Calibration data for FORS2 are taken for two purposes:
assess the instrument status
reduce science data
Generally, FORS2 calibration data are taken as daytime calibrations. On user request, also nighttime calibrations can be obtained as part of the Observing Block which generates the SCIENCE data ("attached calibrations").
Please, refer to Chapter 4 of the FORS User Manual for details on the current calibration plan.
The calibration data taken obtained in the supported modes are processed by QC Garching to obtain calibration solutions. These solutions come as:
All 4-port frames belonging to pipeline-supported modes are processed using FTU (Fits Translation Utility) and a set of IRAF/IDL readable keywords is added to the the FITS headers. In particular a full description of the port structure, including pre-scan regions, is included.
IMPORTANT NOTE: all FORS2 pipeline products do not have the pre-scan regions which are chopped-off during the reduction process. As a consequence, master calibrations/reduced science files start at Y pixel coordinates @5 and stop at @1030. In order to use the pipeline master calibrations with raw frames, one needs to perform the trimming of input images in the same way.
RAW CALIBRATION DATA
The following types of raw calibration data exist for the supported modes:
bias level, RON
flat field of science data
gain, bad pixel map, lamp efficiency, fixed pattern noise
photometric zeropoints, extinction and colour coefficients
bias level, RON
flat field of science data, slit transfer function, 2D dispersion solution and distortion correction
reduce data and apply wavelength correction, correct sky background and extract spectrum
extracted spectrum with errors, sky background, polarization parameters
Like for all astronomical data, the reduction of FORS2 calibration frames requires a cascading scheme where the mutual dependencies of products and raw frames have to be respected. Master Biases can be produced from raw files only, but all the other products require the prior generation of other calibration products. The proper sequence of all these production steps is called the calibration cascade.
After production and certification, the calibration products are renamed after a scheme which includes type, creation date and relevant FORS2 settings.