude of intersetion of az/el axes
SITEELEV=          8.24551E+02 / Height of the intersection of az/el axes
SITESYS = 'NAD83'              / Coordinate system (datum)
SITETYPE= 'GEODETIC'           / Type of position
RADESYS = 'FK5     '           / Coordinate reference frame of major/minor axes
EQUINOX =          2.00000E+03 / Equinox of selected coordinate reference frame
DELTAUTC=          -6.6429E-02 / UT1 - UTC offset for beginning of scan
IERSPMX =          -1.2272E-05 / X Polar motion correction for beginning of scan
IERSPMY =           3.7457E-05 / Y Polar motion correction for beginning of scan
COSVMODE= 'OFF     '           / State of cosVMode parameter
RESTFRQ = 1.40800000000000E+09 / Rest Frequency of source
IFFREQ  = 3.00000000000000E+03 / Specified IF center Frequency
LOOFFSET= 0.00000000000000E+00 / Additional Frequency offset due to Rcvr interna
LOMULT  =           1.0000E+00 / Frequency Multiplier due to Rcvr internal multi
REQDPTOL=           1.0000E+00 / Requested Doppler tracking tolerance in Hz
POWERLVL=           -1.100E+01 / Requested LO Power Level in dBm
SIDEBAND= 'LOWER   '           / i.e. LOFREQ = (SKY+IFFREQ)/LOMULTI-LOOFFSET
END