Analysis of the SCUBA-2 skydips and heater-tracking data from the S2SRO data has allowed calculation of the opacity factors for the SCUBA-2 450 m and 850 m filters to be determined. Full details of the analysis and on-sky calibration methods of SCUBA-2 can be found in Dempsey et al. (2010) .
Archibald et al. (2002)  describes how the Caltech Submillimeter Observatory (CSO) 225 GHz opacity, , relates to SCUBA opacity terms in each band, and . It was assumed for commissioning and S2SRO that the new SCUBA-2 filters are sufficiently similar to the wide-band SCUBA filters that these terms could be used for extinction correction. In the form , the original SCUBA corrections were:
The JCMT water-vapour radiometer (WVM) is now calibrated to provide a higher-frequency opacity value which has been scaled to . The WVM (not the CSO 225 GHz tipper) data were used for this analysis.
The new filter opacities as determined from skydip data are as follows:
The SCUBA-2 filters are different from the SCUBA filters, with the 450 m filter, in particular, significantly narrower than its SCUBA counterpart. The SCUBA-2 filter characteristics are described in detail on the JCMT website6.
The extinction correction parameters that scale from
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
$SMURF_DIR/smurf_extinction.def. We have also added a slight calibration tweak to
WVM-derived values to correct them to the CSO scale. It is worth noting that if an individual
science map and corresponding calibrator observation has already been reduced with the
old factors (and your source and calibrator are at about the same airmass and if the tau
did not change appreciably), any errors in extinction correction should cancel out in the
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. Picard is a post-processing and data combination tool that uses the same
infrastructure as ORAC-DR, but is designed to be used after the initial reduction with the DIMM is
complete. Details of the Picard recipes and how to use them can be found on the ORAC-DR web
A map reduced by the mapmaker 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 (pW) can be multiplied by this FCF to obtain a calibrated map, and 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:
SCUBA2_FCFNEFDtakes the reduced map, crops it, and runs background removal. Surface fitting parameters are changeable in the Picard parameter file.
FLUX_450.MYSRC = 0.050and
FLUX_850.MYSRC = 0.005(where the values are in Jy), for example.
An example of a Picard parameter file (used for reduction of the 850 m calibrators) is shown here: