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Know Where Your Filters Are

Posted by Tom Benedict on 13/02/2015

In an earlier post I described some of what we did at work to fix the damage to the optics on one of our instruments. This instrument has been a constant source of fun and excitement. Prior to the lens damage it had another catastrophic failure in which one of its filters fell out of the filter jukebox and destroyed part of the filter change mechanism. That prompted an immediate instrument change so we could take it apart and fix the damage. But it also prompted a longer-term investigation into how the fault occurred so we could make sure it never happened again.

Something Wrong

As near as we could tell, one of the latches that keep the filters in the jukebox failed, which allowed the filter to slip out and into the space the guide rails occupy. When the jukebox came down, it smashed the tips off the guide rails. We held a brainstorming session to come up with ideas to stop this from happening and to detect if it ever does. The preventative measures were relatively straightforward. The sensing took more work. Some of the approaches were relatively easy to implement, but didn’t give us the level of sensing we were after. One of the more difficult and invasive approaches gave us all of the information we wanted, but would take the most time to implement. Needless to say that’s the option we ran with.

The filter jukebox has eight slots, so the instrument can hold eight filters. We added proximity switches to each slot so we can tell if it’s occupied by a filter. We also added sense lines to each of the latches that act on the filters so we can tell what state they’re in. Finally, we added a ninth proximity switch in the in-beam position so we can tell if a filter is in the beam. This lets us detect filters falling out of the jukebox, like the one that caused the earlier failure. It also lets us sense if the filter change mechanism had a mechanical failure and let go of a filter before it was either fully deployed in-beam or fully stowed in the jukebox. Basically, it lets us know if the mechanism does anything it’s  not supposed to do.

Installing Electronics

The electronics were designed and built by one of my co-workers. I did most of the mechanical work – the proximity switch plate, the light baffle, the modifications to the bulkheads to pass the latch sense signals through, etc. My boss and my co-worker did all  of the cabling and came up with the test plan.

New Filter Sensing Instrumentation

It took almost a week to install the electronics. Some of that was spent building and testing cables. Most of it was spent dealing with what we call “summit issues”: things that go wrong simply because they go wrong. My one contribution to the electronics was to build a ribbon cable. Seems simple enough. I’ve never messed up a ribbon cable. I mean, how hard can it be, right?

When I plugged in my new cable one of the proximity switches self destructed in a most spectacular way. Hissing, a bit of flame, and gouts of brown smoke gushing out from under the proximity switch plate. We took the entire system back to headquarters to replace the damaged parts and figure out what went wrong.

Back at HQ I took the ribbon cable apart, expecting to see a bent pin, a flawed ribbon cable, or any of the other likely causes for such a failure. What I found was an aluminum shaving wedged between two of the pins. Apparently when the mounting holes for the new electronics were being drilled, some metal chips fell into the box of spare parts and one of them wedge inside the IDC connector I used to build the cable. Including the drive to and from HQ and the time I spent cleaning the smoke damage off of all of the filters, total time lost was close to a day… for a metal chip. “Summit issue.”

Happiness is a Working Instrument

We finally finished all the work yesterday. We tested it thoroughly, then tested it again. Then tested it again. My boss was kind enough to pose for me at the end of what for him has been seven straight weeks of work on this instrument. (So if he looks a little stressed, cut the guy some slack.)

Back on the Telescope

With all new rails, jukebox rails, filter frames, and a wonderful new sensing system, it was finally time to put it back on the telescope.

– Tom

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