Primary Header Keyword Definitions

LSTSTART

This lists the calculated local sidereal time in sidereal seconds since local sidereal midnight of the start time for the scan.

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'.

INDICSYS

This keyword describes the type of positions contained in the MAJOR and MINOR axis columns of the binary table. Possible values are GALACTIC, RADEC, HADEC, AZEL or OTHER. In the case of OTHER, the MAJOR and MINOR columns will contain co-ordinates in the user-defined system.

OFFSYS

Analogous to the INDCSYS keyword, this keyword describes the command offset coordinate system.

ORADESYS

Analogous to the RADESYS, this specifies the standard FITS keyword for the offset command coordinate system.

RADESYS, EQUINOX

For RADEC co-ordinate systems, the RADESYS standard FITS keyword specifies the coordinate reference frame for the MAJOR and MINOR axis columns. The EQUINOX keyword denotes the epoch in use, for the MAJOR and MINOR axis columns. These are described in detail in WCS paper II.

DELTAUTC

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

IERSPMX, IERSPMY

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

PFM_CONF, PFM_VERS

These keywords list the pointing focus model configuration name and version.

REFMODEL

The field lists the refraction model type. Values may be one of "NONE", "GbtComMemo16", or "PTCS35.2", meaning refraction is off, based on GBT commissioning memo 16, or PTCS note 35, version 2 respectively.

MNTOFFAZ, MNTOFFEL

These keywords specify the encoder offset values, applied by the Az/El servo system (CCU). The values are in degrees.

LPC_AZ1, LPC_AZ2, LPC_EL

These keywords specify the user provided local pointing correction values for the scan in azimuth, cross-elevation and elevation respectively. The net contribution to pointing by the local pointing correction values can be computed by the relation:

$$az_{lpc} = LPC\_AZ1 + \frac{LPC\_AZ2}{cos(MNT\_EL)}$$ (5)

$$el_{lpc} = LPC\_EL$$ (6)

PNTMODEL

This keyword defines the type of pointing correction model active during the scan. The field currently has two values: 'GBT MEMO 173' or 'NONE'.

PNTAZD00, PNTAZB01, PNTAZD01, PNTAZA11, PNTAZB11, PNTAZC21, PNTAZD21, PNTAZB02, PNTAZD02

Theses keywords represent the values of the traditional model coefficients for the azimuth axis. The values are in radians, are the coefficients are listed in GBT memo 173.

PNTELD00, PNTELC10, PNTELD10, PNTELB01, PNTELD01

These keywords represent the values of the traditional model coefficients for the elevation axis.The values are in radians, are the coefficients are listed in GBT memo 173.

PNTAZH00, PNTAZH01, PNTELH00, PNTELH01

These keywords record the coefficient values for the hysteresis correction in azimuth and elevation. The terms are defined in the following way:

$$\Delta_{az} = H_{00}\tanh(H_{01}\dot{az})$$ (7)

Where $H_{00}$ is the maximum amplitude of the correction in radians, and the term $H_{01}$ is the multiplier of rate, used to control the engagement sensitivity. The term $\dot{az}$ is in radians per second.

PNTAZR00, PNTELR00

These keywords represent the multiplicative coefficient of axis rate, used to derive a friction correction. The term R00 is in units of radians of offset per $\frac{radians}{second}$ .

PNTLAMBDA1 ,PNTLAMBDA2, PNTLAMBDA3

The keywords represent the average values of provided from the track correction tables. The value is in degrees, and is an average value for the duration of the scan. For concise definitions of these corrections, see PTCS PN-53, equation 48.

FTRKXDA,FTRKYDA, FTRKZDA

The GBT uses a focus tracking model of the form:

$$correction = A + B\cos(el) + C\sin(el)$$ (8)

for each of the 3 displacement and 3 tilt axes.

This value indicates the position focus tracking constant coefficient in millimeters, for the X, Y, Z axes.

FTRKXDB,FTRKYDB, FTRKZDB

The value indicates the cos(el) coefficient of the focus tracking model in millimeters.

