Ever wonder how much larger an obstructed system needs to be to match a refractor in light grasp?
I have. And many times have researched coating types, central obstruction diameters and the like, and done comparative transmission/reflection/occlusion analyses of specific scopes. But that's a lot of work. So looking at some of the past analyses I've done, I've come up with a proposed rule of thumb - the "Rule of three 7s". Here's how it works. To determine (roughly) the how a given obstructed scope (SCT, MCT, Newt, etc.) matches in light grasp/throughput to a refractor in terms of equivalent aperture, multiply the aperture of the obstructed scope by 0.777 to derive the aperture of an "equivalent" refractor.
A 90mm MCT would have throughput equivalent to a ~72mm refractor.
A 16" Dob behaves like a ~12" refractor.
An 8" SCT operates like a ~6" refractor.
A C5 works like a 100mm refractor.
Why would the rule of thumb work for both catadioptrics and Dob/Newts? Sure the latter generally have smaller COs and two fewer surfaces, but they also generally have standard mirror coatings whereas most modern SCTs have highly enhanced coatings on both mirrors and correctors. Despite greater occlusion by the secondary, SCTs generally put through more light than equal aperture Dobs with "standard" mirror coatings.
Anyway as a rough rule of thumb what do you think? It works in reverse too. Just divide the aperture of your refractor by 0.777 to get it's peer obstructed aperture in throughput terms.
A TEC 140ED works like a 180mm obstructed scope. In terms of dimmest star seen this is correct in my experience. I had a 180mm Russian MCT, obtained the TEC later, compared them, and found that at similar magnification the TEC actually went as deep if not a hair deeper than the 7" MCT. I offed the MCT since it lacked the TEC's other benefits of rapid cool down, wide field capability and collimation robustness.
Clearly throughput isn't the only relevant consideration when choosing telescope designs, but it is one important criteria when comparing scopes of different designs with different apertures.
Edited by jrbarnett, 17 March 2015 - 10:40 AM.