October 29-30, 2001MEIDEX - Crew Tutorial - Calibration F - 1 MEIDEX – Crew Tutorial Calibration of IMC-201 Adam D. Devir, MEIDEX Payload Manager

October 29-30, 2001MEIDEX - Crew Tutorial - Calibration F - 2 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

October 29-30, 2001MEIDEX - Crew Tutorial - Calibration F - 3 The IMC-201 Parameters

October 29-30, 2001MEIDEX - Crew Tutorial - Calibration F - 4 Filters The IMC-201 is equipped with a filter wheel with 6 filters  Filter #1: CWL= 339.7nm, FWHM=4.1nm  Filter #2: CWL= 380.6nm, FWHM=4.4nm  Filter #3: CWL= 472.1nm, FWHM=25.1nm  Filter #4: CWL= 558.2nm, FWHM=26.5nm  Filter #5: CWL= 665.4nm, FWHM=48.3nm  Filter #6: CWL= 855.5nm, FWHM=53.0nm

October 29-30, 2001MEIDEX - Crew Tutorial - Calibration F - 5 The FOV of the IMC-201 The total FOV of the IMC-201 was measured to be o (H) x o (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 km 2.

October 29-30, 2001MEIDEX - Crew Tutorial - Calibration F - 6 The FOV of the IMC-201 – The PSF Filter 6

October 29-30, 2001MEIDEX - Crew Tutorial - Calibration F - 7 The Required Radiometric Accuracy for Dust Measurements

October 29-30, 2001MEIDEX - Crew Tutorial - Calibration F - 8 Dust Radiance Dust Radiance as Measured for Rural Aerosols (over sea surface) with OD ~ 0.8, 0.3, 0.2 and 0.1

October 29-30, 2001MEIDEX - Crew Tutorial - Calibration F - 9 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

October 29-30, 2001MEIDEX - Crew Tutorial - Calibration F - 10 Radiometric Accuracy – Calibration Aspects

October 29-30, 2001MEIDEX - Crew Tutorial - Calibration F - 11 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 – GL 0. The polynomial dependence – N x t = f 3 (GL 0 ) allows to show that such fit has a residuals <1% 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/cm 2 /nm] all over its aperture.

October 29-30, 2001MEIDEX - Crew Tutorial - Calibration F - 12 Radiometric Calibration of Xybion IMC-201

October 29-30, 2001MEIDEX - Crew Tutorial - Calibration F - 13 Xybion IMC-201 – Absolute Radiometric camera

October 29-30, 2001MEIDEX - Crew Tutorial - Calibration F - 14 FilterSlope %/degree 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.

October 29-30, 2001MEIDEX - Crew Tutorial - Calibration F - 15 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. Sphere illuminated flat-field response Fitted poly-surface Grayscale is 0.8 – 1.0 Difference of flat-field and fitted surface gives the Pixel-to-pixel correction. Scale is +/- 4% Uncorrected sphere illumination Corrected sphere illumination

October 29-30, 2001MEIDEX - Crew Tutorial - Calibration F - 16 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.

October 29-30, 2001MEIDEX - Crew Tutorial - Calibration F - 17 Pixel-to-Pixel Non-uniformity

October 29-30, 2001MEIDEX - Crew Tutorial - Calibration F - 18 The Moon Calibration

October 29-30, 2001MEIDEX - Crew Tutorial - Calibration F - 19 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.

October 29-30, 2001MEIDEX - Crew Tutorial - Calibration F - 20 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.

October 29-30, 2001MEIDEX - Crew Tutorial - Calibration F - 21 The Moon Calibration

October 29-30, 2001MEIDEX - Crew Tutorial - Calibration F - 22 The Moon Calibration

October 29-30, 2001MEIDEX - Crew Tutorial - Calibration F - 23 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

October 29-30, 2001MEIDEX - Crew Tutorial - Calibration F - 24 Radiometric Accuracy – An Example

October 29-30, 2001MEIDEX - Crew Tutorial - Calibration F - 25 Filter # Exposure time (msec) Gain (%) CCD Temp. (C o ) Date (mm/dd/yy) Time (hh:mm:ss) Coded data Radiometric Images of the Sky

October 29-30, 2001MEIDEX - Crew Tutorial - Calibration F - 26 Clear Sky Radiance Measurements  Modeled with Rayleigh atmosphere.  Radiance data show SZA dependence in comparisons.

October 29-30, 2001MEIDEX - Crew Tutorial - Calibration F - 27 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.

October 29-30, 2001MEIDEX - Crew Tutorial - Calibration F - 28 END Crew Tutorial – Calibration of IMC-201