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Focal length won't help if you don't have the aperture to capture the light. You could get an f/15 mak cas as well, but the aperture is tiny.
My 10" was 1250mm FL and my 12" is 1500mm. When I use my 5x powermate, with the extension given by the filterwheel it ends up that i'm imaging at about 10.5m focal length. With the small chip size of the DMK (and ToUcam) (5.6microns) it gives a pretty large image that i'm usually happy with. Of course we always want a bigger image, so sometimes we resample etc.
On nights of very good seeing I can use an extension tube in addition to the extension already given by the filterwheel, and can get up to 14m focal length. For this though, the seeing needs to be very good and the object needs to be bright enough so that you can still use a short-enough exposure. The longer the exposure (to let more light in) the more you lose the benefit of "freezing the seeing" and the features will start blurring together.
If you really want more focal length, get more aperture to go with it. Get a 12" newt or a C14 or something. Best of both worlds.
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