This script reduces a “quadrant jitter” photometry observation with UKIRT imaging data. It takes an imaging observation comprising one or more series of four object frames where the target is approximately centred in each quadrant; and a dark frame to make automatically a calibrated, untrimmed mosaic in the reference frame of the moving target.

It performs bad-pixel masking, null debiassing, dark subtraction, flat-field creation and division, feature detection and matching between object frames, and resampling. See the “Notes” for further information.

Registration is adjusted to track the motion of the moving target using ephemeris data stored in file target_ephem.dat. See “Ephemeris-file Format” for details of this file’s format.

This recipe works well for extended moving sources (comets), whose extent does not exceed 45 arcseconds for UFTI or 10 arcseconds for IRCAM, in moderately crowded fields. Sources may include those with a comparatively bright core embedded in faint extended emission. The object need not be isolated, as the recipe masks objects within the other quadrants, and hence does not introduce significant artifacts into the flat field. This recipe should not be used for frames where the telescope guided on the moving object. In that case reduction should be performed by QUADRANT_JITTER_TELE, which registers using the telescope offsets alone.

• A World Co-ordinate System (WCS) using the AIPS convention is created in the headers should no WCS already exist.

• For IRCAM, old headers are reordered and structured with headings before groups of related keywords. The comments have units added or appear in a standard format. Four deprecated headers are removed. FITS-violating headers are corrected. Spurious instrument names are changed to IRCAM3.

• The bad pixel mask applied is $ORAC_DATA_CAL/bpm.

• Each dark-subtracted frame has thresholds applied beyond which pixels are flagged as bad. The lower limit is 5 standard deviations below the mode, but constrained to the range $-$100 to 1. The upper limit is 1000 above the saturation limit for the detector in the mode used.

• The flat field is created iteratively. First the quadrant containing the object is masked in each object frame. Second an approximate flat field is created by combining the normalised and masked object frames using the clipped median at each pixel. This flat field is applied to the object frames. Sources within the flat-fielded frames are detected, and masked in the dark-subtracted frames. The second stage is repeated but applied to the masked frames to create the final flat field.

• Registration is performed using common point sources in the overlap regions. If the recipe cannot identify sufficient common objects, it matches the centroid of the central source. If this fails, the script resorts to using the telescope offsets transformed to pixels. Once the offsets are determined, they are adjusted for the motion of the target, so that the final mosaic registers the target, not the background stars.

• The ephemeris file is specified by environment variable ORAC_EPHEMERIS, defaulting to $ORAC_DATA_OUT/target_ephem.dat.

• The resampling applies non-integer shifts of origin using bilinear interpolation. There is no rotation to align the Cartesian axes with the cardinal directions.

• The recipe makes the mosaics by applying offsets in intensity to give the most consistent result amongst the overlapping regions. The mosaic is not trimmed to the dimensions of a single frame. Thus the noise will be greater in the peripheral areas having received less exposure time. The full signal will be in the central ninth containing the main object. The mosaic is not normalised by its exposure time (that being the exposure time of a single frame).

• For each cycle of four, the recipe creates a mosaic, which has its bad pixels filled and is then added into a master mosaic of improving signal to noise. The exposure time is also summed and stored in the mosaic’s corresponding header. Likewise the end airmass header and end UT headers are updated to match that of the last-observed frame contributing to the mosaic.

• Intermediate frames are deleted except for the flat-fielded (_ff suffix) frames.

The current format of the ephemeris file is one line per object comprising three space-separated fields in the following order:

• the objectname, which may contain embedded spaces;

• the motion in the plane of the sky in arcsec/second for right ascension then declination.

Note that the right ascension motion is the change in right ascension multiplied by the cosine of the declination. The format will change to include UT and possibly date.

• The integrated mosaic in $<$m$>$$<$date$>$_$<$group_number$>$_mos, where $<$m$>$ is the instrument’s group prefix.

• A mosaic for each cycle of four in
  $<$m$>$$<$date$>$_$<$group_number$>$_mos$<$cycle_number$>$, where $<$cycle_number$>$
  counts from 0.

• The individual flat-fielded frames in $<$i$>$$<$date$>$_$<$obs_number$>$_ff, where $<$i$>$ is f for UFTI and i for IRCAM, and u for UIST.

• The created flat fields in flat_$<$filter$>$_$<$group_number$>$ for the first or only cycle, and flat_$<$filter$>$_$<$group_number$>$_c$<$cycle_number$>$ for subsequent cycles.

USEVAR = LOGICAL

MOVING_JITTER_SELF_FLAT, QUADRANT_JITTER, QUADRANT_JITTER_TELE.

• The processing engines are from the Starlink packages: Ccdpack, Kappa, Figaro, and EXTRACTOR.

• Uses the Starlink NDF format.

• History is recorded within the data files.

• The title of the data is propagated through intermediate files to the mosaic.

• Error propagation is controlled by the USEVAR parameter.