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Thanks Doug.
The Sky 90 with the field flattener/reducer is fast at 4.5 but only 400mm. I sometimes want to get in closer, hence the 2X Canon extender idea to bring it up to 800mm. Guess it would be F9, still doable, but the sharpness of stars worries me. If I was only imaging the moon or planets, I could use the Tak extender 1.6.
Perhaps I need a new scope or try my 10" LXD 55 at 1000mm.
Craig
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