NOD_CHOP_APHOT - Reduces a chopped and nodded observation, and performs aperture photometry

Description:

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 a calibrated, untrimmed mosaic 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. See the ``Notes'' for further information.

The script combines and registers the various chopped and nodded images of the point source and neighbouring background to form to form a single image with four times the signal. Photometry of the point source using a fixed 5-arcsecond aperture is then calculated. The results appear in $ORAC_DATA_OUT/aphot_results.txt in the form of a Starlink small text list. The analysis of each star is appended to this file.

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.

• The combined source image is made by taking symmetrical areas about each source, such that no pixels are duplicated. Thus the divisions occur at midpoints of the chop throw and the nod separations. These are registered using the source centroids.

• The photometry tabulation includes the file name, source name, time, filter, airmass, the catalogue magnitude and estimates of the zero-point with and without the application of a mean extinction. There are headings at the top of each column.

• The photometry uses a multiply clipped (2,2,2.5,3 standard deviations) mean to estimate the sky mode in an annulus about the source. The inner annulus diameter is 1.5 times that of the aperture (7.5 arcsec); the outer annulus is 3.0 times (15 arcsec) for Michelle. The errors are internal, based on the sky noise.

• Intermediate frames are deleted except for the differenced pairs (_dp suffix) frames.

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 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_SELF_FLAT_NO_MASK_APHOT,
BRIGHT_POINT_SOURCE_APHOT, JITTER_SELF_FLAT_APHOT.

Implementation Status:

• The processing engines are from the Starlink packages: CCDPACK, KAPPA, FIGARO, and PHOTOM.

• 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.