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Quality Control and
Data Processing

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A complete set of MUSE calibrations consists of the following frames:

  • bias
  • dark (optional)
  • lamp flat field
  • arc lamps
  • sky flat field
  • standard star
  • astrometry (optional)
  • geometry table (optional)

They need to be processed in a certain order.

First, the bias frames are combined into a master bias. It is further used in subsequent reduction steps.

The combined master dark is not expected to be needed for data processing as the dark current of the MUSE CCDs is small.

The raw lamp flat field frames are combined into a master flat and trace tables, where the slices are located on the image, their edges are traced along the wavelength axis (vertical direction) and bright/dark pixels are identified.

The set of three arc-lamp frames (one lamp each) repeated 5 times (5x3) is required to create wavelenght solution.
Assumption is that to create the lsf-table the arc-lamps' sequence has to be repeated 15 times (15x3). The line spread function is computed for each slice and is later used during the sky subtraction. Currently it is used in a static form.

Originally, the actual pixel data of the combined sky flat was not used in further data reduction. Instead, the integrated flux value was utilized for the later illumination correction as an estimate of the relative throughput of the IFUs. This changed with adopting pipeline version 1.0 (2014-12-08). Twilight correction is routinely applied to the on-sky data.

The response curve for the flux calibration of the science observations is obtained from the standard star observations using catalog of reference spectra of the spectrophotometric standard stars. Also, a static form can be used.

The astrometric calibration is created from the exposure of an astrometric field. It is needed to set the the world coordinate system for the science exposure and assign righ ascension and declination to the spatial elements of the data cube. Since the WCS solution was verified to be stable, this step in data reduction can be considered optional. Currently it is used in static form.

The geometry table calibration is created from 80 dedicated input frames. They are taken occasionally to monitor status of the instrument, typically once a month. The geometry table is closely related to the astrometric calibrations, thus, they have to be processed together. Since it appears to be quite stable with time and requires heavy processing, it is currently used in static form.

In addition, single input frame illumination flats are used for processing on-sky data. They are taken close in time to the science and standard stars' data. Were introduced with pipeline version 1.0 (2014-12-08). Find the graphical representation of the MUSE calibration cascade here.

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