Beam switching and continuum drifts

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Beam switching and continuum drifts

Beam switching is used in the context of continuum drifts. The various positions involved are described in different parts of the FITS and CLASS headers.

In the FITS header, the relevant information from e.g. iram30m-nbc-20220817s29-imb.fits looks like:

      IMBF-scan:
        Header:
          OBJECT   (C)  = Mars                   / Source name
          CTYPE1   (C)  = RA-SFL                 / Basis system (longitude) --
          CTYPE2   (C)  = DEC-SFL                / Basis system (latitude) -- XLAT-SFL
          EQUINOX  (R8) =   2022.62422998199     / [Julian yrs] Equinox
          LONGOBJ  (R8) =   56.1815486637719     / [deg] Source longitude in basis frame
          LATOBJ   (R8) =   18.2461304985798     / [deg] Source latitude in basis frame
          SWTCHMOD (C)  = beamSwitching          / Switch mode
        Table:
          SYSOFF   (C)  = Nasmyth        projection  / WARNING! Added a dummy 'projection' value
          XOFFSET  (R4) = -1.9150140E-04   0.000000  / WARNING! Added a dummy 'projection' value
          YOFFSET  (R4) =  2.6664753E-05   0.000000  / WARNING! Added a dummy 'projection' value

      IMBF-antenna (subscan #1):
        Header:
          SUBSTYPE (C)  = onTheFly               / Subscan type
          SYSTEMOF (C)  = horizontalTrue         / System for offsets
        Table:
          LONGOFF  (R8) = -2.908680211E-04 ... 2.908678739E-04 / [rad] long. offset from source in user native f
          LATOFF   (R8) =   0.00000000     ...  0.00000000     / [rad] lat. offset from source in user native fr

Figure: Schematic representation of a continuum drift (horizontal in this example). The red components show the scan HDU information. The black components show the antenna HDU information. Scan HDU provides the projection center, projection kind, and reference position used to fill the position section in CLASS. Antenna HDU provides the drift offsets used to fill the continuum drift section and the RX array in CLASS.

The scan HDU is used to build the position section in the CLASS header. In this example, the reference position is taken from LONGOBJ and LATOBJ (in RA-Dec equinox 2022.6 coordinates in this example). According to the scan table, the position offset are (0,0) here (but they could be non-zero) with the radio projection (SYSOFF is projection).

The antenna HDU is used to build the drift offsets from the position described above. The table provides the whole drift offsets. It is stored in the CLASS RX array as it is not regularly spaced in the general case. Its associated system of coordinates is saved in the drift section (ctype, horizontal system in this example3.2). Its unit (angles here) is described in the general section (xunit parameter).