Characterization of Cameras for the COSMO K-Coronagraph

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Thursday, October 11, 2018

Digital image sensors are ubiquitous in astronomical instrumentation and it is well known that they suffer from issues that must be corrected for data to be scientifically useful. Alfred De Wijn present discussion on errors resulting from digitization, and characterization of non-linearity and ADC errors of the PhotonFocus MV-D1024E cameras selected for the K-Coronagraph of the Coronal Solar Magnetism Observatory.

Digital image sensors image
Digital image sensors are ubiquitous in astronomical instrumentation and it is well known that they suffer from issues that must be corrected for data to be scientifically useful. Non-linearity is traditionally addressed with a global correction. This figure shows a comparison for the PhotonFocus MV-D1024E used for the COSMO K-Coronagraph of the raw data (left), standard correction with a global non-linearity, bias, and gain that leaves residual systematic errors after dark and gain correction of up to 1% of the signal (middle), and a 4th-order polynomial correction to each pixel. The plots show the normalized averaged intensity value for 100 pixels near the middle of the detector at 40 illumination levels. The raw data shows structure with large amplitudes that is highly correlated between illumination levels. The standard correction uses the unilluminated exposure as the dark frame and 22nd level as the gain. These levels consequently show a perfect correction. The 4th-order fit shows substantially improved correction. It is now being implemented in the K-Coronagraph data reduction.

I derive an analytic expression for quantization errors. The MV-D1024E camera has adequate bit depth that quantization error is not an issue. However, I show that this is not the case for all cameras, particularly cameras with deep wells and low read noise that output data at low bit depth in order to run at high frame rates. The impact of non-linearity and ADC errors on science observations of the K-Coronagraph is analyzed using a simplified telescope model. Errors caused by the camera ADCs result in systematic errors in the measurement of the polarimetric signal of several times 10−9 B, which is around the desired accuracy of the instrument and about an order of magnitude above the desired sensitivity. I demonstrate a method for post-facto data correction using a lookup table and derive parameters from camera characterization measurements that were made with a lab setup. Non-linearity is traditionally addressed with a global correction. I show through analysis of calibration data that for the MV-D1024E this correction leaves residual systematic errors after dark and gain correction of up to 1% of the signal that I attribute to small differences in the characteristics of pixels that are not unexpected in this type of sensor. I demonstrate that a pixel-wise correction of non-linearity reduces the errors to below 0.1%. These corrections are necessary for the K-Coronagraph data products to meet the science requirements. They have been implemented in the instrument data acquisition system and data reduction pipeline. While no other instrument than the K-Coronagraph or camera than the MV-D1024E is discussed here, the results are illustrative for all instruments and cameras.

 

 

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