Expert Amateurs

I haven't been doing photography lately because I've been heavily engaged at work. Which is typically not a bad thing for me. Even so, this last week has been rough. We've been coating a 4m class telescope mirror; a process that takes three days when things are really clicking, and longer if they aren't. This one took three very full days. I left the house at 5:30am each morning, and typically got home after 9pm each evening. Added to that, all the work happened at 14,000' above sea level. It takes a toll on a body. Trust me.

By the end of a schedule like that, everyone is tired, a little loopy, and ready for the whole thing to be over. The catch is this: not every coating comes off first-try. Any contamination, even the size of a fingerprint, can cost an entire coating. What's worse, you never know until the whole process is done and you take the mirror back out of the vacuum chamber. At that point you get to find out if you go home and wake up to a new day the next day, or whether you go home knowing you'll be waking up the next morning to strip the bad coating off, re-prep the equipment, order new chemicals, and start over. It takes a toll on a body. Trust me.

As the chamber was pumping down, an odd thought occurred to me: Here was a whole team of experts in our field, but mirror coatings happen so infrequently, in essence we were all amateurs. What's worse, practically every monolithic professional telescope mirror is coated by amateurs. There's really no escaping it.

Most professional telescope mirrors are coated with bare aluminum. Since telescopes are used outside by definition, they're exposed to the elements. Wind, dust, fog, and rain (yes, even rain...) slowly destroy the coating, so most mirror coatings only last three to five years. Then they must be stripped and replaced. For a telescope with a monolithic mirror this typically means the whole place is shut down once every three to five years so the telescope can be taken apart, and the mirror removed for re-coating. It doesn't make sense to have people on-staff solely to coat mirrors, so this usually means engineers and technicians who are otherwise occupied 99% of the time will be the ones doing the job. A job they have to re-learn every three to five years.

Yeah...

The place where I work is in a unique position. We're one of the older 4m class telescopes, so as newer ones were built in the same area, they had the option not to include coating equipment at their facility and to use ours instead. Several of them took this route, so we coordinate coatings for a number of telescopes. Running the numbers, we do about one major coating every year or so. This doesn't happen like clock-work, so some years we'll do two in a given year, and at other times we won't do any for years at a stretch. We've got more experience coating mirrors than most of the people in the area, but even so it's something we re-learn each time.

This past year we've spent weeks running experiment after experiment, trying to refine our technique: what chemicals we use, how we perform each step, which steps to include, etc. We're trying to make the process safer for the people involved and for the environment, and at the same time we're trying to build in as much assurance as we can that the process will produce a good coating every time. Leading up to this coating we had done dozens of small mirrors. If you needed to find someone with practice and experience, we were it.

Even so, there's a huge difference between coating a 12" mirror and coating a mirror more than ten feet across. The last one we'd done that size was our own, a year ago. Experts? Yes. But expert amateurs nonetheless.

We still don't know how this one turned out. The thickness was good, and the metal stuck to the glass quite well. But there's a blemish in the coating roughly half a meter square. We won't know how this affects the performance of the mirror until next week, when we can measure reflectivity and scatter in that area. At that point the people responsible for the mirror will make the call: Keep it as-is, or do it again and hope it comes out better next time and not worse.

Which brings up the reason we're all fine remaining amateur experts: Getting in more coatings per year typically means coating the same mirror twice. Coating the same mirror twice means things didn't work out right the first time. If that's the only way to become an expert, it's not worth it.

Tom