Appendix B
SCUBA-2 data calibration

 B.1 Flux conversion factors (FCF)
 B.2 Extinction correction

B.1 Flux conversion factors (FCF)

Primary and secondary calibrator observations have been reduced using the specifically designed dimmconfig_bright_compact.lis. The maps produced from this are then analysed using tailor-made Picard recipes. For instructions on applying the FCFs to your map see Section 8.1.

A map reduced by the map-maker has units of pW. To calibrate the data into units of janskys (Jy), a set of bright, point-source objects with well-known flux densities are observed regularly to provide a flux conversion factor (FCF). The data (in pW) can be multiplied by this FCF to obtain a calibrated map. The FCF can also be used to assess the relative performance of the instrument from night to night. The noise equivalent flux density (NEFD) is a measure of the instrument sensitivity, and while not discussed here, is also produced by the Picard recipe shown here. For calibration of primary and secondary calibrators, the FCFs and NEFDs have been calculated as follows:

The Picard recipe SCUBA2_FCFNEFD takes the reduced map, crops it, and runs background removal. Surface-fitting parameters are changeable in the Picard parameter file.
It then runs the Kappa beamfit task on the specified point source. The beamfit task will estimate the peak (uncalibrated) flux density and the FWHM. The integrated flux density within a given aperture (30-arcsec radius default) is calculated using Photom autophotom. Flux densities for calibrators such as Uranus, Mars, CRL 618, CRL 2688 and HL Tau are already known to Picard. To derive an FCF for other sources of known flux densities, the fluxes can be added to the parameter file with the source name (in upper case, spaces removed): FLUX_450.MYSRC = 0.050 and FLUX_850.MYSRC = 0.005 (where the values are in Jy), for example.
Three different FCF values are calculated, two of which are described below.
The arcsecond FCF
FCFarcsec = Stot Pint ×Apix (B.1)

where Stot is the total flux density of the calibrator, Pint is the integrated sum of the source in the map (in pW) and Apix is the pixel area in arcsec2, producing an FCF in Jy/arcsec2/pW.

The beam FCF
FCFbeam = Speak Ppeak (B.2)

producing an FCF in units of Jy/beam/pW.

The measured peak signal here is derived from the Gaussian fit of beamfit. The peak value is susceptible to pointing and focus errors, and we have found this number to be somewhat unreliable, particularly at 450μm.

B.2 Extinction correction

Analysis of the SCUBA-2 secondary calibrators has allowed calculation of the transmission relationships for the SCUBA-2 450 μm and 850 μm pass-bands to be determined. Full details of the analysis and on-sky calibration methods of SCUBA-2 can be found in Dempsey et al. (2013) [8][9].

Archibald et al. (2002) [1] describes how the Caltech Submillimeter Observatory (CSO) 225 GHz opacity, τ225, relates to SCUBA opacity terms in each band, τ450 and τ850. The JCMT water-vapour radiometer (WVM) uses the 183 GHz water line to calculate the precipitable water vapour (PWV) along the line-of-sight of the telescope. This PWV is then input into an atmospheric model to calculate the zenith opacity at 225 GHz (τ225). This allows ease of comparison with the adjacent CSO 225 GHz tipping radiometer. The opacities have been as:

τ450 = 26.0 × (τ225 0.012); (B.3)


τ850 = 4.6 × (τ225 0.0043). (B.4)

The SCUBA-2 filter characteristics are described in detail on the JCMT website1.

The extinction correction parameters that scale from τ225 to the relevant filter have been added to the map-maker code. You can override these values by setting ext.taurelation.filtname in your map-maker config files to the two coefficients ‘(a,b)’ that you want to use (where filtname is the name of the filter). The defaults are listed in $SMURF_DIR/smurf_extinction.def.