Hafnium is still applied to Starbright XLT OTA's, and the corrector plate actually has a very good transmission of ~97%.
The thing we must not forget is that Celestron provided cumulative system reflectivity/transmission numbers. Meaning how much of the light that reaches the scope, actually ends up in your eyepiece (focal plane).
To do this we have to calculate the light loss at the corrector plate, primary mirror and secondary mirror.
For Celestron XLT, the corrector plate has 97% average transmission, primarry mirror is average at 94% rsflectivity and secondary the same at 94%. I say average because the reflectivity varies over the wavelength spectrum.
This means that roughly 86% of the light that gets into the aperture of the scope, pass the secondary obstruction, ends up in the eyepiece of your fancy Celestron SCT with XLT coatings.
If we take the most widespread newts, lets say synta, they have average reflectivity around 91-92%. If we say 92% for primary and secondary:
That means that your average asian newt has roughly 85% system reflectivity which is on average 1% less than the SCT with XLT. But 1% is pretty much nothing, so we can say that the same amount of passing light per aperture ends up in your eyepiece. The actual amount/volume of light is modified by central obstructions.
This is light % at the focal plane. From here to your eyes, it has to go through an eyepiece. And it is the quality of that eyepiece that usually has a bigger impact on the image, rather than a few % difference in reflectivity.
Edited by Recretos, 28 December 2018 - 05:56 AM.