Usually the off-axis stop Newt will incur an image brightness penalty of about 15-20%.
Given a guess of about 90% reflectivity for the average mirror coatings, you have 90% x 90% = 81% total reflectivity for the primary and secondary of your Newt. Enhanced coatings will of course improve the numbers. Old coatings will degrade the numbers.
The usual modern APO (multi) coatings are more like 95% or better overall for all 4 (or 6 for triplets) surfaces. Better for oiled objectives - only two surfaces involved. Even old style Mag fluoride coatings will get you approx. 1.5% loss per surface. No coatings? About 4% loss per surface ~ 96% transmission each. .96 x 0.96 x 0.96 x 0.96 = 85% - so old style, uncoated doublet is a closer match for your average off axis stop Newt brightness. Kinda makes you think about the importance of good coatings in your favorite eyepieces, huh?
Edited by siriusandthepup, 17 April 2019 - 01:54 AM.