This script reduces a chopped and nodded observation, currently just for Michelle data. It takes an imaging observation comprising a multiple-of-four object frames to make automatically a calibrated, untrimmed mosaic in the reference frame of a moving target.

It performs a null debiassing, creation and propagation of data variance, difference the integrations for each AB chop beam pair, bad-pixel masking, difference adjacent nodded pairs, registers the frames, and forms a mosaic. 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.

• A variance array is created for each beam, first using the read noise, and once the bias is removed, Poisson noise is added.

• A bias frame selected from the calibration system is removed from each beam in CHOP read mode. If no bias frame is available in the CHOP mode, the recipe subtracts a null bias, so the errors will be overestimated in the CHOP read mode; the data array will be unaffected once the beams are differenced. The ARRAY_TESTS recipe files a suitable short-exposure dark as a bias in the calibration system.

• The integrations of the two beams are differenced, the first subtracted from the second in each pair.

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

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

• Registration is performed using the telescope offsets transformed to pixel, 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 integer shifts of origin. 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 mosaic is not normalised by its exposure time (that being the exposure time of a single frame).

• For each cycle of object frames, 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 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 differenced pairs (_dp 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 object frames in
  $<$m$>$$<$date$>$_$<$group_number$>$_mos$<$cycle_number$>$, where $<$cycle_number$>$
  counts from 0.

• The differenced pairs in $<$i$>$$<$date$>$_$<$obs_number$>$_dp, where $<$i$>$ is the frame prefix.

NUMBER = INTEGER

USEVAR = LOGICAL

NOD_CHOP, MOVING_JITTER_SELF_FLAT, MOVING_QUADRANT_JITTER.

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

• Uses the Starlink NDF format and multi-NDF HDS container files.

• 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.