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SCT focal reducer and Canon EOS 40D

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I'm failry new to imaging and am having problems with my set up. I bought a focal reducer from a star party last year - marked as 'Series 4000 f/6.3 focal reducer/field flattener multi-coated China' in yellow caps (no brand).

If I connect this to the threads on the rear cell of my f10 CPC925, then connect a Celestron off-axis guider, T-adapter and canon EOS 40D, I get f3 (and a lot of vignetting!).

If I replace the off-axis guider with an Antares SCT to T adapter (puts less than 20mm between the reducer and the T-adapter), I'm still at f5.

Have I got a duff reducer or am I missing something?

Thanks in advance


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