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FORS2 Pipeline:
Calibration Data

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INTRODUCTION

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 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:

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


MASTER CALIBRATION DATA AND PRODUCTS

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.


 
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