Post-hoc derivation of MDI flat fields

The SOHO satellite now offers a unique perspective on the sun as it is the only space based instrument that can provide large, high resolution data sets over an entire 11 year solar cycle. This unique property enables detailed studies of long term variations in the sun. One significant problem when looking for such variation in solar properties is determining what component of any variation is due to deterioration of the instrument, and what is due to the sun itself. One of the key properties that changes over time is the apparent sensitivity of individual pixels in the CCD array. This can change considerably as a result of optics damage, and radiation damage and aging of the sensor itself. In addition to reducing the sensitivity of the telescope over time, this damage significantly changes the uniformity of the flat field of the instrument, a property that is very hard to recalibrate in space. For procedures such as feature tracking, and intensity analysis this can cause significant errors. We present a method for deriving new flat fields for high resolution MDI continuum data, using analysis of existing continuum and magnetogram data sets. This method allows us to reduce the residual flat field error by up to a factor of 10, enough to significantly change the results from correlation tracking analysis. One significant advantage of this method is that it can be done using archived data, without requiring any special satellite operations.

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