Hi Chris,

"... Still needed DOS4GW.EXE file for some reason..."

That's very definitely needed. Charon is a 32-bit DOS
program, which needs an extender stub. DOS4GW is that stub.

"...Whenever I run Charon on any image (FITS file) I'm
getting a warning that the sun is up?"

Charon uses the time stored in the FITS image, plus the
location you specify in its Settings menu, to compute where
the sun would be at that time as seen from that location.
Either the FITS time specified in the image is wrong, or
the MPC code specified in Charon's Settings menu is wrong.

Setting the computer's clock wouldn't change anything;
it's the time at which the image was taken that matters
(which should make sense; after all, you may have taken
your images at local midnight but not gotten around to
processing them until the next morning... what matters
isn't when you run the image through Charon, but when
you took it.)

You mention that the FITS time is UT, so you're probably
all OK there. My guess would be a wrong MPC station.

-- Bill