Discussion of stray light in Juno Earth flyby images Options

[Gerald]

This is now a first step of a somewhat sophisticated analysis of the ghosts, the success of which is to find out.
The ghosts are best feasible outside the bright Earth. So I've masked all efb12 subframes to just show the ghosts in front of dark background.
Example:

This is the average of all these masked subframes, constraint to the non-black pixels:

This average shows remnants of the source images. Most of this can be averaged away by decomposing the image in a horizontal and a vertical mean brightness function.
For averaging, the brightness values are temporarily gamma-corrected with gamma = 2, considering the square-root encoding of the raw image.
The two functions can then be composed to result in this mean ghost image:

This image is intended to serve as a 0th approximation of a flatfield for the ghosts.

[mcaplinger]

Here's a dimensioned drawing of the Junocam color filter array (dimensions in mm). In looking at EFB12, I would venture to guess that a lot of the stray light features are actually interline smear from the clear areas in the CFA adjacent to the CH4 region, not all of which are occluded by the light shield. These are exacerbated by the short exposure times we had to use in EFB.

Reflections off the light shield are not very plausible, as the shield and all of the internals of the optics are bead-blasted and black anodized. The AR coatings on the glass surfaces are as good as we could obtain but certainly there are paths from the filter edges, from the CCD die, bond pads, metallization, etc.

Since the instrument is not intended to be radiometrically precise and the stray light is only especially visible off the limb, I'm not thinking this is going to be much of an issue for most applications. I'm still more concerned about band-to-band registration, for which no perfect model yet exists.

Mods: again, I suggest this material be moved to a subforum as it's unlikely to be very interesting to most.

[moderator note: A Juno subforum will probably be created soon and the Juno thread split and/or reorganized when this happens]

[Gerald]

Thanks for the technical plan. This clarifies several of the observed effects, e.g. the narrow horizontal substripes in EFB03 due to the small gap between the color stripes.

I'm not thinking this is going to be much of an issue for most applications.

Here a 16x enhanced crop of EFB01, probably showing the sharp ghost of our Moon:

Since it's TDI 1, I've been interpreting the elongated shape as reflected light rather than smear (with some uncertainty).
It seems, the light of the moon happened to hit only one (causing the sharp ghosts) of at least three areas causing ghosts.
I'm expecting this to cause issues mainly in cases, when a bright target is displaced about 18 degrees (vertically) relative to a dark target of interest, think e.g. at attempts to observe auroras, or at areas which are dark in one spectral band.

... there may be some imaging during approach and earlier on the first orbit ...

Some images similar to the Moon image during approach addressing specifically the possibly reflecting or smearing areas would certainly help calibration.

I'm still more concerned about band-to-band registration, for which no perfect model yet exists.

Pinning this down to subpixel precision looked much easier to me than quantifying the fuzzy stray light and smear, so I was inclined to do the difficult things first.
But just to be sure, I'll be going to shift priorities towards image geometry, since this needs to be elaborated anyway, including the geometry of the ghosts.