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Flat Tuning Adjustment Optimization

Andrew J
in General
Hi Adam/All.

I really appreciated Adam's video demonstrating how to Tune Flats for over-correction. I suspect that flat fielding errors are a rather common occurrence for people just getting into the hobby. I have had this issue for years and never knew that it was being caused by my flats being insufficiently bright. Adding 0.02 or 0.03 pedestal to my flats has helped a lot. However, determining just how much pedestal to add has proven to be challenging, requiring a lot of trial and error for each filter. I was wondering if there Is a more "scientific" approach to calculating the amount of pedestal to add to each flat? 

I have experimented with using the Pixel Math formula  "$T +(.5 - mean($T))" to automatically adjust the mean value of my master flats to 0.5 and this seems to work OK, but not sure if this is the best approach. Having an optimized formula that would automatically determine the amount of pedestal to add to each flat would be very useful. I fully understand that the best answer it is to expose flats for the appropriate ADU levels in the first place thus eliminating the need for a pedestal. However, I have found this to be difficult to do on repeatable basis using the Alnitak Flip-Flat Flat Fielder panel I use. This is particularly true for my 3nm Narrow band filters. 

Probably wishful thinking, but maybe in WBPP 3.0 they could add an option to enter (or Calculate) a flat pedestal, similar to the pedestal adjustment that is available for NB Dark Frames. As it is now, I need to create Master Flats with the pedestal before I can use WBPP to calibrate my lights. The current trial and error method of determining the best tuning adjustment value for each flat may mean needing to calibrate, register, and integrate many times before finding the "correct" pedestal amount that will give me the best integrated images. I am thinking there must be a better way...

Andrew J

Comments

  • BlockHeadBlockHead
    Generally properly calibrated flats will not have an offending pedestal that causes them not to work.
    Thus, the offender isn't the flats or something about their brightness- it is the biases/darks that are the problem. Flats are supposed to characterize the optical system- the moment you touch them you lose that relationship. The "trick" demonstrate is only a bandaid for the improper calibration of the flats.
    -the Blockhead
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