The bit encoding for the long (64-bit) IDs that are used as unique keys in the
SDSS catalog tables is described here.
PhotoObjID
The encoding of the photometric object long ID (objID in the photo
tables) is described in the table below. This scheme applies to the fieldID
and objID (objid bits are 0 for fieldID).
| Bits | Length
(# of bits) | Mask | Assignment | Description |
| 0 | 1 | 0x8000000000000000 | empty | unassigned |
| 1-4 | 4 | 0x7800000000000000 | skyVersion | resolved sky version (0=TARGET, 1=BEST, 2-15=RUNS) |
| 5-15 | 11 | 0x07FF000000000000 | rerun | number of pipeline rerun |
| 16-31 | 16 | 0x0000FFFF00000000 | run | run number |
| 32-34 | 3 | 0x00000000E0000000 | camcol | camera column (1-6) |
| 35 | 1 | 0x0000000010000000 | firstField | is this the first field in segment? |
| 36-47 | 12 | 0x000000000FFF0000 | field | field number within run |
| 48-63 | 16 | 0x000000000000FFFF | object | object number within field |
SpecObjID
The encoding of the long ID for spectroscopic objects is described below.
This applies to plateID, specObjID, specLineID, specLineIndexID, elRedshiftID
and xcRedshiftID.
| Bits | Length
(# of bits) | Mask | Assignment | Description |
| 0-15 | 16 | 0xFFFF000000000000 | plate | number of spectroscopic plate |
| 16-31 | 16 | 0x0000FFFF00000000 | MJD | MJD (date) plate was observed |
| 32-41 | 10 | 0x00000000FFC00000 | fiberID | number of spectroscopic fiber on plate (1-640) |
| 42-47 | 6 | 0x00000000003F0000 | type | type of targeted object |
| 48-63 | 16 | 0x000000000000FFFF | line/redshift/index | 0 for SpecObj, else number of spectroscopic line (SpecLine) or index (SpecLineIndex) or redshift (ELRedhsift or XCRedshift) |
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