I have made extensive tests with bias and darks on a KAF-8300. I may make the results available as time permit.
The bottom line is that because the chip has a very low dark current compared to its read noise, subtracting the bias from the dark often results in negative values for many pixels (at least for darks of less than 10 min at -30C). These values are truncated to zero and make the calibrated dark pretty bizarre.
There multiple technical solutions when doing a calibrated dark, like adding pedestal, not using the dark or the bias, and so on. I need to investigate more to see the impact of each solution (more by curiosity than by need, dithering seems to be solving most of the problems anyhow and the dark is not critical for short exposures).
Now the question:
Even when doing the ImageCalibration of lights with 'calibrate' (so dark/bias/flat done in one operation), it seems that the ImageCalibration truncates the intermediate result 'dark - bias'. Is this right ? Or did I not interpret the results correctly ?
If ImageCalibration does the truncation, should it not do the truncation only on the calculated end image ? Because with the light added, the result is always positive, so there would be no truncation in the process. Or should it log a message like '1234567 pixels of the dark truncated to zero' to attract attention to that specific problem?
thanks for any info
-- bitli
The bottom line is that because the chip has a very low dark current compared to its read noise, subtracting the bias from the dark often results in negative values for many pixels (at least for darks of less than 10 min at -30C). These values are truncated to zero and make the calibrated dark pretty bizarre.
There multiple technical solutions when doing a calibrated dark, like adding pedestal, not using the dark or the bias, and so on. I need to investigate more to see the impact of each solution (more by curiosity than by need, dithering seems to be solving most of the problems anyhow and the dark is not critical for short exposures).
Now the question:
Even when doing the ImageCalibration of lights with 'calibrate' (so dark/bias/flat done in one operation), it seems that the ImageCalibration truncates the intermediate result 'dark - bias'. Is this right ? Or did I not interpret the results correctly ?
If ImageCalibration does the truncation, should it not do the truncation only on the calculated end image ? Because with the light added, the result is always positive, so there would be no truncation in the process. Or should it log a message like '1234567 pixels of the dark truncated to zero' to attract attention to that specific problem?
thanks for any info
-- bitli