Keywords: mrtcal-prog-manual

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# 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

Next: Interpolation of calibration products Up: Positions Previous: In practice: Effect on Contents