Ok, that is comforting to hear then. In that case I will most likely leave it as is. Thank you very much; that is exactly the feedback I was looking for.
Also, a correction. That image *was* taken with the Crayford in, because the focal length (measured by the image scale) was exactly 1000 mm. Without the Crayford in the focal length is exactly 805 mm. It was focused without a Bahtinov, however.
Good, am pleased too. Glad to be of help.
So you are shooting direct to imager - to get the 805mm FL you have to have the image plane about an inch past the rear flange of the C90. I've a scale on the helical focuser to tell the infinity focus focal length at those positions, since it changes considerably.
You may wish to take another image with the Bahtinov and see if you have the same color cast.
In the originally posted version, if you look at the upper limb, you can see color fringing. Interestingly, you can see on the dark side of the bright upper limb that it is blue on one side of a stitch in the mosaic and red on the other side of the stitch. So it was clearly blue when shooting one panel and red when shooting the other. I am not sure what could cause that; perhaps a slight focus shift between panels? This would be even more apparent at the original resolution, before I downsampled the final image by ~50%. In any event you can also see some color in the craters at the terminator.
I saved the picture and maxed out the saturation. Don't see red at all. See a suffuse yellow cast on the outer limb of three quadrants, with the remaining first quadrant having a blue suffuse cast.
The C90's corrector is not achromatic, so inside/outside focus does yellow/blue a bit, a side effect of the thin corrector design choice. At a guess, could a slight miscollimation, or corrector wedge, or perhaps dirt in the corrector's front mating surface to the tube, "tilt" it so that focus is slightly skew?
The C90 wasn't always assembled with due care, as I've found foibles. My own needs to be stored for long times vertically, as horizontally causes it to be "off". So I store it corrector face down and it's just fine for any length of time. I think the adhesive in the base that holds the mirror in collimation (I use nylon thumbscrews in place of the cap screws to touch up collimation) isn't fully cured and sags otherwise.
Oh, and the moon does have subtle brown hues I can see with my C90 and refractors - in part why I did the baffle improvements.
For all I know some of this is atmospheric dispersion, though.
Depends on the altitude of the moon in the sky for this shot. Usually the gradient spans the picture, and it forms a line. Don't see that here.
Nice to here from someone else with a good experience with the C90, instead of the usual bashing. I'd never in several decades had a good one until recent, and in my pursuit of seeing how good the good one could be made, came away with an entirely different view of their flaws and virtues than prior experience.
Best of luck with your AP.