Here is the place to talk about all those equipment(Telescope, Mounts, Eyepieces, etc...) you have. Not sure which scope/eyepiece is best for you? Trash it out here!
I thought the central obstruction of big newtonian is bigger than the same size SCT?
Yes, if you mean those Newtonian design for Astrophotography. But basically almost all Newtonian will have smaller central obstruction than SCT of the same aperture.
While central obstruction of SCT are quite constant (which is usually at around 34%), central obstruction of non-astrograph Newtonian is usually less than 25% and some well design large Newtonian can have central obstruction of less than 20%.
Have a nice day.
Yang Weixing "The universe is composed mainly of hydrogen and ignorance."
I guess the general idea is that, the larger the f ratio, the smaller is the obstruction. (Visualise it in terms of light rays.) Newtonians that are made for astrophotography (like Eplisons, and R200SS) have very large obstructions because they have small f ratio and they are intended to illuminate the CCD fully.
Hehe, that's why I think for visual observers, it's not good to get Newtonians faster than f5, especially for planetary viewing. Just my personal opinions.
"Hehe, that's why I think for visual observers, it's not good to get Newtonians faster than f5, especially for planetary viewing. Just my personal opinions."
However, as the aperture goes up, a smaller secondary can be used. So it need not really follow that f/5 rule. The problems are that the faster scope will be harder to collimate and the mirrors will be harder to be figured to the same standard as a longer focal length mirror. But.. the larger aperture in the end will give higher resolution, better contrast (due to the bigger exit pupil..higher Signal Noise Ratio) and also is not practical to have a large aperture scope with longish focal lengths.
I have read that some people think that a perfect 6" f/8 reflector can beat a 20" reflector (with good optics) in planetary by saying the smaller obstruction will win that huge reflector in contrast. I guess people who have these opinions are living in countries with really horrible seeing. Also, neither a 6" perfect apo refractor is going to beat a 20" reflector with good optics when the seeing and thermals are good. At same magnification, the planet will be brighter in the larger scope and that would give a huge advantage in low contrast details...
"The universe is composed mainly of hydrogen and ignorance." 
