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Muddy,
With imaging the planets the higher the f number the better. In your case the f13 is fine. The diameter of the scope will prevent large image scales being obtained, but you should get a good start. Like suggested I would get a 2x barlow. Don't bother with a powermate yet as a 2.5 will push you over the photographic limit of the scope. A scope can go much higher in magnification under the photographic limit than visually.
If you have a 102mm scope with f13 with a 2x barlow your magnification will be 442x. If you consider that you will get visually around 40x per inch of scope and the 102 is 4 inches then 160x is max for visual and 442 will be near max for photographic.
I would not buy a 5x powermate for that scope. It will produce bad results. It will be too powerful. So I think you should get a TV 2x barlow (they are good quality) and use the neximage directly into the barlow, you should get some useful imaging practice. Or you could get a 1.5x barlow (TV does not make these) and this would lessen your magnification and produce slightly better results given the diameter of the scope.
In terms of rules for photographic magnification I have found that with my scopes of around FL2350-2500mm f10 with a 2.5x powermate a mag around 1000x is the upper most limit in average seeing. Anymore than that the image breaks down very quickly. That means effectively a 10" will support something around 100x mag per inch for photographic use in average seeing. In excellent seeing this may go as high as 125x. Only seen this once in the last year. For a 9.25 the effective photo mag is just a bit over 100 times in average seeing.
So to sum up, get a 2x barlow and try for saturn or Jupiter or the moon and see how this goes. If your unsure get a slightly smaller barlow first and see if you can borrow a 2x from someone else.
Best of luck.
Paul
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