4 Jiggle maps

 4.1 The bare minimum

The basic SCUBA jiggle map reduction consists of 4 steps, each of which is done with a separate task: reduce_switch, flatfield, extinction and rebin. For a complete reduction, you will also need some or all of the following tasks: change_flat, change_pointing, change_quality, despike, remsky, scuclip, scuover, and scunoise. For interactive despiking you will also need sclean or dspbol. For scan map data one additionally needs the tasks despike2, scan_rlb, and restore or remdbm.

4.1 The bare minimum

We now try our first map (scan 86), which is a single observation (3 integrations) of IRC+10216, one of our secondary calibrators, obtained on December 8 1997. Normally we would do the first three steps with the script scuquick, i.e. scuquick -tau=0.185 -sub=lon IN 86 out=i86, but here we do it step by step, so that you can see what is involved. First we execute reduce_switch, and flatfield:

  % reduce_switch
  IN - Name of input file containing demodulated data > 86
  SURF: Opening 19971208_dem_0086
  SURF: run 86 was a MAP observation of object IRC+10216
  SURF: file contains data for 2 switch(es) in 4 exposure(s) in 3
  in 1 measurement(s)
  OUT - Name of output file to contain reduced switch data /’o86’/ > i86
  % flatfield
  IN - Name of input file /@o86/ > i86
  SURF: run 86 was a MAP observation of IRC+10216
  OUT - Name of output file /’i86_flat’/ >
  SURF: applying flatfield from lwswphot.dat

Next we want to correct for the atmospheric attenuation of the signal. At present there are two measures of the atmosphere’s opacity - τCSO, and skydips. A large amount of effort has gone into understanding the relationship between these two over the past few semesters and the results are presented in Archibald et al. [11]. The relationships found for the wideband filters now used are:

τ850 = 4.02 × (τCSO 0.001) (1)
τ450 = 26.2 × (τCSO 0.014) (2)

A careful observer will examine the τCSO data for the night, note whether it appears stable or not (it commonly ‘spikes‘ on nights with poor atmospheric conditions) and compare the results with the skydips obtained (one can use the Surf routine skydip to reduce the data if you want or use ORAC-DR). The fits made to 450-dips are not always particularly good, there seems to be a problem with the minimization routine, and so the recommended method of calculating opacity at 450 is always to convert from the 850 or τCSO. Which you use is certainly open to debate; the skydips may well have the advantage of being made at the same azimuth as your source, but the τCSO is measured more regularly (every ten minutes or so). The preferred method used at the JAC is to fit a polynomial to the τCSO data, and convert the value from the fit to opacity at 450 and 850, these fits are regularly made and available at the JCMT’s calibration web page ORAC-DR will make use of these fits if one configures it to do so.

Before getting too involved in correcting for sky-opacity it is worth keeping in mind that at 850, and particularly for faint sources, the exact value is not necessary – other uncertainties are likely to dominate. We therefore adopt a value τ850 = 0.185 for this observation. The extinction correction is applied by running extinction on the flatfielded data.

  IN - Name of NDF containing demodulated data /@i86_flat/ > i86_flat
  SURF: run 86 was a MAP observation with JIGGLE sampling of object
  SURF: file contains data for 4 exposure(s) in 3 integration(s) in 1
  SURF: observation started at sidereal time 9 41 05 and ended at 9 50
  SURF: file contains data for the following sub-instrument(s)
   - SHORT with filter 450
   - LONG with filter 850
  SUB_INSTRUMENT - Name of sub-instrument to be extinction corrected
  /’SHORT’/ >
  FIRST_TAU - First zenith sky opacity measured /0/ > 0.185
  FIRST_LST - Sidereal time of first opacity measurement; hh mm ss.ss
  /’0.0’/ >
  SECOND_TAU - Second zenith sky opacity measured /0.185/ >
  SECOND_LST - Sidereal time of second opacity measurement; hh mm ss.ss
  /’0.0’/ >
  OUT - Name of output NDF /’i86_lon_ext’/ >

Note that extinction allows you to supply two values of opacity measured at different times if you want – in this case we have bypassed this option. This is also where the long wavelength array gets separated from the short wavelength one. When we want the short wavelength data we have to run extinction again and choose short as the SUB_INSTRUMENT. Our data are now extinction corrected, but still in instrumental units (Volts). In order to have a feeling for the true signal and noise level in our data, we therefore need to apply a scaling factor, FCF (Flux Calibration Factor), that converts the instrumental units to Jy/beam or Jy/arcsec2. Calibration is discussed in detail in Section  7. For just a quick look we ignore the intricacies of calibration and use nominal FCSs, which for the current filters (850 W & 450 W) is 220 and 310 Jy/beam/V for 850 and 450 μm, respectively. Since this map was taken with the old 850 μm filter, 850 N, we use a different calibration factor, FCF = 280 Jy/beam/V, which is more appropriate. To scale our extinction corrected data we use the Kappa command cmult.

  IN - Input NDF data structure /@i86_lon_ext/ >
  SCALAR - Multiplication constant > 280
  OUT - Output NDF > i86_lon_cal

Here we gave the calibrated data set the extension _cal. Now we are ready to convert our extinction corrected and calibrated data onto a spatial grid using the Surf task rebin.

  REBIN_METHOD - Rebinning method to be used /’LINEAR’/ >
  OUT_COORDS - Coordinate sys of output map; PL,AZ,NA,RB,RJ,RD or GA
  /’RJ’/ >
  SURF: output coordinates are FK5 J2000.0
  REF - Name of first data file to be rebinned /’i86_lon_cal’/ >
  SURF: run 86 was a MAP observation of IRC+10216 with JIGGLE sampling
  SURF: file contains data for 4 exposure(s) in 3 integrations(s) in 1
  WEIGHT - Weight to be assigned to input dataset /1/ >
  SHIFT_DX - X shift to be applied to input dataset on output map
  (arcsec) /0/ >
  SHIFT_DY - Y shift to be applied to input dataset on output map
  (arcsec) /0/ >
  IN - Name of next input file to be rebinned /!/ >
  SURF Input data: (name, weight, dx, dy)
     -- 1: i86_lon_cal (1, 0, 0)
  LONG_OUT - Longitude of output map centre in hh (or dd) mm ss.ss
  /’+09 47 57.38’/ >
  LAT_OUT - Latitude of output map centre in dd mm ss.ss format /’+ 13
  16 43.7’/ >
  OUT_OBJECT - Object name for output map /’IRC+10216’/ >
  PIXSIZE_OUT - Size of pixels in output map (arcsec) /3/ >
  SURF: Initializing LINEAR weighting functions
  SIZE - Number of pixels in output map (NX,NY) /[70,65]/ >
  OUT - Name of file to contain rebinned map /’i86_lon_reb’/ >
  WTFN_REGRID: Beginning regrid process
  WTFN_REGRID: Entering second rebin phase (T = 0.03516951 seconds)
  WTFN_REGRID: Entering third rebin phase (T = 0.1326885 seconds)
  WTFN_REGRID: Regrid complete. Elapsed time = 0.1400405 seconds

The resulting map can be viewed with Kappa’s display

  % display axes clear i86_lon_reb

or by using Gaia. The resulting map does not look particularly nice, because we have not yet blanked out any noisy bolometers, done sky noise reduction or despiking.