Washing a 16″ mirror December 20, 2006Posted by dorigo in astronomy, personal, science.
Yes, that’s right. Every once in a while, even telescope mirrors require washing.
True, most of the dust that over time deposits on the surface does not do too much harm, either in terms of reflectivity or in terms of resolution. The dirtiest ones I’ve seen were abominable, but they still delivered at above 95% of their nominal light-collection power.
Nevertheless, a clean mirror is better than a dusty one, especially if you need that extra bit of contrast and luminosity that gives you a tad better glimpse of a galaxy far away. So today, being home for several reasons, I decided to take some time to do my own very special laundry.
My mirror does not have a quartz coating on top of the aluminum layer. It is a choice dictated by wanting the most reflectivity possible: the quartz coating takes away some 1-2% of the possible photons that your eyepiece collects. On the other hand, the protective effect of the quartz over the very thin aluminum treatment is overestimated: pure aluminum will produce by itself a thin oxide layer at contact with air, which is very strong and is enough protection, especially if you are going to make a fresh aluminum coating every couple of years or so – which is my case, since Romano Zen does it at an affordable price.
So, is it dangerous to wash your mirror ? Yes and no. You have certainly to be very careful to use tools which are perfectly clean, lest you damage the surface. But water and soap will do with a soft sponge.
That is what I did this morning: I placed the mirror on the floor of my shower box, and gently brushed it with the water-dripping sponge and a little soap. Then I rinsed and let it dry at a small tilt angle. The result ? All the dust is gone, and most of a bad oil stain as well. I estimate that the reflectivity is back to some 99.995% of its original value (since about 6 mm^2 of the 125600 of the surface are still affected by stain or black dots), while before washing it the non-reflective surface had brought that number down to 99% or so.
Big deal ? Well, not really, but when you own an instrument that allows you to travel through space like a big dobsonian reflector, you want to squeeze the most out of it. Every photon counts! Last week, as I was observing M77, a giant galaxy which is about 60 million light years away from us, I thought that the 70x magnification power and 4000x light gathering power provided by the optical system I was using was effectively allowing me to be looking at the galaxy as if I had traveled for 59 million years at light speed toward the galaxy. I was looking at it as if I had been only 1 million light years away!
So, a 1% increase in light gathering power might seem ridiculous, but think about it: it means getting a light increase as if going from 1 million light years away to 0.9999 light years of distance. That means having traveled in space for one light-hour, or about a billion kilometers closer!