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MEIDEX Crew TutorialCalibration of IMC-201
Adam D. Devir, MEIDEX Payload Manager
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Calibration of Xybion IMC-201

• Camera Parameters
• Filters
• FOV
• The Required Radiometric Accuracy for Dust
  Measurements
• Dust Measurements
• Radiometric Accuracy Requirements
• Radiometric Accuracy Calibration Aspects
• Radiometric Calibration of Xybion IMC-201
• Xybion IMC-201 Absolute Radiometric Camera
• Temperature Effect on the Absolute Calibration
• Flat Field Calibration
• Pixel-to-Pixel Non-uniformity
• The Moon Calibration
• An Example
• Radiometric Images of the Sky

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The IMC-201 Parameters
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Filters

• The IMC-201 is equipped with a filter wheel with
  6 filters
• Filter 1 CWL 339.7nm, FWHM4.1nm
• Filter 2 CWL 380.6nm, FWHM4.4nm
• Filter 3 CWL 472.1nm, FWHM25.1nm
• Filter 4 CWL 558.2nm, FWHM26.5nm
• Filter 5 CWL 665.4nm, FWHM48.3nm
• Filter 6 CWL 855.5nm, FWHM53.0nm

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The FOV of the IMC-201

• The total FOV of the IMC-201 was measured to be
  13.93o (H) x 10.66o (V).
• The total FOV was measured to be 699 (H) x 481
  (V) pixels.
• The dimensions of each pixel are 0.33mrad (H) x
  0.37mrad (V).
• At flight altitude of 300km, each pixel will
  cover 0.1 km (H) x 0.11 km (V).
• The PSF of the IMC-201 was measured to be
  3pixels (see next slide).
• Correspondingly, from radiometric point-of-view,
  the minimal area that can be measured (in the
  nadir) will be 0.3 x 0.3 km2.

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The FOV of the IMC-201 The PSF
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The Required Radiometric Accuracy for Dust
Measurements
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Dust Radiance
Dust Radiance as Measured for Rural Aerosols
(over sea surface) with OD 0.8, 0.3, 0.2 and 0.1
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Radiometric Accuracy

• In order to be able to calculate the aerosol
  parameters from the radiometric measurements of
  the solar radiance reflected from the dust (above
  the Mediterranean sea surface), two measurements
  have to be done
• Measurement of the radiance of the sea surface
  free from the dust.
• Measurement of the radiance of the dust above the
  sea.
• Both measurements have to be done with accuracy
  of ? 1/.
• For this we need to have an accurate calibration
  of the Xybion camera that will enable us to
  calculate the radiance with that accuracy.
• The main factors that affect the calibration
  accuracy are
• Radiometric Calibration Absolute calibration
• Calibration of the Temperature Effects on the
  Calibration
• Flat Field Calibration
• Pixel-to-Pixel Non-uniformity

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Radiometric Accuracy Calibration Aspects
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Absolute Radiometric Calibration

• The radiometric calibration of the Xybion camera
  is based on measuring the radiance (R) of an
  aperture of an integrating sphere () with
  different exposure times t msec and for all
  filters.
• The product of the (N x t) is shown as a function
  of the Level of the video signal of the aperture
  expressed in gray-level units GL0.
• The polynomial dependence N x t f3(GL0)
  allows to show that such fit has a residuals lt1
  over most of the dynamic range of the camera for
  all filters.
• Normalizing this polynomial dependence for all
  filters shows that the radiometric response of
  the camera is the same for all filters.() An
  integrating sphere is a device that has a rather
  large aperture with a constant spectral radiance
  N Watt/str/cm2/nm all over its aperture.

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Radiometric Calibration of Xybion IMC-201
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Xybion IMC-201 Absolute Radiometric camera
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Temperature Effect on the Absolute Calibration

• System sensitivity decreases with an increase in
  its temperature (this is characteristic of all
  bi-alkali photo-cathodes.
• Correctable to 0.5 level after initial warm-up
  period of 25 minutes.

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Flat field Calibration by Integrating Sphere

• Slowly varying component is removed via
  polynomial surface fit.
• Residual variations are due to fiber optic and
  pixel gain variations.

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Pixel-to-Pixel Non-uniformity

• Fitted surface images
• The 20 variation of the center-to-edge
  asymmetry is mostly apparent in channel 6 and
  probably is due to internal scattering.
• Residual non-uniformity.
• Fiber bundle variations and pixel gain variations
  are /- 4 and are similar for all the channels.

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Pixel-to-Pixel Non-uniformity
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The Moon Calibration
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The Moon Calibration
The long-term stability of the calibration was
tested. The variations in the stability were
found to originate in Gain changes of the MCP
(due to the use of unregulated voltage supply)
and to aging of the integrating sphere.
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The Moon Calibration

• In-flight calibration is the only indication that
  the Xybion calibration was not affected by any
  deposition on the window of the canister.
• The MEIDEX payload has no internal calibration
  sources to be used for such in-flight on-board
  radiometric calibration of the Xybion camera.
• The only in-flight calibration options are
• Using calibration sites on the earth (that depend
  on their exact albedo and the sun attenuation
  through the atmosphere).
• Using moon calibration.
• Two Moon calibrations made in-fight as part of
  MEIDEX primary mission (one at the beginning of
  the mission and one towards its end) will give us
  the indication that the Xybion calibration was
  not affected during the mission.
• Since the moon diameter is rather small
  (8.7mrad) and the PSF of the camera (1mard ) is
  not very small compared to it, it was decided to
  test the accuracy of the moon calibration by
  placing a variable iris (with known angular
  diameter) in front of the aperture of an
  integrating sphere.

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The Moon Calibration
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The Moon Calibration
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The Moon Calibration

• The radiometric calibration of the moon was found
  to be good. The deviation from normalized
  response of one are reasonable since
• There was no flat-field correction and especially
  no pixel-to-pixel correction. Such correction
  will affect very much the radiometric response of
  the camera especially for small targets.
• There is some jitter in the Run Mode exposure time

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Radiometric Accuracy An Example
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Radiometric Images of the Sky
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Filter Exposure time (msec) Gain () CCD Temp.
(Co) Date (mm/dd/yy) Time (hhmmss) Coded data
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Clear Sky Radiance Measurements

• Modeled with Rayleigh atmosphere.
• Radiance data show SZA dependence in comparisons.

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Sky with small amount of Aerosol

• Better SZA agreement is obtained by adding 0.05
  optical depth aerosol.
• Both measured 340nm and 380 nm radiance values
  are lower with respect to the model which is
  consistent with stray light in the calibration.

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ENDCrew Tutorial Calibration of IMC-201