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Hi Jeff
I'm not sure which ED80 you are referring to but if it is the Skywatcher f/7.5 the following may be of some help.
Based on my own experience with using focal reducers and tele-extenders with the GStar camera and using SkyTools to marry that data with the specifications for the Skywatcher ED80, I believe that you will likely get the following FOVs :
With 2x barlow – 18':55” x 13':40”
At prime focus – 37':53” x 27':22”
With x0.5 focal reducer (it replaces the nosepiece) – 49':11” x 35':35”
That gives a fairly useful imaging range. For example you could almost, but not quite, capture the whole of the Sculptor Galaxy NGC253 when using the barlow. And at the other end of the scale you would need a 2 x 3 panel mosaic to mostly cover the Pleiades M45.
I can't opine on what, if any, vignetting you might get using the focal reducer.
David
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