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Old 13-06-2018, 11:15 PM
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Wavytone is offline
Join Date: Jul 2008
Location: Killara, Sydney
Posts: 4,147
Bill how much mirror grinding experience do you actually have ? What you propose is not a small undertaking even for those with considerable experience and a grinding machine to do the hard work.

Two suggestions:

1. Make the primary with a long focal ratio - say f/7, and use the 8” as a flat secondary to reflect the beam back down the OTA. A secondary that big will result in an OTA around 2.5 metres which would be manageable as a pole truss.

From there use a second diagonal flat to bring the beam out to the side. Although it’s not a nasmyth, this would put the focus at a pretty reasonable height for most observers - and without needing ladder.

2. Make it a 20” f/5 conventional newtonian. A whole lot easier, and a lot more practical to observe with as well. Buy the elliptical flat secondary to match. Turn the 8” into an 8” newtonian, say f/4, and it would serve nicely as a super finderscope on one side.

Even when mounted as a dob you’ll need a ladder to use this... and ladders are very very dangerous in the dark.
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