FTRKXDC,FTRKYDC, FTRKZDC

The value indicates the sin(el) coefficient of the focus tracking model in millimeters.

FTRKXTA,FTRKYTA, FTRKZTA

This value indicates the focus tracking tilt constant coefficient in degrees, for the X, Y, Z axes.

FTRKXTB,FTRKYTB, FTRKZTB

The value indicates the cos(el) coefficient of the focus tracking model in degrees.

FTRKXTC,FTRKYTC, FTRKZTC

The value indicates the sin(el) coefficient of the focus tracking model in degrees.

LFC_X, LFC_Y, LFC_Z, LFC_XT. LFC_YT, LFC_ZT

These keywords describe the local focus correction coefficients used during the observation. Displacements are in millimeters, and rotations in degrees.

DIABMODE

This value indicates the state of whether or not diurnal aberration corrections are active.

POLARMTN

This value indicates the state of whether or not polar motion corrections are active.

COSVMODE

This value indicates the state of the cosVMode parameter.

AMBTEMP

Ambient temperature used for refraction calculations in degrees C

AMBPRESS

Ambient pressure used for refraction calculations in milliBars

AMBHUMID

Ambient humidity used for refraction calculations as a fraction (i.e. a range of 0..1.0)

WEATHSTA

This keyword describes the source of weather information, and may take on the values: User_Input, Weather_Station_1, or Weather_Station_2

OPTICSMD

This value indicates the optical path configuration of the telescope. Values are 'PRIMEFOCUS OPTICS', 'GREGORIAN OPTICS' or 'STOW OPTICS'.

FOCTRMOD

This value indicates the state of the focusTrackingMode parameter. Values may be either "OFF" or "ON".

FOCPATM

This value indicates the state of the parallacticAngleTracking parameter. Values may be either "OFF" or "ON".

TRCKBEAM

This value indicates the ASCII name of the tracked beam.

TRCKBMXE

This value indicates the cross-elevation offset in degrees, with respect to the normal antenna boresight. In other words this is the offset from the position which would have been used, had the feed been placed in the center of the receiver mount. Note that the actual azimuthal offset is elevation dependent, and varies during a scan. The Az offset is defined as:

$$AzOffset = \frac{TRCKBMXE}{\cos(El)}$$ (9)

Note: This value is already accounted for in the (MAJOR,MINOR) and (RAJ2000,DECJ2000) columns, and is included for reference only.

TRCKBMEL

This value indicates the elevation offset in degrees, with respect to the normal antenna boresight. In other words this is the offset from the position which would have been used, had the feed been placed in the center of the receiver mount.

Note: This value is already accounted for in the (MAJOR,MINOR) and (RAJ2000,DECJ2000) columns, and is included for reference only.

PLSCALXT

The keyword indicates the number of arc-min of the change in elevation per degree of subreflector X tilt.

PLSCALZT

The keyword indicates the number of arc-min of the change in cross-elevation per degree of subreflector Z tilt.

SDMJD

Describes the time associated as the center of scan, in modified Julian day format.

SOBSC_AZ, SOBSC_EL

This keyword records the refracted topocentric azmuth/elevation command at the scan midpoint. (Time indicated by the SDMJD keyword.) This position is relative to the position of the beam on the sky, rather than the position of the main reflector. Values are in degrees.

SMNTC_AZ, SMNTC_EL

These keywords records the encoder azimuth/elevation command at the midpoint of the scan. (Time indicated by the SDMJD keyword.) The values are in degrees, and are representative of the commanded position of the structure with respect to the track, rather than a beam position on the sky.

DYN_ENA

This keyword describes whether the antenna manager is enabling external corrections.

SDYN_AZ1,SDYN_AZ2,SDYN_EL

These keywords record the dynamic pointing correction in effect for azimuth, cross-elevation and elevation respectively at the midpoint of the scan. Values are recorded in degrees.

SDYN_FTKYP

This keyword records the Y-axis dynamic focus correction in effect at the midpoint of the scan in millimeters.