[Doomsayer]

Hello
I've just completed construction of two near identical 12.5" f6.7 Ritchey Chretien astrographs. Yesterday, I put #one onto my Paramount for a the first time - certainly looks the part - see pics below. They are based on 'budget' pyrex optics from Star Instruments and include a 2.7" field flattener doublet and a big 6" secondary mirror. They resemble the RCOS carbon fibre truss 'scope configuration, but are basically my own build design, based on sparse information and advice from diverse sources and a few ideas of my own. Anyone who has considered an RC ATM project would be aware of the limited 'how to' info out there. Early inspiration came from another ATMer in NZ who has the same 12" optics, plus the bloke who has posted detailed pics of his impressive looking 20"RC on Cloudy Nights.

If people are interested, I can post some more pics of the various stages of the process.

They combine off the shelf parts such as a VSI secondary focuser (massive!) and spider, plus A-P 2.7" barrels. I have a conical secondary baffle and an unusual conical profile primary baffle tube of my own design which has stepped rings on the outside of the baffle to mimic a conical baffle. There are several knife stop baffles inside the main baffle throat, plus flocked paper elsewhere. The rest is CNC or hand machined 6061 plate, custom anodised, plus some custom made threaded fittings. I had the advantage of CAD plus 1:1 print layouts to check the design as I went - this allowed me to avoid or solve problems. No significant mistakes as yet.

If I did it again I would use more CNC to lighten up the Al-plate a lot more and maybe look at building a smaller secondary focuser. The two OTAs are beefy at close to 30kg each. I have carefully tested squareness and concentricty of the OTAs with a laser and paper targets - all basically bang on - which is a relief!

I am now of course waiting for the clouds to part so I can start the tuning & testing process with my ST8. The other OTA should get some testing with a bigger chip at the end of the month. I expect that these will deliver a large flat field - to be confirmed.

cheers
guy