Flats for full/partial disk
Posted: Sun Feb 04, 2018 11:14 pm
Hi all,
I've been experimenting with masking off the sky background in Photoshop in order to get partial flat frames to work for full or partial disk images. Basically I create a layer mask on the partial flat, using a histogram stretched copy of the flat itself. This allows me to reveal an underlying layer which I fill with a gray level selected based on the desired correction. It's pretty easy to use the healing brush to fade out details I wish to preserve if they show up on the pseudoflat. Since I'm only generally needing to lighten dark areas such as limb darkening or Newton rings, I try to make the final mean match the max of the flat; I would think that matching the mean of the flat should yield roughly equal lightening and darkening. I then apply the flat to the source image in AutoStakkert (not Photoshop, since it doesn't do true flat field correction).
This seems to work reasonably well on the few test images I've run so far using pseudoflats created by blurring the source image, I would think it should work for "real" flats as well even if they contain the limb of the solar disk. I'm wondering if anyone else out there has an alternative technique, or any thoughts or suggested refinements on this approach. Obviously better not to need to use a flat, but might be a useful means to salvage /tweak some data. Thanks!
Example on full disk for limb darkening - raw image pseudoflat with fill layer Calibrated image
I've been experimenting with masking off the sky background in Photoshop in order to get partial flat frames to work for full or partial disk images. Basically I create a layer mask on the partial flat, using a histogram stretched copy of the flat itself. This allows me to reveal an underlying layer which I fill with a gray level selected based on the desired correction. It's pretty easy to use the healing brush to fade out details I wish to preserve if they show up on the pseudoflat. Since I'm only generally needing to lighten dark areas such as limb darkening or Newton rings, I try to make the final mean match the max of the flat; I would think that matching the mean of the flat should yield roughly equal lightening and darkening. I then apply the flat to the source image in AutoStakkert (not Photoshop, since it doesn't do true flat field correction).
This seems to work reasonably well on the few test images I've run so far using pseudoflats created by blurring the source image, I would think it should work for "real" flats as well even if they contain the limb of the solar disk. I'm wondering if anyone else out there has an alternative technique, or any thoughts or suggested refinements on this approach. Obviously better not to need to use a flat, but might be a useful means to salvage /tweak some data. Thanks!
Example on full disk for limb darkening - raw image pseudoflat with fill layer Calibrated image