Each sequence of four frames expected in each spatial position are tabulated below.

For each spatial set, the recipe performs a null debiassing, bad-pixel masking, dark subtraction, pairwise frame differencing, flat-field division, integer shifts of origin to register, and mosaicking. The wavelength-shifted mosaic is given by

where $Fn$ is the bad-pixel masked and dark subtracted frame $n$. In practice, the flat field is applied to each differenced pair, such as ($F1-F2$), when the pair becomes available, rather than waiting until all four frames have been observed.

Finally the recipe registers all the wavelength mosaics spatially, and forms a untrimmed mosaic, combined using the median to reduce stellar artifacts.

• 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.

• 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.

• You should use SKY_FLAT_FP to make the flat field.

• Registration is performed using the telescope offsets transformed to pixels.

• There is no resampling, merely integer shifts of origin.

• For each set of four, the recipe creates a wavelength mosaic. For each cycle of spatial positions the wavelength mosaics are registered to form a spatial mosaic. For repeat cycles the spatial mosaic is then added into a master mosaic of improving signal to noise. The exposure time is also summed and stored in the master 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 recipe makes the mosaics by applying offsets in intensity to give the most consistent result amongst the overlapping regions. No mosaic is trimmed to the dimensions of a single frame, thus the noise will be greater in the peripheral areas of the spatial having received less exposure time. Each mosaic is not normalised by its exposure time (that being the exposure time of a single frame).

• Intermediate frames are deleted except for the flat-fielded (_ff suffix) frames.

• The integrated mosaic in gf$<$date$>$_$<$group_number$>$_mos, where $<$date$>$ is the UT date in yyyymmdd format.

• A mosaic for each cycle of eight in
  gf$<$date$>$_$<$group_number$>$_mos$<$cycle_number$>$, where $<$cycle_number$>$
  counts from 0.

• The individual flat-fielded frames in f$<$date$>$_$<$obs_number$>$_ff.

NPAIRS = INTEGER

NUMBER = INTEGER

USEVAR = LOGICAL

• 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.