I'm not complaining about the measurements, I'm just curious. So we can see what's causing the scatter. I can see in the top diagram that none of the light should be reaching the camera (only scatter will). The mirrors shown on the right in my two dark-field images there are perfectly fine, and look virginly magnificent in normal daylight and in use. planet-finder coronagraphs) the optics must be simple and obsessively pristine. For rare, highly specialized applications (e.g. For most applications, filthy-looking mirrors perform as well as pristine ones. This includes both qualitative mapping of the flaws and quantitative metrology of their Total Integrated Scatter (TIS). Dark-Field illumination of mirrors and systems in order to characterize cosmetic flaws. I actually did an ~Independent Study~ white-paper on this a while back. The "Flashlight Test" is great for revealing dust, gray, etc. Whatever the source, I bet there's a good chance that the reason a freshly cleaned mirror still looks dirty when shining a bright light down the tube is because the mirror coating has begun degrading in select spots. So either there is stubborn contamination on the surface that even warm soapy water and finger tip swishing can't get rid of, or the act of cleaning with water begins the oxidation process of the mirror almost immediately, and what you see when you shine the light down the mirror are imperfections in the protective overcoat that allowed the aluminum to start to degrade in those specific spots (or the contamination that was sitting on the mirror was acidic, such as pollen, and that was eating the coatings). Nowhere near as pristine as when it was new. It was totally, 100% clean.Īfter a recent cleaning, when re-installing the mirror, it indeed has a mild sparkle effect - not to the same degree as the 3 year old mirror it replaced, but it's there. It looked like a light shining from the bottom. When I first installed my new mirror about 9 months ago, aside from one or two motes of dust, you could not even tell there was a mirror at the back of the scope. I suspect that either the cleaning procedure is not thorough enough (we tend to be very gentle with our mirrors for good reason), or the "dirt" is actually the beginning of oxidation. I thought it might be nice to understand WHY it looks so dusty like that, as well as a word of caution to those of us who forget what we've been told time after time! I'm kinda over that now, yet I still get that itch to clean the mirror and have to talk myself out of it.
Before understanding this condition, I would shine a light down tube and think "Dang! That mirror got dirty really quickly! I just cleaned it!!!" I feel like I have been able to bear with the dust better since, but it still creeps back into my mind when I have those nights of less than favorable seeing or poor transparency. I've seen guys warn about assessing mirror cleanliness in this manner, after I fell for it once in the beginning. I have to admit I've shined a red light, or even a white flashlight, down tube at night ( in the field ) and the mirror looks like it's bathroom-floor-behind-the-toilet level dirty, only to be spotless when removed from the tube!