Checking SCUBA-2 calibrator data

A new PICARD recipe has been added called SCUBA2_CHECK_CAL which can be used to assess the quality of the calibration by calculating flux conversion factors (FCFs) and estimating the beam size to compare with standard values.

It can be run on any processed observations of standard sources (except, obviously, focus data). The input files must be uncalibrated, either the output from running makemap by hand or from the pipeline and then processed with the PICARD recipe UNCALIBRATE_SCUBA2_DATA.

Run it as any other PICARD recipe:

% picard -log sf -recpars myparams.ini SCUBA2_CHECK_CAL calfiles*.sdf

The recipe trims the image, optionally removing a background before it fits the source to estimate the beam.

The next step is calculating the FCFs which are reported to the screen. The map is then calibrated using either a standard FCF or one of those just calculated (controlled by a recipe parameter). The noise in the map is calculated from this calibrated image.

The recipe writes a log file called log.checkcal which contains various parameters. The values of interest include the total flux and uncertainty derived from the uncalibrated (input) map, the noise in the calibrated version of that map, the FCFs and associated uncertainties and the beam FWHM. In the future there will also be an estimate of the error beam. The log file can be read into Topcat so values can be plotted as a function of time, elevation or any other parameter (if there are enough of them).

The total flux is measured using aperture photometry with an aperture radius of 30 arcsec. The aperture size may be adjusted using a recipe parameter, but be aware that comparison with the standard ARCSEC FCF will require correcting for the different aperture. See the SCUBA-2 calibration paper for further details.

By default the recipe can only estimate FCFs from known calibrators. However, it also be used to estimate the FCF from non-standard sources if the flux is known to the user. Recipe parameters exist to allow this to be specified (on a per-source basis if desired). Note that the sources should still be unresolved, point sources.

The FCF calculations can be compared with the standard values (given in the calibration paper) to determine whether or not there may be particular issues with the calibration on a given night. The BEAM FCF is more sensitive to focus than the ARCSEC value, so if the former is out of spec, while the latter is not, it could indicate that the telescope was not as well focussed as it could have been. If both are far out of spec (indicated by red text in the recipe output) then it may suggest that a non-standard FCF is in order. It is also important to look at the trend in the FCFs over the night, and not just a single value.

The main recipe parameters of interest are:

• APERTURE_RADIUS - radius of photometry aperture in arcsec. Default is 30.
• USEFCF - a flag to denote whether the derived FCF should be used to calibrate the data. Default is 0 (use the standard FCF).
• FLUX_850 - total flux of a point source at 850 um. The corresponding 450 um parameter is FLUX_450. Fluxes may be specified for multiple sources by adding the source name (upper case with spaces removed) to the parameter - e.g., FLUX_850.MYSOURCE1, FLUX_850.MYSOURCE2 etc.
• REMOVE_BACKGROUND - a flag to denote whether or not a background should be removed. Default is 0 (do not remove a background). If true, then there are other parameters available to control the background fitting and removal process.

The full list of recipe parameters is described in SUN/265 and will be available in the upcoming Starlink release.