Telescope Pointing Information


Telescope pointing information is a set of values recorded by the antenna computer. This information is used to relate each backend readout to the exact position of the telescope beam on the sky when it was taken. It is especially useful in the OTF (on-the-fly) mapping mode with a non-facility instrument, where the telescope beam is on a constant move during data acquisition and the coupling between the antenna computer and an instrument control computer is loose.

Telescope pointing information is being sampled at 100 Hz and written into disk files for later retrieval. These files are named using four-digit MOD (minute of day) for the first entry as 0000, 0001, 0002, ..., 1438, and 1439. The first entry in the files is always on the minute (:00.00) and the last entry is for 10 msec before the next whole minute (:59.99). There are thus 1440 files and each contains 6000 entries. For example, the file 0000 contains entries from 00:00:00.00 to 00:00:59.99 UTC, the file 0720 contains the entries from 12:00:00.00 to 12:00:59.99, 1439 from 23:59:00.00 to 23:59:59.99, etc. They will be overwritten once the MOD rolls over. A lock file of the form xxxx.lck is created to indicate that the file xxxx is currently being updated. Clients are free to access all the other files but the one with the lock. Disk file PLOG is no longer available.

In addition to disk files, the same information is also available through a TCP service running on the antenna computer. When connected, the service returns telescope pointing information in the same format as disk files. Currently there are three services with different data rate (100, 10, and 1 Hz). Data update rate is 1 Hz in all three services, meaning in case of the 100 Hz service 100 entries come out at once every one second.

The current telescope pointing information contains the following values:

Two different formats, packed binary format and integer array format, have been used in the past. The current format is the integer array format with some additional flags: