SKY_FLAT - Creates and files a flat field derived from jittered frames

Description:

This recipe makes a sky flat for UKIRT imaging from a series of sky or object frames which are combined using one of a selection of statistics. It expects one dark frame followed by jittered sky frames.

It performs a null debiassing, bad-pixel masking, and dark subtraction before combining normalised frames pixel by pixel using the median. Details of the flat are filed in the index of flats for future selection and use of the flat. See the ``Notes'' for further details.

For best results the field observed should contain few stars and no bright ones. In contaminated sky regions, recipe SKY_FLAT_MASKED will greatly reduce artifacts appearing in the resultant flat.

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.

• Intermediate frames are deleted.

• Sub-arrays are supported.

Output Data :

• 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 recipe cycles. Token <filter> is the filter name, <group_number> is the frame number of the group, and <cycle_number> is the number of the cycle, counting from one.

• The flats are filed in $ORAC_DATA_OUT/index.flat.

Configurable Steering Parameters:

NUMBER = INTEGER

The number of frames in the jitter. If absent, the number of offsets, as given by header NOFFSETS, minus one 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 :

SKY_FLAT_FP, SKY_FLAT_MASKED, SKY_FLAT_POL.

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 the intermediate file to the flat.

• Error propagation is controlled by the USEVAR parameter.