The telescope pointing information recorded during Bolocam Phase 1 engineering run is in packed binary format. Each entry consists of 30 bytes: 2-byte unsigned integer is followed by 28-byte packed binary structure. The tables below explain the format in detail. Note that the byte ordering is big-endian, and MSBs (most significant bits) and LSBs (least significant bits) are shown as b7 and b0, respectively:
| Offset (bytes) |
+0 | +1 | +2 | +3 | ||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| b7 | b6 | b5 | b4 | b3 | b2 | b1 | b0 | b7 | b6 | b5 | b4 | b3 | b2 | b1 | b0 | b7 | b6 | b5 | b4 | b3 | b2 | b1 | b0 | b7 | b6 | b5 | b4 | b3 | b2 | b1 | b0 | |
| 0 | × | × | × | #ticks (10 msec) | #days since Jan. 0, 2000 (= MJD - 51543) | |||||||||||||||||||||||||||
| 4 | #minutes from midnight | Flags | × | × | Local apparent sidereal time (10 msec) | |||||||||||||||||||||||||||
| 8 | Apparent right ascension (10 msec) | |||||||||||||||||||||||||||||||
| 12 | Apparent declination (100 mas) | |||||||||||||||||||||||||||||||
| 16 | Parallactic angle (100 mas) | Azimuth (100 mas) | ||||||||||||||||||||||||||||||
| 20 | Elevation (100 mas) | |||||||||||||||||||||||||||||||
| 24 | Error in azimuth (100 mas) | |||||||||||||||||||||||||||||||
| 28 | Error in elevation (100 mas) | |||||||||||||||||||||||||||||||
It is proven to be really tricky to decode this format. Here is a reference implementation (plog2_read.c) that retrieves and decodes entries in a relatively portable way (it still assumes that integers are 4-byte wide and two's complement format is used for negative values). Note that LSTs and RAs may have negative values, i.e., recoded value range may be [-12:00:00.00, +12:00:00.00), instead of [00:00:00.00, 24:00:00.00). If a negative LST or RA is found, 24 hours should be added to obtain a true value.
| Offset (bits) |
Width (bits) |
Type | Description | |
|---|---|---|---|---|
| From | To | |||
| 0 | 0 | 1 | Not used | |
| 1 | 1 | 1 | Not used | |
| 2 | 2 | 1 | Not used | |
| 3 | 15 | 13 | unsigned integer | #ticks in the minute (10 msec) |
| 16 | 29 | 14 | unsigned integer | #days since Jan. 0, 2000 (= MJD - 51543) |
| 30 | 40 | 11 | unsigned integer | #minutes from midnight |
| 41 | 41 | 1 | boolean | TRACKING |
| 42 | 42 | 1 | boolean | ACQUIRED |
| 43 | 43 | 1 | boolean | SCANNING |
| 44 | 44 | 1 | boolean | TRANSITED |
| 45 | 45 | 1 | boolean | CELESTIAL |
| 46 | 46 | 1 | Not used | |
| 47 | 47 | 1 | Not used | |
| 48 | 71 | 24 | signed integer | Local apparent sidereal time (10 msec) |
| 72 | 95 | 24 | signed integer | Requested apparent right ascension (10 msec) |
| 96 | 118 | 23 | signed integer | Requested apparent declination (100 mas) |
| 119 | 142 | 24 | signed integer | Parallactic angle (100 mas) |
| 143 | 167 | 25 | signed integer | Requested azimuth (100 mas) |
| 168 | 190 | 23 | signed integer | Requested elevation (100 mas) |
| 191 | 215 | 25 | signed integer | Error in azimuth (100 mas) |
| 216 | 239 | 24 | signed integer | Error in elevation (100 mas) |