The SCUBA-2 data acquisition (DA) system writes data files every 30 s, one file for each of the pixel subarrays. In addition, each observation can be preceded or followed by calibration frames such as darks or flatfield ramps. For example, observation 15 on 2009-12-14 of Uranus produced the following 16 files with the 450 m array that was operational at the time (s4a):
The fifth column gives the file sizes in bytes, and shows that the first and last files, which were dark observations, have nearly identical lengths. Similarly files 2 to 14 also have identical sizes as they each contain precisely 30 s worth of the on-source integration. File 15 is shorter, as the end of the integration lasted less than 30 s. As noted in the introduction, the first two files containing data from the actual scan of Uranus (numbers 2 and 3) are included with this Starlink release.
Kappa tasks such as fitslist and ndftrace can be used to see the FITS headers and dimensions of the data. In this example, the first and last files are dark observations, and all the other files are produced by a single continuous scan of Uranus. The main data arrays of each file are cubes, with the first two dimensions enumerating columns and rows, and the third time slices (sampled at 200 Hz).
Raw SCUBA-2 data are stored as integers (uncalibrated digitized units). The Smurf task flatfield can
be used to scale raw data to units proportional to picowatts (pW, as double precision floating points)
using the results of measurements of a flatfield calibration. The calibration can be stored internally in
the data file (see the HDS extension
MORE.SCUBA2.FLATCAL) or can be calculated dynamically from the
flatfield ramps bracketing each science observation. The DIMM will always use a flatfield ramp if
available. For example, use the command
to produce flat-fielded versions of the files that contain bolometer data taken during the scan across Uranus, with the dark observations (1 and 16 in this example) automatically filtered out. Other non-science files will also be ignored2. Note that the single quotes around the wildcards for the input and output files are necessary since Starlink routines expand them internally, rather than using the shell. However, it is not generally necessary to use flatfield before proceeding with map-making. Smurf will flatfield the data internally by default.
2Use the SEQ_TYPE FITS header to determine whether a file is part of the main data acquisition sequence or a support file