NOD_CHOP_FAINT - Reduces a chopped and nodded observation of a faint source, combining images and smoothing

Description:

This script reduces a chopped and nodded observation of a faint point or compact source, currently just for Michelle data. It takes an imaging observation comprising a multiple-of-four object frames to make a calibrated, smoothed combined image of the source automatically.

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 containing positive and negative images of the source. Column and row patterns are filtered.

The script extracts the various chopped and nodded images of the source and neighbouring background from the mosaic. It combines them using a median filter at each pixel to form to form a single image of the source with four times the signal. This combined frame is smoothed to enhance the visibility of faint sources.

See the ``Notes'' for further information.

Notes:

• 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 pixels.

• 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.

• Pickup and bias variation patterns, evident as ripples in the rows or bands in the columns respectively, are removed by subtracting the median along each column or row from the pixels in that column or row.

• The combined source image is made by taking symmetrical areas about the expected position of each source (derived from the chop throw and the nod separations), corrected for a shift of the base location from its nominal position. The shift comes from centroiding on bright sources with recipe NOD_CHOP_APHOT. The areas extend such that no pixels are duplicated. Thus the divisions occur at midpoints between the four images.

• The combined source image is smoothed using a 4-by-4 pixel block-average filter.

• Intermediate frames are deleted except for the differenced pairs (_dp suffix), and the bias- and pickup-corrected frames (_cpc and _rpc suffices).

Output Data :

• 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 combined source image and neighbourhoods in
  <m><date>_<group_number>_cab. The smoothed combined image in
  <m><date>_<group_number>_scab.

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

Configurable Steering Parameters:

NUMBER = INTEGER

The number of frames in the nod pattern. If absent, the number of offsets, as given by header NOFFSETS, minus one is used. If neither is available, 4 is used. An error state arises if the number of jittered frames is fewer than 4 and not a multiple of 4. []

USEVAR = LOGICAL

Whether or not to create and propagate variance arrays. [1]

Related Recipes :

NOD_CHOP, NOD_CHOP_APHOT, NOD_SELF_FLAT_NO_MASK.

Implementation Status:

• 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.