Correcting for pointing drifts with PICARD

If you're lucky enough to have a bright source in your map, you may be able to improve the image fidelity by correcting for pointing offsets between observations. The pipeline now has this capability and it is applied automatically for calibrators, but not for most science targets. I've added a new PICARD recipe, called SCUBA2_REGISTER_IMAGES, which allows you to shift your images to line up at a common reference position before mosaicking. Here's how to do it.

First step, upgrade to the latest Starlink version via rsync.

Step two, create a recipe parameter file (say, params.ini) and add the following lines to it:

[SCUBA2_REGISTER_IMAGES]
REGISTER_IMAGES = 1
REGISTER_RA = HH:MM:SS.S
REGISTER_DEC = +DD:MM:SS.S

where "HH:MM:SS.S" and "+DD:MM:SS.S" are the RA and Dec of the source you wish to use.

Then run

% picard -log sf -recpars params.ini SCUBA2_REGISTER_IMAGES myfiles*.sdf

and if all goes well, you will end up with a series of output files, one for every input file, which end in "_reg.sdf". These files can then be coadded using MOSAIC_JCMT_IMAGES.

The recipe works by fitting a 2-d profile to the brightest source within a 2x2 arcmin² box centred on the given reference position. The WCS is adjusted to place the reference position at the position reported by the fit.

However, note that this method will only work if the source you're using is present in every image, is compact and clearly resolved from other nearby sources and is the brightest source within the 2x2 arcmin² search box in every input image. It won't work for deep extragalactic maps, nor does it correct for any pointing drifts that occur within an observation. Correcting for those drifts is much more involved, but it is something we'll investigate over the coming months. It's been tested on HL Tau, OMC-1 and AFGL4029 in W5-East.

It's also possible to do the registering and coadding with a single call to MOSAIC_JCMT_IMAGES. In this case you'd want to add the following to a recipe parameter file (in addition to any other coadding parameters):

[MOSAIC_JCMT_IMAGES]
REGISTER_IMAGES = 1
REGISTER_RA = HH:MM:SS.S
REGISTER_DEC = +DD:MM:SS.S

Here's an example using some observations of HL Tau data taken in December 2009. The corrections were as much as 8.5 arcsec in RA (the Dec corrections were less, up to 3 arcsec). The improvements are modest at 850 um (of order 10% increase in peak flux and a few per cent reduction in beam size) but much more significant at 450 um. The peak flux after correcting for pointing drifts is more than 20% higher than before, and the FWHM has decreased by 15% (9.2" to 7.9"). Furthermore, the source elongation (ratio of major and minor axes) is reduced from 16% to less than 3%. Before and after images are shown below.

Left: HL Tau at 450 um, coadd of 9 separate observations from 20091213 and 20091214. Right: same data after registering each image before coadding. Both images are scaled between the same limits.