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Another thing to keep in mind is that the f.l. and hence the f-ratio of a Cassegrain is not fixed and depends on how far the EP is placed from the back. A 2" diagonal will slow it down some. So e.g. instead of 20mm you might want to go with 22-24mm. For same reason, you might also find that the 2x powermate is greater than 2x.
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