Brightness, coatings, optical design...etc.

Hey all, I just read an add in S&T, and thinking also about earlier posts on here about how a 9Omm etx is "dimmer" than an 8Omm refractor....makes me wonder, Is there a way to calculate approx. how "bright" a scope is or should be? The add I read was about Meades new ultra coatings, saying how the surfaces are upwards of 97% as opposed to regular enhanced coatings which are 89%. Then it said how on a CAT scope you have 4 surfaces. the front of the corrector plate, the back surface of the corrector plate, and both mirrors. My line of thinking is...if you have 1OO% light coming into the scope, then the front of the corrector lets through 89% now the rear of the corrector plate would let through 89% of the remaining which would be 79.2% of the original, then the primary would let 89% of that which is 7O.4% of the original, then the seconday would bring it down to 62.7% of the original, not mention the loss on the eyepeices!!! Now a doublet refractor would have 4 sufaces also, so would be the same? A newt only has 2 surfaces b4 focus so would let through 79.2% ? Of course we have the central obstuction....!!!! What is prompting me to this, is I read a few times that a 4 inch refractor is about equivalent to a 6 inch reflector, but that seemed kinda harsh towards the reflector, but the post about the 8Omm refractor being brighter than the 9Omm Mak, made me wonder? To any who suffered through this long post, thanks much! And sorry so long! Ron

I think the most basic MgF coatings pass 96% of incoming light on average per surface, depending on index of refraction of the glass. Multicoatings can get up to 99% or better. Then you lose a tad to glass absorption, but normally a refractor should pass about 95% before reaching your diaganol. The oilspaced Astro Physics triplets are very good because the oil seems to suppress reflections all the more, removing what would have been 4 more air to glass surfaces within the triplet lens group. Yep, that's why refractors seem to perform better on deep sky than like aperture reflectors or catadioptrics, plus not losing a small percent of light to the central obstruction.