How do I find the noise, NEFD and observation properties of a SCUBA-2 observation?

We have a PICARD  recipe named SCUBA2_MAPSTATS which will calculate various properties of the observation, its noise and its average NEFD given a single reduced SCUBA-2 observation.

You can run this recipe as follows (in a terminal, after setting up the Starlink software):

picard -log sf SCUBA2_MAPSTATS scuba2_reduced_file.sdf

This will produce an output file named ‘log.mapstats’ in the directory specified by the environmental variable¬† ‘ORAC_DATA_OUT’ (if set), or in the current directory if that is not set.

This file currently contains entries for each of the following columns:

# (YYYYMMDD.frac) (YYYY-MM-DDThh:mm:ss) () () () (um) (deg) () () () () (s) (s) () () () () (") (") () () ()
# UT HST Obs Source Mode Filter El Airmass Trans Tau225 Tau t_elapsed t_exp rms rms_units nefd nefd_units mapsize pixscale project recipe filename

with some of the units indicted in the top line (ones that are always the same), and others specified as a column (these can be different depending on what was applied to the dataset).

These tell you information about the observation — the time it was started (UT and HST), the observation number (column labeled Obs, i.e. was it the 1st, 2nd,3rd etc SCUBA-2 scan of the night), the sourcename, the Mode (Daisy or Pong), the filter (450um or 850um), the Elevation, Airmass, Transmission, Tau225 and Tau at the start of the observation (taken from the fits headers of the file). The total time spent on source is given in t_elapsed, and the average exposure time for a pixel is given by t_exp.

The RMS is calculated from the variance array of the observation, and its units are explicitly given. The NEFD is calculated from the data in the NDF extension ‘MORE.smurf.nefd’, and its units are explictly given.

The RMS, exposure time and nefd are calculated over the central square region of the map, defined by the MAP_WDTH and MAP_HGTH headers.

If multiple files are run through SCUBA2_MAPSTATS, either in a single call of PICARD or by repeatedly running PICARD in the same terminal on different files, the results will be appended to the existing log.mapstats file. The final columns — project, recipe and filename — are given to ensure it is clear to users which line of the logfile corresponds to which input file.

SCUBA2_MAPSTATS is only designed to work on reductions of a single observations. On coadded observations it could produce misleading results, or even fail completely to work.

If your data is calibrated, this will assume that the units are correctly set in the file, and that the noise and NEFD extensions have also been correctly updated. This can be done by using the PICARD recipe CALIBRATE_SCUBA2_DATA.

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