SKY_AND_JITTER_APHOT - Reduces a ``combined jitter'' photometry observation, and performs aperture photometry

Description:

This script reduces a ``combined jitter'' photometry observation with UKIRT imaging data. It takes an imaging observation comprising one or more sets of frames, each set containing a sky frame, followed by jittered object frames; and a pre-determined flat-field frame to make a calibrated, trimmed mosaic automatically.

This recipe performs bad-pixel masking, null debiassing, sky subtraction, flat-field division, feature detection and matching between object frames, and resampling. See the ``Notes'' for details.

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 is suitable for moderately faint point sources.

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.

• You may use SKY_FLAT or SKY_FLAT_MASKED to make the flat field.

• 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 most-recent sky frame is used for the sky subtraction.

• Where automatic registration is not possible, the recipe matches the centroid of central source, and should that fail, it 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 mosaic by applying offsets in intensity to give the most consistent result amongst the overlapping regions. The mosaic is trimmed to the dimensions of an input frame. The mosaic is not normalised by its exposure time (that being the exposure time of a single frame).

• For each cycle of jittered frames, the recipe creates a mosaic, which 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. There are headings at the top of each column.

• The photometry uses the mode calculated from 3 $\raisebox{-0.4ex}{\rm *}$ median - 2 $\raisebox{-0.4ex}{\rm *}$ mean and Chauvenet's rejection criterion to estimate the sky level in an annulus about the source. 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.

• Sub-arrays are supported.

Output Data :

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

• 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.

Configurable Steering Parameters:

NUMBER = INTEGER

The number of frames in the jitter, excluding the sky frame. If absent, the number of offsets, as given by header NOFFSETS, minus two is used. If neither is available, 5 is used. An error state arises if the number of jittered frames is fewer than 3. []

USEVAR = LOGICAL

Whether or not to create and propagate variance arrays. [0]

Related Recipes :

JITTER_SELF_FLAT_APHOT, NOD_SELF_FLAT_NO_MASK_APHOT,
SKY_AND_JITTER, SKY_FLAT, SKY_FLAT_MASKED.

Implementation Status:

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

• 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 :

SKY_AND_JITTER5_APHOT.