Jeff Morgan - Wile E. Coyote School of Telescope Making
Celestron 8SE Dobstuff 13.1": Swayze refigured Coulter mirror, 6 pt mirror cell (2 pt edge support) and CF focuser board made by me StarBlast 4.5 ST80/PortaMount II Zhumell 20x80/Oberwerk 15x70 on a Seronik-style tripod boom mount Hubble Optics 18 inch F/4 mirror.
A 60mm department store refractor started it all....
Quote:It's a curious notion that one would swivel one'e eye all over a 100 degree AFoV but not so for a 70--or even a 50--degree AFoV.
Quote:Seems like your "style of viewing" would have an effect here. People using 100 degree EPs who customarily shift their head to look around the whole field of view (and thus presumably often look directly at the edge) would be more likely to notice edge dimming.
For someone using 50-72 degree EPs, who customarily views the center of field, and only uses peripheral vision most of the time for the edge, wouldn't notice dimming at all since the peripheral vision is more sensitive to light anyway.
Quote:But if one shrinks the UTA too much, it defines the maximum true field of the scope, instead of your low power eyepiece. Perhaps that is worthwhile, I'll have to think it through a little more.
Quote:70% is about 1 F-stop or ~1/1.414. Seems to me like the MTF would be the best tool to quantify the change in image contrast due to changes in secondary size. The rest appears to be about personal perceptions akin to personal perceptions in the audiophile world. IMO vacuum tubes sound best.Don
Quote:Then I realized that the field lens diameter on one the most common low power eyepieces (the 31 Nagler) is not 2", but rather 42 mm (1.65").
Quote:You wont find a high end secondary less than 1.30".
Shane in black-zone New Mexico
Quote:Not really - most of the mirror is still capable of looking well off axis no matter what the UTA, so long as the UTA is at least the size of the primary.
Quote:I worked out a lot of the tradeoffs involved while first analyzing geometrically exactly what size the secondaries should be for large ultrafast mirrors, retaining only full illumination for a small central area. And an associated issue - how close should the barrel of a Paracorr 2 come to the edge of the primary light path, vs the larger secondary required to get it completely clear...Best,Mark
Quote: I should have realized that, but had never thought about it before. My thinking has always been it's worth the extra inch to keep my body warmth away from the tube - but that is my perception, I have never seen it tested. It should be easy enough to do on a cold night.
Quote: Quote: I should have realized that, but had never thought about it before. My thinking has always been it's worth the extra inch to keep my body warmth away from the tube - but that is my perception, I have never seen it tested. It should be easy enough to do on a cold night. One of the advantages of a refractor is that the light spends most of it's time quite distant from the tube. A tight UTA assembly would seem to compromise the thermal issues.Jon
Quote:Well, who says you need to illuminate your lowest power eyepiece to 70% at the edge?
Home-built Newt: 10" f/4.5 Royce conical primary, Skywatcher NEQ6 (Astrotroniks tuned) iEQ45, Duo-T, 8" f/4 Skywatcher Quattro CF 120 ED f/7.5 Skywatcher Black Diamond 80ED Stellarvue
Quote: You are correct in that other than Protostar, I was unable to find another source.... complete with test certificates.
Quote:Just don't make the UTA small enough to vignette the field--why ADD the vignetting of the UTA to the vignetting of the secondary if you're going to skirt the thin ice of "too small" on the secondary.
Quote:Reducing the UTA diameter allows a focal plane closer to the secondary, just like a shorter focuser does. The (beneficial) effect on secondary vignetting is the same.
Quote:Another issue is eyepiece height. The closer the focuser is to the secondary, the taller the scope. In some situations, a larger secondary can turn a scope that requires a stool into a flat foot scope.Jon
Quote:I would not reduce the tube diameter to make decrease the secondary size - at least 1" is needed in addition to the radius of the primary to ensure proper air flow to deal with thermal currents in the tube. In my view, the biggest failing in commercial solid tube Newts is tubes that are too narrow.
Quote: Another issue is eyepiece height. The closer the focuser is to the secondary, the taller the scope. In some situations, a larger secondary can turn a scope that requires a stool into a flat foot scope.Jon