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Posted by John E. Siers on August 15, 2001 21:03:03 UTC

The presence of a "corrector lens" in a Newt's optical path doesn't necessarily indicate a spherical mirror (though it may in a cheap one). The real purpose is to let you get a higher focal ratio out of a shortened tube. Orion does, in fact, sell a similar telescope -- their ShortTube 4.5 EQ.

This is sort of like building a Barlow lens into the path. The mirror itself may be f/4 or lower, but the lens "multiplies" it. Otherwise, a 6" f/7 would need to have an optical path 42" long. Orion's ShortTube is an effective f/8.8 (1000mm) and would need an optical path almost 40" long -- kind of hard to pack into an 18" tube. It's the corrector lens that makes it possible, and you can figure that the mirror itself is probably an f/4.

On the other hand, you are right in one sense. It is much more difficult to make a parabolic mirror in a short focal ratio. At long focal ratios, the difference between a spherical curve and a parabolic curve is slight, but that's not so of ratios around f/4 or f/5. Any curvature errors in the mirror will be much more noticeable.

Clear skies,
John Siers

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