Antenna indicated positions are those derived from the values read back from the encoders. We will provide three versions of each position; MOUNT coordinates, J2000, and the co-ordinate system in which the antenna was commanded. MOUNT co-ordinates are those returned by the servo computer; they are currently the same as raw encoder units apart from the addition of the encoder offsets (which will also be stored in the antenna FITS file).
In principle any one of these three co-ordinate systems could be generated from any other off-line, given the complete description of the pointing model, the weather conditions at the time of the observation, and so on. However, to do this correctly would require not only knowing the parameters of the pointing model, but the full definition of the terms, the order in which they were applied, and so on. In practice, it will be simpler and less error prone for the antenna manager to provide all three systems (they are being calculated anyway); observers can always reproduce the calculations if they wish.
The process of going from commanded to mount co-ordinates is commonly referred to as the ``downstream'' pointing transformation. The reverse process, of going from mount to commanded (or equatorial) co-ordinates is referred to as the ``upstream'' pointing transformation.
Currently only traditional methods are being used to determine the pointing model for the telescope. Therefore, processing the main axis encoder values through the upstream pointing transformation is the best we can do. In the future, the laser rangefinders (or other metrology systems) may be used to provide additional pointing corrections, either in real-time for use by the servo, or after the fact to be applied at the data reduction stage. Methods for incorporating these corrections into the upstream pointing flow, and representing the data in an appropriate FITS file, is deferred to a later date.
One point should be made explicitly here. We have not yet fully thought through how offset positions for multi-beam receivers will be handled, nor fully de-coupled pointing and focus tracking effects. At this stage, the positions recorded in the antenna FITS files are therefore implicitly those for the receiver beam (prime or gregorian focus) that was used to derive the antenna pointing model in use, assumed to be at the nominal focus.
As noted, it is possible (usually for engineering purposes) to disable certain terms in the downstream pointing transformation. In this case, the upstream transformation will not be correct, and the reported positions in J2000 and the commanded co-ordinate system will not be strictly correct.
The three sets of co-ordinate pairs will be provided in a binary table, along with the time stamp and ancillary information (secondary optics positions for example) as described in the antenna FITS file document. The column names are: RAJ2000, DECJ2000, MOUNT_AZ, MOUNT_EL, MAJOR and MINOR. The first four represent J2000 and mount co-ordinates as described above. The last pair provide the antenna longitude and latitude in the commanded co-ordinate system.
Analagous to the description of the commanded co-ordinate system contained in the GO FITS file, the antenna FITS file will use the keywords INDICSYS, RADESYS, EQUINOX and DATE-OBS to fully describe the type of co-ordinate system being represented by the MAJOR, MINOR columns. The values of INDICSYS, RADESYS and EQUINOX will be provided as follows:
command co-ord INDICSYS RADESYS EQUINOX
system
GALACTIC GALACTIC absent absent
J2OOO RADEC FK5 2000.0
B1950 RADEC FK4 1950.0
JMEAN RADEC FK5 nnnn.n
GAPPT RADEC GAPPT absent (Note 1)
HADEC HADEC absent absent
AZEL AZEL absent absent
USER OTHER absent absent (Note 2)
SOLAR SYSTEM OTHER absent absent (Note 2)
Note 1: For geocentric apparent co-ordinates, the indicated values provided are referred to the epoch of of the equator and equinox of the start of the observation, as given by DATE-OBS, not for the time for which the position is reported (the difference is negligble).
Note 2: We have not yet decided how to represent the USER and SOLAR SYSTEM co-ordinate systems in the FITS file. In the case of these, the MAJOR and MINOR columns in the table will simply be filled with zeroes.
DATE-OBS is the standard FITS keyword which is included in all GBT FITS files to indicate the start of the observation.