Above, the primary mirror is lifted, very carefully, off its mount revealing the complicated mechanisms we don't often get a chance to see. At the top right is the centre of the mirror with its ventilation system for mirror cooling. We blow cold air over the primary mirror during the day to keep it cold and matched to the anticipated air temperature at night. This has had a huge and positive effect on our image quality, especially at the start of the night. The stuff around it are simply reflections of the telescope structure above.
The primary is slowly moved away from the mount revealing even more of the structure it normally rests on. The rings of small devices are the actuators we use to bend the mirror by tiny amounts to correct for the effects of gravity as we slew the telescope around the sky.
It's a painstakingly slow process but this is not cheap optics like my camera. Several tonnes of glass are gently being moved to the truck which will take it over to the CFHT for its new coating. There is no replacement for the primary mirror, we can't afford to make any mistakes.Looks a bit like a flying saucer, doesn't it?
It's on the truck and now a protective cover is placed on top for the few hundred yards journey to the CFHT. The whole process shown in the pictures took a couple of hours, but it's already three days into the project just to get this far. This isn't something you can do quickly.
With the primary mirror on its way I took the opportunity to take a few pictures of the mount and the actuators. Getting these to perform correctly with WFCAM has been a large part of my job over the years and it was a good chance to get a close look at them before they were all tested later in the day. As you can probably tell from the valves, they are operated pneumatically under some very precise control via software - a system I've come to understand rather well!
A closer look at one of the actuators and pneumatic line. The primary rests on top of these and they move up and down as the telescope moves in order to reshape the primary mirror and keep image quality consistent no matter where we're looking in the sky.
I don't know what that material is on top of the actuators but it looks like cork! I'll have to ask.
Finally, the top-end or the mechanism that supports the secondary mirror which is usually at the top of the telescope but now sits in the lab. This is another part of the telescope that I've been responsible for over the last few years as we use it to move the secondary mirror to compensate for gravitational-induced flexure in the WFCAM instrument as well as compensating for atmospheric turbulence (often moving and adjusting its position 100 times a second). It's been giving us the odd problem recently so we'll be having a close look at it over the next couple of weeks.There's been a lot of effort by our engineering crew to make this project work and there's more to come, but it was great to see yet again what a professional and hard-working bunch of people they are. And a lot of fun to work with!
8 comments:
Fascinating photos...really well-done too; a lot of detail can be seen even without my reading glasses, lol.
that is so cool!
Tom, thanks for the great pictures.
Maren - wish I could have taken better pictures but it was a little busy up there!
Anon - I couldn't agree more.
Beep - for once I had to forget taking really nice pictures and just snap away instead.
Tom
Cool pictures! Thanks for a look into this process!
I have a question: What is the mirror actually made of? Because when you talk about bending it using the actuators, I realize the mirror can't be like mirrors in a home: Made out of solid glass. Or can it?
Keera - great question and to be honest I don't know the answer. The mirror is pretty much normal glass although there may be some small chemical differences. Unfortunately the person who would have been able to give you a quick answer passed away a few months ago.
The bending is tiny, you wouldn't notice it with the naked eye. The wavelengths of light we observe are of the order of a few microns (1000 times smaller than a millimetre) and to correct for gravity the changes we need to make to the mirror's surface are in the micron range.
I can imagine your picture of a brittle piece of glass but the primary is actually very thick (you can get an idea of how thick it is from the UFO picture in my post). We only need to bend it a minute albeit precise amount which comes nowhere near to stressing it.
Try pressing your hand on one of your windows at home (gently, please!). I bet you can bend it more than we change the shape of our primary.
Tom
I did read years ago that glass is actually a very, very thick liquid; it never completely solidifies. Still, what I drink water out of feels pretty solid and unyielding to me. Not going to try the window thing because my windows are some kind of acrylic. But now I know your mirror is a thick slab of glass.
I've always thought of glass as the in between substance between a solid and liquid. A thick liquid is a nice description. Glass does flow like a liquid except the process is very slow, much longer compared to our lifetime, which is why your wine glasses are probably safe for now!
Tom
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