The current telescope pointing information is recorded in integer array format. The size of each entry is 52 bytes. Each entry contains 13 elements. They are all of size 4 bytes and in network byte order. Thus they can simply be read as an array of integers on big-endian systems (e.g., 68k and SPARC). On little-endian systems (e.g., x86 and VAX), their byte order must be swapped. In C/C++, that can be done in a portable way by calling ntohl() on each element. An example code for the network version is available.
| Offset (bytes) |
Size (bytes) |
Type | Description |
|---|---|---|---|
| +0 | 4 | unsigned integer | UTC modified Julian day at midnight |
| +4 | 4 | unsigned integer | #ticks from midnight (0.01 s) |
| +8 | 4 | booleans | Flags (see below) |
| +12 | 4 | unsigned integer | Local apparent sidereal time (0.01 s) |
| +16 | 4 | unsigned integer | Requested apparent right ascension (0.01 s) |
| +20 | 4 | signed integer | Requested apparent declination (0.1 ") |
| +24 | 4 | signed integer | Parallactic angle (0.1 ") |
| +28 | 4 | signed integer | X offset (0.1 ") |
| +32 | 4 | signed integer | Y offset (0.1 ") |
| +36 | 4 | signed integer | Requested azimuth (0.1 ") |
| +40 | 4 | signed integer | Requested elevation (0.1 ") |
| +44 | 4 | signed integer | Error in azimuth (0.1 ") |
| +48 | 4 | signed integer | Error in elevation (0.1 ") |
Right ascensions, declinations and parallactic angles are not valid if the Celestial flag (see below) is not set.
Static equatorial and Galactic offsets like RAO /FIELD and GLO /MAPPING are not included in the X-offset and Y-offset but in the requested apparent right ascension and declination. Static altazimuthal offsets like AZO and ZAO /MAPPING are neither in the X-offset and Y-offset nor in the requested apparent right ascension and declination but only in the requested azimuth and elevation. These static offsets are available through a separate remote procedure call (RPC) mechanism.
The 32-bit flag field is defined as follows:
| Offset (bytes) |
b7 | b6 | b5 | b4 | b3 | b2 | b1 | b0 |
|---|---|---|---|---|---|---|---|---|
| +8 | TTL Input 8 (RX2-16) |
TTL Input 7 (RX2-15) |
TTL Input 6 (RX2-14) |
TTL Input 5 (RX2-13) |
TTL Input 4 (RX2-12) |
TTL Input 3 (RX2-11) |
TTL Input 2 (RX2-10) |
TTL Input 1 (RX2-9) |
| +9 | TTL Output 8 (RX2-8) |
TTL Output 7 (RX2-7) |
Output 6 | Output 5 | Output 4 | Output 3 | Output 2 | Output 1 |
| +10 | Equatorial Offset | OFF-beam | ON-beam | Chopping | ||||
| +11 | Tracking | Acquired | Scanning | Transited | Celestial |
The Output 1, Output 2, Output 3, Output 4, Output 5 and Output 6 flags do not have corresponding physical output lines yet. All the intput and output flags are accessible from UIP using FLSIGNAL and FLWAIT.
The OFF-beam flag is set when the chopping azimuth offset (AZO /CHOPPING) is positive. The ON-beam flag is set when the chopping azimuth offset is negative.
If the Equatorial Offset flag is set, the X-offset and Y-offset are in the apparent equatorial coordinates of date, otherwise they are in the horizontal coordinates of date. In special observing modes where both the equatorial and the altazimuthal scans are in effect, e.g., raster-scanning in the equatorial coordinates while Lissajous-sweeping in the altazimuthal coordinates, the X-offset and Y-offset hold the altazimuthal offsets, and the equatorial offsets are added to the requested right ascension and declination.
The Tracking flag in the telescope pointing information is different from the TTL TRACKING (/IDLING) signal. For historical reasons, the TTL TRACKING (/IDLING) signal is high when the Tracking flag is true but the Scanning flag is false.