having much trouble pushing C8 to F40-50 for Mars captures

Hi:

As with the 2003 Mars opp, I'm again using my C8, Toucam Pro, and Televue 3x
barlow lens for captures. However, Mars seems quite a bit smaller this time
and I've been trying to increase the system magnification to F40-F50 for a
larger image. I do this by extending the length between the end of the
barlow and the Toucam CCD. If I add about 50 mm, for example, the F ratio
increases to F40 and add another 30-40 mm, F ratio becaome approx F50.
However, I have been running into some problems:

1) Believe it or not, even at the native F30 with a Televue 3x, 1.25" size,
and Toucam Pro, I see vignetting in the form of a brighter circle of light
somewhat centrallized within the field of view. This doesn't show up easily
unless I'm looking at a bright target such as the moon. Even then, it has
never been severe enough to warrant any kinds of correction measures
unless......

2) Now, I increase the system amplification to F40 by adding about 50 mm or
so between the barlow and CCD. Now, the circle of light becomes much more
prominent and I start to see coma outside the middle of the field. There is
also light dropoff of the planet away from the center of field.

3) At F50, when I add 75mm plus between the barlow and CCD, the
aforementioned problems become severe. I must keep Mars centrallized. Any
deviation from center of field and light dropoff becomes great and field
coma is intense.

I should point out that the center of light cone isn't always at the center
of the field. Sometimes it can be in another area of the CCD frame. I
presume this is because of sagging of the optical train, causing the barlows
light cone to fall onto a different area of the CCD.

My question is this: how do I eliminate the light cone and edge of field
problems? Is there anything I can do with my current system to increase the
width of the light cone so it falls on the entire CCD?

I'm considering changing my magnification system. I would appreciate any
suggestions for obtain higher F ratios without vignetting and field
curvature with a C8/ Toucam system.

Thanks,
Nick