Thanks David, Rick, John and Tom, I appreciate your comments.
@John – this is the 1st time ever that I have read of an astronomer wanting a shorter focal length!
I took a series of exposures from 1 sec right up to 60 secs and have since measured the flux around the expected position of Phobos. Unfortunately, the background levels in the region of Phobos are some 5 to 6 times brighter in the 1 sec frames that the expected brightness of Phobos, so I’ve got Buckley’s chance of digging it out!
Cheers
Dennis