Checking the noise properties of SCUBA-2 science data

Ever wondered if there was an easy way to assess the noise properties of the data going in to your SCUBA-2 maps? Well now there is. A new recipe called ASSESS_DATA_NOISE has been added to the SCUBA-2 pipeline and it performs standard quality-assurance analysis (as done by the quick-look pipeline at the telescope) to give you an idea of the noise properties of the sciences data.

Running ASSESS_DATA_NOISE

To run it, first put all the files you wish to process into a text file (called myfiles.lis in the example below). Create a recipe parameter file if necessary (myparams.ini below). Then initialize the pipeline as usual (not necessary if done previously):

% oracdr_scuba2_XXX -cwd

where XXX is the wavelength of choice. Finally, run the pipeline, adding the name of the recipe at the end of the command line:

% oracdr -loop file -files myfiles.lis -recpars myparams.ini \
  -log sf ASSESS_DATA_NOISE

The recipe displays its results in a KAPPA window, showing the focal-plane layout with the noise for each bolometer in the top left corner, a histogram of the noise in the lower left, and the focal-plane image of the noise equivalent power (NEP) in the lower right. (If you want to turn off this display, add -nodisplay to the ORAC-DR command line above.)

By default the results are calculated for each subscan (i.e., each 30-second file) separately. As described below, options exist to just use the first subscan or to calculate the noise properties for the entire time stream.

The focal-plane mosaics for each subscan are stacked to create a 3-d cube of noise as a function of time (the third axis is MJD). The corresponding NEP images are written into this file under the .more.smurf.nep extension. This file has the suffix _scinoistack.

The results are written out to a log file called log.scinoiseXXX (where XXX is the wavelength as above), which can be loaded into TOPCAT to display the number of bolometers, noise or weighted NEP as a function of time or subscan number for each subarray.

The recipe also writes out a file called log.bolonoise, which contains more results (with the results for each subarray on a separate line), plus index.noise and index.nep. From these files it will be possible to determine which files failed the QA tests, and thus can be excluded from the input list to the map maker.

Of course, the down side to removing data is that it will not be possible to achieve the intended noise level, and may reduce sensitivity to extended structure (as the time series may be broken into smaller chunks). However, it should be easy to determine whether or not using some fraction of the best data gets as close as could be expected.

Customizing

Like most recipes, a number of recipe parameters are available:

• NOISE_CONFIG - the config file you wish to use when calculating the noise. It is recommended that you use the same as the one used when making maps. If not given, the standard noise config file is used.
• NOISE_CALC - may be "quick", "each" or "full" which will use the first thirty-seconds of data (for a quick check), or for every thirty second data file, or use the entire time stream to calculate the overall noise properties. The default is "each".
• FREQRANGE - frequency range over which to calculate the noise. Default is 2-10 Hz.
• FREQLO - lower frequency at which to calculate the background. Default is 0.5 Hz.

See the SMURF documentation for the FREQRANGE and FREQLO before adjusting them (SUN/258).

This is just the first release and its usefulness and output data relies on user testing and input. Please try it out and let me know.