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Ok Brendan just a though as I believe the Nasmyth design is somewhat underated and underused
In fact does not require the primary to be reconfigured and you would only need one additional fixed mirror.
Oldham Optical Astro Page The extra cost of the elliptical flat is not significant when the cost of the other two mirrors is considered.
If the elliptical flat is towards the primary end of the tube, there are no extra optical obstructions and as an extra bonus, the baffling needed in standard Cassegrains is not required.
Stray light has a lot more difficulty in reaching the focal surface with this design.
Effectively halving the length of your optical tube.
JM2BW
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