Chapter 4
POL-2 Data Reduction – Running pol2map

The previous chapter, Chapter 3, described how pol2map produces $I$, $Q$, and $U$ maps from raw POL-2 data. It showed that this reduction process—-which uses pol2map—comprises three steps.

As with the other Python scripts in Smurf, it is possible to get more information about the available parameters by running either:

or the

command.

4.1 How to use pol2map

Before running pol2map directly, it is necessary to ensure that the Starlink environment has been initialised and the Smurf package started (see Section 1.3.2 and Section 1.3.3).

This chapter describes how to run pol2map firstly to produce an initial $I$ map; and then again to produce the final $I$, $Q$, and $U$ maps and a vector catalogue, as described in Section 4.2.

To run pol2map, values should normally be supplied for the following command-line parameters¹, in order to produce the initial intensity image. Note that if a parameter description ends with a value in square brackets, it is the default value that will be used for the parameter if no value is supplied on the command line.

Some additional command-line parameters are required when pol2map is used for the second time—as discussed in Section 4.3—to produce the final $I$, $Q$, $U$ maps and vector catalogue².

The pol2map command provides many other parameters that may be used to modify its behaviour in various ways. To see a full list, type the following.

4.2 pol2map – producing the initial I map

As discussed in Chapter 3, pol2map must first be run on the raw data to produce an initial $I$ map. In this first step:

where the file myfiles.lis contains a list of the raw data files to be included in the map, and could (for instance) look like this.

This uses all available data from all four 850 µm sub-arrays, for Observation 43 taken on 2016 January 25³. In addition, the data used in this example also come from Observations 56 and 59 taken on 2016 January 11 (UT).

Note that qout and uout are set to null values, as no $Q$ or $U$ maps are required to be produced during this initial Step 1 reduction stage.

The following shows the output from running this initial pol2map command.

The files and folders produced by this reduction process are described below.

The output $I$ map, iauto.sdf, can be opened and viewed with Gaia.

Figure 4.1: The $I$ map, iauto.sdf, as viewed with Gaia.

The maps folder contains the individual $I$ maps from each separate observation:

and the qudata folder contains these files.

4.3 pol2map – producing the I, V, U maps and catalogue

As discussed in Chapter 3, the $I$ map output from the initial run of pol2map is used to derive the final $I$, $Q$, and $U$ maps. If requested, a vector catalogue is also produced.

The second and third steps of the POL-2 data reduction process can be run via a single command.

The following shows the output from running this second pol2map command. First, pol2map produces new $I$ maps for each map, correcting the position using the correction stored in the old $I$ map⁴, and then co-adds all the observations.

As pol2map continues, the $Q$ and $U$ maps are produced, again with pointing corrections. This is followed by the creation of the output vector catalogue.

The output of this final run of pol2map is as follows.

Figure 4.2: Left: $I$ map, iauto, as produced by the automask on the first pass of pol2map. Right: Final $I$ map, iext, as viewed with Gaia. The flatter background is due to the increase in pca.pcathresh.

Figure 4.3: Left: $Q$ map, qext.sdf. Right: $U$ map uext.sdf, as viewed with Gaia.

The maps folder now contains individual $Q$ and $U$ maps, alongside the existing $I$ maps listed below.

4.4 Output vectors from pol2map

The output vector catalogue contains a range of values derived by pol2map for each pixel contained within the $I$ map. Intensity values and errors in the catalogue are expressed in units of mJy/beam. If desired, it is possible to switch the catalogue to units of pW by using Jy=no on the pol2map command line. The columns are listed below.

4.5 POL-2 FCFs

Inserting POL-2 in front of SCUBA-2 reduces the throughput to SCUBA-2. POL-2 is not a perfect polarimeter. Its wire grid absorbs and scatters incoming signal so the modulation amplitude is lower than for a perfect polarimeter. In addition cross polarisation and depolarisation decreases the modulation amplitude without decreasing the power in the transmitted signal. The first type of inefficiencies can be measured by comparing normal SCUBA-2 maps with and without the polarimeter inserted. Such observations have been done on Uranus, Mars and Jupiter. The second type of losses can be measured with a source of known polarisation.

To convert POL-2 data to astronomical units such as mJy/beam a Flux Conversion Factor, FCF, must be applied to the data. For POL-2, the FCFs are quoted in terms of the non-polarimetric SCUBA-2 FCFs.

POL-2 FCFs are now applied automatically within pol2map when catalogue columns in mJy/beam are required by the user, i.e. if the pol2map parameter JY is set to TRUE (the default option). The current version of pol2map uses the new, revised POL-2 FCFs for 850 µm and 450 µm and takes account of cases where the supplied raw data span one of the dates at which the FCF changed (20180630 and 20161101).

4.6 Changing pixel size in pol2map

Inevitably, as with unpolarised SCUBA-2 data reduction, users will sometimes find it necessary to fine-tune the pol2map reduction process for specific situations.

The bin size within the final vector catalogue is controlled by the BINSIZE parameter in the Smurf pol2map command.

Changing the catalogue bin size in this way does not change the pixel size of the maps created pol2map. Instead, the maps are binned up to the requested bin size before the catalogue is created. There is another parameter, called PIXSIZE, which controls the map pixel size, but it is usually advisable to leave this at its default value. This is because the map pixel size can affect the behaviour of the iterative algorithm used to create maps.