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Archive for July, 2017

Vacuum Chamber Camera Mounts

Posted by Tom Benedict on 26/07/2017

August 1st we’re taking our telescope apart so we can re-coat the mirror. To date I’ve helped coat mirrors from four telescopes on the mountain including those from CFHT (M1 and M2), Gemini North (M2), IRTF (M1), and UKIRT (M1). Later this year we’re coating the 2m M1 from the Las Cumbres Observatory Global Telescope Network’s Faulkes telescope on Maui. Next year (we think) we’ll add a fifth Big Island telescope to the list when we coat the M1, and two M2 mirrors from the University of Hawaii 88″.

All of these are done in the basement of the CFHT Observatory, my home away from home.

CFHT Aluminizing Room

Mirror coatings offer a unique opportunity to photograph a really cool process: going from a bare piece of glass to a mirror in under 60 seconds. The only problem? The coating chamber itself is one huge, opaque piece of steel.

The Chamber

Luckily, the chamber has two windows installed in it. One lets you view the mirror. The other lets you view the filaments that evaporate the aluminum onto the glass. They’re relatively small, with a clear aperture of seven inches, but with some careful arrangement it’s possible to position cameras to peer through them and photograph the flash.

But it’s not easy. In years past I’ve balanced tripods on relatively small platforms, bolted makeshift 80/20 frames above and below the windows to try to support cameras, and even resorted to using tape. At times my solutions have been downright ugly, but they’ve worked well enough to let me photograph the transition from glass to mirror.

Before and After - Telescope Mirror

This year I wised up. I asked my boss if I could spend a little money and solve this problem once and for all. “Sure,” was his reply. That one word solved everything.

The windows are held in place by a ten inch diameter ring and six 3/8″ bolts. I couldn’t modify the rings, but I could install longer bolts and add tabs that let me attach additional hardware.

With the tabs in place I was able to install four 11″ articulated arms that I ordered from Amazon (two per window). If I had the option I’d install more, but even with two the windows become crowded quickly.

Because the two windows offer such different views of the chamber, the photographic requirements differ as well:

The window on the top of the chamber – the one that offers the view of the filaments – is used for diagnostics: Did all of the filaments fire? Did the aluminum wick onto the filaments without dropping? Did the aluminum vaporize without spatter? These require a relatively wide angle view, so I mounted a GoPro on one of the arms. On the other I mounted a Pulnix low-light video camera that’s wired into a monitor down at the controls to the coating chamber.

Cameras on the top porthole

Because outside light will reflect off the chamber window, causing glare, when we’re using the cameras we have to keep them covered.

Top porthole black-bagged

The window on the side of the chamber – the one that offers a view of the mirror being coated – is used to observe the coating process. Was there arcing during the glow discharge stage? Did the glow discharge generate a usable plasma? Did all of the filaments fire? Did the glass get a coating of aluminum?

This window is a little more complicated. To observe the glow discharge and the filaments, we installed a convex mirror at the base of the window. A Gopro aimed at this mirror can see, albeit in a very distorted way, most of the filaments and almost all of the glow discharge cathode. To observe the mirror, I used a DSLR with an 11mm lens pressed flat against the window.

Cameras on the side porthole

The side window also needs to be protected from stray room light, and will be black-bagged the same way the upper window was. Before the next coating I’d like to make proper blackout bags to cover the windows, but for now the cloth-and-bungee method will work fine.

This year I hope we get some really excellent video and stills of our mirror coating without having to resort to duct tape and bailing wire!

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