150mm MCT f/13, 31% CO
"People say I'm in denial. I disagree."
18" f/4.2 homebuilt truss 12.5" f/6 hybrid tube dob
Quote:You should think also of the mirror cell.Indeed the 3 points cell needs to be avoided.But consider also the fact of the 6 or 9points cell supporting a thin mirror. The flexions involved by the system may not even perceptible in perfect seeings. An excellent mirror say ptv 10 may be destroyed even with a such cell that does at final only a ptv5 for example. This should be analysed carefully, not only by computing but on sky. 200mm is easy to push at 500x even under average skies.Thin mirror cool faster indeed and the tube design is also important.You may think a clear open optic will solve the situation as the zambuto photo. This can work under dry nights with no moisture deposition and no observer close to the optics.This is not a solution.I found personnally the plain tube made of resin (parks for instance) the best solution and closed with a parallel faces window at the front for limiting the air exchanges with outside. With injection of dry neutral gaz inside, you solve almost the instrumental seeing, the air currents for the most.After the thickness of the mirror has a relative importance hidden by the tube design.When the scope is permanently installed outside, temperature equilibrium becomes not a problem.I did this on a 200mm Cave in the 80ies and this worked fine, and the problem was rejected to the site observation quality and the local seeing only. A step of the seeing levels was almost annealed. Seeing being composed with the instrumental, the local and the altitude seeings.I think what is usually attributed to the site observation is actually more coming from the instrumental seeing, in fact the more nefast step.Yesterday evening I was with the 235 cassegrain with an open tube always settled outside, images were 4-5/10 only, almost no air currents inside the tube (short) and jupiter was at 200x almost stable at the eyepiece and the contrasts blurred by the high altitude seeing. GRS was colored and distinct with the tropical band with lot of festons, etc...Not necessary to go to the 305mm this was with less features.The cassegrain mirror is a glass bloc that doesnot be subject to flexions.A last point, little ptv involves more sensitivity to seeing that is normal.Hope this is an help for anybody.Stanislas-Jean
Quote:Hi Pete,Interesting report, thanks for posting it. Sounds like the fans are a big help in dealing with the boundary layer.Best,Eric Jamison