The world coordinate frameset used to map the spatial and frequency coordinates of the observation are written to an NDF extension in the output file(s). The frameset consists of a mapping from the GSD CELL coordinates to both tracking RA/Dec and Az/El coordinates. The third axis represents the frequency scale of the observation.
The mapping from GSD CELLs to ACSIS TCS tracking and AZEL coordinates is created using the frameset from the GSD CELL headers and the GSD CENTRE coordinates given for the source’s position at the time of the observation, in combination with the LST and CELL offsets of each time step.
In order to create this frameset, several components from the original GSD file are required:
GSD’s CELL coordinates describe a grid placed upon the sky with a centre cooresponding to the location described in the CENTRE_RA1950 (C4ERA), CENTRE_DEC1950 (C4EDEC), CENTRE_RA (C4RADATE), CENTRE_DEC (C4DECDATE), CENTRE_RA2000 (C4RA2000), CENTRE_DEC2000 (C4EDEC2000), CENTRE_GL (C4GL), CENTRE_GB (C4GB), CENTRE_AZ (C4AZ), and CENTRE_EL (C4EL) coordinates. These pairs of coordinates are assumed to correspond with the time given in the OBS_UT1D (C3DAT) and OBS_UT1H (C3UT) headers.
The CENTRE_CODE (C4CECO) (coordinate system of the tracking frame) along with the corresponding coordinate pair from those listed above is used as the RA/Dec input, along with the TEL_LONGITUDE (C1LONG) and TEL_LATITUDE (C1LAT) to create the frameset.
The description of the CELL coordinate frame comes from the GSD CELL headers:
The third axis of the WCS frameset is the frequency scale. The GSD values used to generate this frameset are:
The WCS frameset requires two further inputs in order to be created. These are the velocity and the velocity definition (’LSRFLG’). For total power and cross correlation observations, the velocity is assumed to be 0, while for other observations the value in VELOCITY (C7VR) is used. The velocity definition is a flag which defines both the rest frame of the source velocity (topocentric, kinematic LSR, heliocentric, or geocentric, given by the bottom 4 bits of ’LSRFLG’) and the source velocity definition (radio, optical, or relativistic, given by bits 5 and 6 of ’LSRFLG’).
The WCS frameset is used to generate the TCS tracking and AZEL base and offset coordinates found in the JCMTState NDF extension. At each time step of the observation, the time can be obtained from the SCAN_TABLE1 (C12SCAN_TABLE_1)array (see TCS_TAI
). The current cell offset is found in the list of xy coordinates stored for each scan in the MAP_TABLE (C14PHIST)array. Base coordinates are generated using a cell offset of 0, 0.