Description

Task sdcal implements a single-dish data calibration scheme similar to that of interferometry, i.e., generate calibration tables (caltables) and apply them. Available calibration modes (calmode) are 'ps', 'otfraster', and 'otf' for sky (reference) calibration; and 'tsys' for $T_{\rm sys}$ calibration. Caltables can be applied to the data with calmode='apply'. Each mode generates a caltable except for calmode='apply'. A combination of mode keywords is also supported, e.g., calmode='ps,tsys,apply' to calibrate sky and $T_{\rm sys}$ on-the-fly. Calibration is available even for fast-moving sources like the Moon (see the note relating to the 'otf' mode below).

The calibration mode must be set in accordance with the observing mode of the data. The modes are as follows: 

  • 'ps': position switching (including OTF) with explicit reference (OFF) spectra
  • 'otfraster': raster OTF scan without explicit OFFs
  • 'otf': non-raster OTF scan without explicit OFFs

Thus, if the data contains explicit reference spectra, 'ps' should be used. Otherwise, 'otfraster' or 'otf' should be used.
In 'otfraster' and 'otf' modes, specific edge regions of the observation pattern are automatically marked as reference (OFF) spectra.
These specific regions are:

  • in 'otfraster' mode: regions near the beginning and end of the raster scan lines.
  • in 'otf' mode: regions near the periphery of the observation pattern.

NOTE: Although the 'otfraster' mode is designed for OTF observations without explicit OFF spectra, it should work even if explicit reference spectra exist. In that case, the OFF spectra are ignored and spectra identified by an edge marker are used as the reference.

NOTE: Detection of periphery scans in 'otf' mode is available for "ephemeris" sources, e.g., the Sun and Moon. Often, antennas will track these ephemeris sources so that the target source is always at the map center. For such observations, a periphery search is done in the source frame of the ephemeris source, so the observing target maintains its position in the map. For calibration in the 'otf' mode, higher order pointing interpolation has been implemented to get the pointing direction for each spectral data more appropriately.

Apart from the way reference spectra are selected, the procedure to derive calibrated spectra is the same for all modes. Selected (or preset) OFF integrations contiguous in time are identified, averaged in each segment, and then interpolated to timestamps for ON integrations. Effectively, it means that OFF integrations are averaged by each OFF spectrum for 'ps' mode, and averaged by either ends of each raster row for 'otfraster' mode. Spectra are calibrated by:

$T_{\rm sys}  \frac{ ON - OFF } { OFF }$.

NOTE: If outfile is unset and calmode doesn't include "apply", a default names of calibration tables are generated based on the infile and a predefined suffix  ('_sky' for sky, '_tsys' for $T_{\rm sys}$).