NOD_SELF_FLAT_NO_MASK_APHOT - Reduces a ``nod jitter'' photometry observation, and performs aperture photometry

Description:

This script reduces a ``nod jitter'' photometry observation with UKIRT imaging data. It takes an imaging observation comprising a multiple-of-four object frames and a dark frame to make a calibrated, untrimmed mosaic automatically.

It performs a null debiassing, bad-pixel masking, dark subtraction, difference adjacent pairs, flat-field creation and division, feature detection and matching between object frames, and resampling. See the ``Notes'' for further information.

Photometry of the point source using a fixed 5-arcsecond aperture is calculated for each jitter frame and the mosaic. 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.

This recipe works well for faint sources in moderately crowded fields.

Notes:

• 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 by combining normalised object frames using the median at each pixel. There is no cleaning of extreme outliers.

• For ISAAC, residual bias variations along the columns are largely removed from each flat-fielded frame. The recipe first masks the sources, then collapses the frame along its rows to form a profile, whose clipped mean is subtracted. The resultant profile reflects the bias variations. The recipe subtracts this profile from each column of the flat-fielded frame.

• Registration is performed using common point sources in the overlap regions. If the recipe cannot identify sufficient common objects, the script resorts to using the telescope offsets transformed to pixels.

• 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 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, 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.

• 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. To discriminate between the various results, the positive images have suffix _pos after the frame name and the negative images have a _neg suffix. 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. This is not unduly biased by the presence of the self-flat artifact in the pixel histogram. The inner annulus diameter is 1.3 times that of the aperture (6.5 arcsec); the outer annulus is 2.5 times (12.5 arcsec) for UFTI and twice the aperture (10 arcsec) for IRCAM. The errors are internal, based on the sky noise.

• Intermediate frames are deleted except for the flat-fielded (_ff 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 individual flat-fielded frames in <i><date>_<obs_number>_ff, where <i> is the frame prefix. The naming format is slightly different for some non-UKIRT instruments.

• For ISAAC, the individual bias-corrected frames in isaac<date>_<obs_number>_bc.

• 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.

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. [0]

Related Recipes :

BRIGHT_POINT_SOURCE_APHOT, NOD_SELF_FLAT_NO_MASK.

Implementation Status:

• 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.

Deprecated Variants :

NOD4_SELF_FLAT_NO_MASK_APHOT, NOD8_SELF_FLAT_NO_MASK_APHOT.