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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:

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.
Image imbfits-longoff-latoff

On the other hand, for horizontalTrue offset system, the offsets drift. In the horizontalTrue offset system, LONGOFF are actually offsets along azimuth, corrected by $cos(elevation)$, and LATOFF are offsets along elevation. If we look at the example in Fig. [*], we can see that:

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 up previous contents
Next: Interpolation of calibration products Up: Positions Previous: In practice: Effect on   Contents
Gildas manager 2023-06-01