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GMOS Data Format and Reduction

This section describes both the data format for all Gemini instruments, including GMOS, and the Gemini IRAF Package, which supports imaging, as well as long-slit, multi-object, and IFU spectroscopic data from both GMOS-N and GMOS-S.

Raw data

The raw images from GMOS are MEF files with a PHU and one or more pixel extensions.

Each of the three chips in the GMOS-S Hamamatsu arrays is read out through 4 amplifiers. Thus, in the 12-amplifier mode currently used for science applications, the data file contains 12 extensions in total. The overscan region is 32 pixels wide. See diagram below. 

Each of the three chips in the GMOS-N and GMOS-S original EEV arrays could be read out through one or two amplifiers (left and right). In the two-amplifier mode the output data file then contains 6 fits extensions (see diagrams below). Each of the six image sections has an accompanying overscan region 32 pixels wide. In one-amp mode the data file contains three extensions, one per chip, again with a 32-pixel wide overscan section. The one-amplifier mode has been the most used for science applications with the original EEV detectors. With the upgraded e2v deep-depletion GMOS-N (e2vDD) array all data is taken in two amp mode.

Binning: When pixel binning is used, the resulting datafile still contains overscan regions that are 32 samples wide.

              Layout of GMOS-S detector harray
  Layout of GMOS-S Hamamatsu detector array and structure of the output data file when reading out in 12-amp mode, with unbinned pixels.

 Layout of GMOS-N detector array
Layout of GMOS-N EEV or e2vDD detector array and structure of the output data file, when reading out in 6-amp mode, with unbinned pixels.

Layout of GMOS-N detector array
Layout of GMOS-N original EEV detector array and structure of the output data file, when reading out in 3-amp mode (using the 'best' three amps, R,R,L). Note the overscan regions are not in the same place for each chip.

Processed data from GMOS

Imaging data

As part of the data quality assessment most GMOS imaging data are processed as follows.

  • Subtraction of the overscan level
  • Subtraction of bias image
  • Correction for the differences in gain for the three detectors
  • Flat field correction
  • Mosaicing of the images from the three detectors into one image

After these processing steps the output image is a MEF file with a PHU and one pixel extension.

For observations that consist of several exposures at different dither positions, the images are registered and co-added. In the process of co-adding the images are cleaned for cosmic-ray-events and bad pixels, including the gaps between the detectors if the dither steps are of sufficient size. An example of a reduced co-added image is shown on the figure below.

Note that Gemini does not distribute reduced science imaging data with the exception of imaging taken for MOS mask design (aka pre-imaging).

GMOS imaging data
A reduced and co-added GMOS imaging observation. For this observation a guide star was used inside the imaging field of view. Thus, the OIWFS is vignetting a small part of the field.