Somewhere near the beginning of his "star-testing" book, Dick Suiter reports that the two finest telescopes he has looked through were Newtonians. I have been a strong refractor advocate for nearly 10 yrs, but don't have much hope of owning a 8-10" refractor, and mounting it. I seek opinions on this topic.
I've been shopping by 'net for a maker of fine telescope mirrors, and believe I have found one. I am starting a long term project -- a 10" f/8 Newtonian, a super-planetary instrument, possibly with an optical window to support the secondary mirror instead of sheet metal vanes. This cuts down on the diffraction 'smearing' of the image, yielding beter resolution on low contrast objects, ie planets. An optical window also seals the scope, keeping out crud, and possibly suppressing the air turbulence within the tube as the mirror cools toward ambient temp. It also leaves the door open for experiments in helium* filling of the tube, again to eliminate 'tube currents'. Helium has very high thermal conductivity, and low viscosity. This leads to excellent mixing within the tube and reduced variation in refractive index of the 'air' due to temp. variation. It also should help cool the mirror quicker. Helium's refractive index varies with temp. much less than air. I fgure that even a part-helium tube filling (eg. 50% He) would help immensely.
I don't envision using helium every observing session, just those rare nights that have excellent seeing. I know the helium will leak out and must be replenished each session, unless I'm very lucky in sealing the tube.
A super planetary scope is also excellent as an all-round scope, but not vice versa. I'm thinking of making a foam-core tube with epoxy and glass/kevlar. A sonotube will be the mould, and stripped out later. The tube is going to be 14" dia x 100" long. Mounting this will be a challenge, due to the large angular momentum it will have when vibrating. I have been thinking of a reinforced concrete 'moveable' 3-legged pier, with a very solid German equatorial mount ( 2" or large dia shafts). I have no interest in astrophotography.
*helium filling: see Texereau, pg 192, 2nd Engish Ed.
Best wishes, Larry Manuel, Calgary, AB.
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