Depending on the observing mode, the telescope is expected to observe 1 or 2 positions on the sky:
Those positions are not described directly in the IMB-FITS. Instead,
the positions of each dump is available in the LONGOFF
and LATOFF columns of the antenna slow table. By using
equivalence classes of the X and Y offset pairs, it should
straightforward to group the dumps by position.
For projection offset system, i.e. when offsets are described
along a sky system of coordinates (equatorial or galactic), this
approach is correct: the offsets are all strictly identical for a
given position and for both coordinates. All digits are the same,
there are no numerical round-off errors.
![]() |
On the other hand, for horizontalTrue offset system, the offsets
drift. In the horizontalTrue offset system, LONGOFF
are actually offsets along azimuth, corrected by
, and
LATOFF are offsets along elevation. If we look at the
example in Fig.
, we can see that:
E-SUBSCAN>LIST>BUILD, Scan has 6 positions (expected 1 or 2): E-SUBSCAN>LIST>BUILD, Position #1: -119.82903552 0.00000000 E-SUBSCAN>LIST>BUILD, Position #2: 119.82643276 0.00000000 E-SUBSCAN>LIST>BUILD, Position #3: -119.81881083 0.00000000 E-SUBSCAN>LIST>BUILD, Position #4: 119.81592996 0.00000000 E-SUBSCAN>LIST>BUILD, Position #5: -119.80748135 0.00000000 E-SUBSCAN>LIST>BUILD, Position #6: 119.80419607 0.00000000