I could not find an optics forum, so I'm posting this here. I'm trying to learn about the optics of imperfect real mirrors and prisms. I dug out my old physics book, but it just barely touches on this. 1. I read that most aluminum coated telescope mirrors reflect about 90% of the light that hits them. Where does the other 10% go? How much is absorbed? How much is reflected at the wrong angle? Does the 90% only count the light that is within 0.1 degree of accurate? I asked this question on an astronomy forum, but they were not interested. 2. I read that if the surface roughness is less than 1/4 wavelength (1/4 of 500 nm), an aluminized mirror reflects 90% of light, and further smoothing has little effect. I also read online that with surface coatings, 99% of light passes through a lens. Does this mean that the lens has better light gathering for it size than the mirror? (forget about any secondary mirror for now) How accurately does a lens focus monochromatic light, compared to a similarly smooth mirror? Assume both are hyperbolic, but only smooth within 1/4 wavelength. 3. Finally, I read that a 45 degree diagonal inside of a prism reflects about 100% of light that hits it. Is that true? How can it be better than a mirror, if both have surface imperfections? I have other questions, but that will do for now. Thank you very much. In order for me to see dim galaxies with binoculars, the binoculars need to be good. Any light that does not make it through accurately can scatter and cause ghost images and haze that make it hard to see the galaxy. The lower brightness is a non-issue compared to the scatter.