In-flight calibration of the ELU gain ratios

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Introduction


The output circuit of the OMI detector modules are equipped with cascaded switching amplifiers which are used to optimize the signal to noise ratio (SNR) of the measured signal. The electronics unit (ELU) allows for three gain switches during read-out of each CCD frame. These gain switches are pre-determined for a specific switching column of the read-out register (ROR), and are defined in each instrument configuration, uniquely described by an instrument configuration identifier (ICID). Effectively this means that four different gains can be used during read out of a CCD frame, allowing to optimize the output signal of specific column -bound regions on the CCD. The gains themselves are constant due to the design of the ELU. The cascaded combination of the amplifiers allow four different relative gains, approximately 1x, 3x, 10x and 40x, identified by the so-called gain-codes 2, 3, 0 and 1 respectively. The overall gain of the system is determined for the lowest gain (gain-code 2), and all other gains are calibrated to this lowest gain, i.e. they are determined relative to gain-code 2. Accurate knowledge of these relative gains is required for the calibration of each measurement.
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Measurements


The gain ratios can be determined using two measurements of a constant signal source. One of these measurements is taken with relative gain 1 for all columns of the image; the second measurement is exactly identical apart from the fact that four different gains are used during read-out of the signal. Dividing the measurement using four gains by the measurement with a single gain yields a data set containing the relative gain ratios. The gain ratios discussed here are derived using the onboard LED calibration source. Four measurements ID's are reserved for this purpose: ICID 32, 33, 34 and 35. ICID 34 measures the unbinned LED image using four different gains at approximately 200 colums intervals; the dedicated dark current background measurement is performed with ICID 32. ICID 35 measures with exactly the same settings as ICID 34 but now using only one gain; the corresponding background measurement is ICID 33. For each ICID 10 images are measured once per month.
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Calculation of the gain ratios


The calculation of the gain ratios is based on collection 3 data and are calculated per pixel. After selecting the appropriate column boundaries on the CCD, statistics can be applied to increase the accuracy of the gain ratio calculation because the gain ratios should be pixel independent. If the selected pixels show a Gaussian distribution, the gain ratio can be taken as the mean of the distribution and the error in the mean can be readily calculated. The gain ratios are calculated by correcting each measurement for its dark current. Onwards, the measurement with four gains is divided by the image with one gain. Each gain area is extracted by their corresponding switch colums, and histograms of the distribution are calculated together with the mean, standard deviation and the median value. Flagged pixels are explicitely excluded from the distribution. The distribution functions of the calculated gain ratios is shown below for the UV and VIS channel, based on the last available measurement.
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Evolution of the gain ratios


The evolution of the gain ratios is shown below for the UV and VIS channel by plotting the mean value (red) with its error (blue) and the median value (green). The figures on this page are based on all 44 measurements available to date.
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Conclusion


The LED calibrations source is very good for this purpose due to its very high short term stability, and allows for the derivation of the gain ratios with high accuracy and precision. The evolution plots of the gain ratios only show minor change in the behavior of the ELU.
For more information contact Quintus Kleipool or Marcel Dobber
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Figures


Last measurement of the UV channel gain ratios


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Last measurement of the VIS channel gain ratios


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UV channel gain ratio evolution


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VIS channel gain ratio evolution


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