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MIDI Quality Control:
Reference Pixels

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The reference pixels of MIDI are the two pixels of the detector onto which the centroids of the target images must fall in order to ensure a proper beam overlap.

MIDI offers different instrument modes (HIGH_SENS and SCI_PHOT). For the HIGH_SENS mode, 2 windows are recorded on the detector, when for the SCI_PHOT mode 4 windows are used. A triple-pinhole is used.

The position of the reference beams (beam A and beam B) are calculated for these 2 modes on the different windows and additionnally for the "OPEN" mode (without the beam combiner).

They are measured by a 2-D gaussian fit using the center of a pinhole. For each mode, each beam and each detector window the pipeline gives the coordinates X and Y.  

Pipeline steps

  • For each file which correspond to an instrument setup and one of the two beam, the pipeline calculates the averaged image over the N frames
  • Using a 2D Gaussian fit find the coordinates of the target in the given averaged image. The number of targets depend on the type of observation. For Interferometry (HIGH_SENS or OPEN) there are 2 groups of 3-pin-holes. The coordinates of the central pinhole for each group are always given.
  • Additionally the size of a search window is also given.
  • The targets are expected to be around the cetral pinholes. The result of the 2D Gaussian determines the accuracy of the beam positions.
  • or Photometry (SCI_PHOT) there are 3 groups of 3-pin-holes. Hence three sets of coordinates are always given .
  • QC1 parameters

    The QC1 parameters recorded in the QC1 database (table midi_refpix) and used for trending are as follow.

  • beam A of OPEN (Imaging): open_beamA_X, open_beamA_Y, open_beamA_size
  • beam B of OPEN (Imaging): open_beamB_X, open_beamB_Y, open_beamB_size
  • beam A of HIGH_SENS window 1: hs_beamA1_X, hs_beamA1_Y, hs_beam_A1_size
  • beam A of HIGH_SENS window 2: hs_beamA1_X, hs_beamA2_Y, hs_beam_A2_size
  • beam B of HIGH_SENS window 1: hs_beamB1_X, hs_beamB1_Y, hs_beam_B1_size
  • beam B of HIGH_SENS window 2: hs_beamB1_X, hs_beamB2_Y, hs_beam_B2_size
  • beam A of SCI_PHOT window 4:sp_beamPA_X, sp_beam_PA_Y, sp_beam_PA_size
  • beam A of SCI_PHOT window 2: sp_beamA1_X, sp_beamA1_Y, sp_beam_A1_size
  • beam A of SCI_PHOT window 3: sp_beamA2_X, sp_beamA2_Y, sp_beam_A2_size
  • beam B of SCI_PHOT window 2: sp_beamB1_X, sp_beamB1_Y, sp_beam_B1_size
  • beam B of SCI_PHOT window 3: sp_beamB1_X, sp_beamB2_Y, sp_beam_B2_size
  • beam B of SCI_PHOT window 1:sp_beamPB_X, sp_beam_PB_Y, sp_beam_PB_size
  • Trending

    The left plot shows the trending of the position of the X coordinates for the two beams (red and black circles) in the detector window 2 for the HIGH_SENS mode. The same kind of plost are shown for the Y coordinates and all the other modes.

    The right image, shows the pipeline product for the SCI_PHOT mode for one of the beam. In each window of the detector (here window1, 2, and 3), there is the image of the triple-pinhole. The coordinates are calculated on the central pinhole. This triple pinhole is one of the MIDI instrument setting, it allows to have simultaneous observations of the object (in the central pinhole) and of the sky (in the two extreme pinholes).

    We also plot the same data over 1 year to identify slow modifications of the position of the reference pixels


    Up to now, there have been no issues with the trending of reference pixels.