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:

• calibration product FITS file frames (e.g. master BIAS or FLATFIELD frames)
• calibration product FITS tables (e.g. wavelength dispersion tables, photometric tables)

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 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.

Since 2015-04-22 new, significantly modified FORS2 pipeline version v.5.0 has been adopted. Master calibrations created by previous pipeline versions are not compatible any more and the pipeline requires modified static calibrations.

RAW CALIBRATION DATA

The following types of raw calibration data exist for the supported modes:

IMAGING

Frame Type	DPR Type	DPR Tech	Purpose
BIAS	BIAS	IMAGE	bias level, RON
DARK	DARK	IMAGE	dark current
SKY_FLAT_IMG	FLAT,SKY	IMAGE	flat field of science data
SCREEN_FLAT_IMG	FLAT,LAMP	IMAGE	gain, bad pixel map, lamp efficiency, fixed pattern noise
STANDARD_IMG	STD	IMAGE	photometric zeropoints, extinction and colour coefficients

LSS

Frame Type	DPR Type	DPR Tech	Purpose
BIAS	BIAS	IMAGE	bias level, RON
SCREEN_FLAT_LSS/LAMP_LSS	FLAT,LAMP/WAVE,LAMP	SPECTRUM	flat field of science data, slit transfer function, 2D dispersion solution and distortion correction
STANDARD_LSS	STD	SPECTRUM	spectrophotometric calibration

MOS/MXU

Frame Type	DPR Type	DPR Tech	Purpose
BIAS	BIAS	IMAGE	bias level, RON
SCREEN_FLAT_MOS/LAMP_MOS, SCREEN_FLAT_MXU/LAMP_MXU	FLAT,LAMP/WAVE,LAMP	MOS, MXU	flat field of science data, slit transfer function, 2D dispersion solution and distortion correction
STANDARD_MOS	STD	MOS	spectrophotometric calibration

PMOS

Frame Type	DPR Type	DPR Tech	Purpose
BIAS	BIAS	IMAGE	bias level, RON
SCREEN_FLAT_PMOS/LAMP_PMOS	FLAT,LAMP/WAVE,LAMP	POLARIMETRY	flat field of science data, slit transfer function, 2D dispersion solution and distortion correction
STANDARD_MOS/PMOS	STD	POLARIMETRY	polarimetric calibration

The following types of master calibration data are produced for the supported modes:

IMAGING

Frame Type	Recipe	Description	N	Products
BIAS	fors_bias	create a master BIAS from set of N input raw frames	5	master BIAS
DARK	fors_dark	create a master DARK from set of N input raw frames	5	master DARK
SKY_FLAT_IMG	fors_img_sky_flat	create a master SKY FLAT from set of N input raw frames	3-5	master SKY FLAT
SCREEN_FLAT_IMG	fors_img_screen_flat	create a master SCREEN FLAT from set of N input raw frames	2	master SCREEN FLAT
STANDARD_IMG	fors_zeropoint	reduce STANDARD STAR images and obtain photometric ZEROPOINT of the night	1	reduced image zeropoint table

Reduced STANDARD star images (PSTI) can be used for own extraction and photometric calibration programmes. They are reduced in the same way as SCIENCE files.

ZEROPOINT tables (PZPI) contain extracted fluxes and data for these standard stars. They have been produced from reduced standard star images using the tool SExtractor for the flux extraction.

LSS

Frame Type	Recipe	Description	N	Products
BIAS	fors_bias	create a master BIAS from set of N input raw frames	5	master BIAS
SCREEN_FLAT_LSS & LAMP_LSS	fors_calib	creates a 2D dispersion solution, correction for spatial distortion, and normalized flat-field	2	normalized master FLAT, dispersion coefficient tables, wavelength map, spatial map
STANDARD_LSS	fors_science	reduce data and apply wavelength correction, correct sky background and extract spectrum	1	extracted spectrum with errors, sky background

MOS/MXU

Frame Type	Recipe	Description	N	Products
BIAS	fors_bias	create a master BIAS from set of N input raw frames	5	master BIAS
SCREEN_FLAT_MOS & LAMP_MOS, SCREEN_FLAT_MXU & LAMP_MXU	fors_calib	creates a 2D dispersion solution, correction for spatial distortion, and normalized flat-field	2	normalized master flat field, dispersion coefficient tables, wavelength map, spatial map
STANDARD_MOS	fors_science	reduce data and apply wavelength correction, correct sky background and extract spectrum	1	extracted spectrum with errors, sky background, response and efficiency curves

PMOS

Frame Type	Recipe	Description	N	Products
BIAS	fors_bias	create a master BIAS from set of N input raw frames	5	master BIAS
SCREEN_FLAT_PMOS & LAMP_PMOS	fors_pmos_calib	creates a 2D dispersion solution, correction for spatial distortion, and normalized flat-field	2	normalized master flat field, dispersion coefficient tables, wavelength map, spatial map
STANDARD_MOS	fors_pmos_science	reduce data and apply wavelength correction, correct sky background and extract spectrum	2-4	extracted spectrum with errors, sky background, polarization parameters

CALIBRATION CASCADE

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.

NAMING SCHEME

After production and certification, the calibration products are renamed after a scheme which includes type, creation date and relevant FORS2 settings.