I have worked with the EFM method for removing scattered light, testing it on a synthetic moon image from night JD 2455923.
The “true” scattered light is represented by an image:
S_true = observed – pedestal – usethisidealimage
The best guess scattered light is represented by the end result from the fit minus the pedestal:
S_efm = trialim – pedestal
I have investigated the mean value of S in the 3 boxes shown below for 100 S_true and corresponding S_efm images. It is the same synthetic image that is the basis of all the “observed” images, but they have slightly different noise added when they are convolved with the PSF.The results:
It is clear that we have significant bias. Not all the stray light is removed with the EFM method. On the positive side, the standard deviations of the fits are small. The counts here can be compared to a typical DS signal of 5 counts.