# Guessing ON and OFF positions

Depending on the observing mode, the telescope is expected to observe 1 or 2 positions on the sky:

- Position switch: 2 positions (ON + OFF)
- Wobbler switch: 1 or 2 positions
- Frequency switch: 1 position

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.

Figure: LONGOFF and LATOFF vs MJD for iram30m-fts-20150429s199-imb.fits (whole scan). Observing mode is tracked wobbler switch, using horizontalTrue offset system. During the scan, elevation changes from 82.3 to 83.6 degrees.

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:

- LATOFF offsets are all null: this means that all the dumps were sharing the same elevation as the reference (source) position. The telescope is tracking the source, so that the elevation changes (this can be seen in the CELEVATIO column), but not the offset in this direction.
- LONGOFF offsets are drifting, slowly decreasing for both positions.

Because of this drift, MRTCAL uses a centi-arcsecond tolerance when the equivalence classes are computed. If we reuse the example in Fig. , this tolerance is still not enough and results in 6 different positions:

      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