CHOP_SKY_JITTER - Reduction of alternating sky-target jitters using interpolated sky subtraction

Description:

This recipe reduces a moderately extended source using near-infrared imaging data. The data comprise alternating blank-sky and target frames commencing and ending with a blank sky. Both the sky and target frames are jittered. The recipe makes a sky-subtracted untrimmed mosaic automatically.

The script performs bad-pixel masking, null debiassing, dark subtraction, flat-field division, sky subtraction, registration, resampling, and mosaicking. The ``Notes'' give more details.

It is suitable for extended objects where the object fills or nearly fills the frame, so sky estimation within the frame is impossible or unreliable, but the extended mapping of the target is not required.

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.

• For INGRID, the pre- and post-exposure images are subtracted. A non-linearity correction is then applied.

• 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 derived from the sky frames as follows. The mode (sigma-clipped mean) is used to offset each sky frame's mode to that of the first sky frame. The corrected sky frames are combined pixel by pixel using a median of the values in each frame. The resultant frame is normalised by its median to form the flat field. This frame median is subtracted from the source frames after they have been flat-fielded. A flat field is created from all the jittered sky frames, and applied to all the target frames.

• 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.

• The sky subtraction comes from linear interpolation of the sky modal values of the two flat-fielded sky frames which immediately bracket the target frame.

• The field distortion of ISAAC is corrected in the target frames using the mappings documented on the ISAAC problems web page.

• Registration is performed using common point sources in the overlap regions. If the recipe cannot identify sufficient common objects, it then tries the crosshead offsets. If these are null, the script resorts to the telescope offsets.

• 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 noise will be greater in the mosaic's 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).

• At the end of each cycle of sky and object frames the full mosaic of target frames is created and displayed. The mosaic has its bad pixels filled by interpolation. On the second and subsequent cycles the full mosaic is 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 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 jittered target 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 target frames in the jitter pattern. If this is not set, a value is derived from the number of offsets, as given by header NOFFSETS. The formula is NOFFSETS / 2 - 1. An error results should NOFFSETS be odd. If neither is available, 9 is the default. An error state arises if the number of jittered frames is fewer than 3. For observations prior to the availability of full ORAC, header NOFFSETS will be absent. []

USEVAR = LOGICAL

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

Related Recipes :

CHOP_SKY_JITTER_BASIC, EXTENDED_3x3, QUADRANT_JITTER.

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.