Tuesday, April 12, 2011

Day 14, 04/12/2011 -- At last, helium.

With the arrival of Franky from McGill yesterday, today he set about setting up the bolometer readout crates (hereafter: BROs). After first assembling the final crate, he identified and fixed a power issue related to a bent pin that caused us to damage 2 (very expensive) readout boards back in Minnesota. The main problem he and Kyle were having was failure to communicate -- no, not because of his Quebecois accent, but because they were having network problems between the computers and the BROs. In the end it turned out to be something misconfigured in one of our network switches, but it took a half day to figure out what the problem was and how to fix it.
In the meantime, Kyle, Jeff, and I started thinking about some of the upcoming calibration experiments -- how to mount a source on the water tower and be able to aim it, how to suspend a styrofoam bucket with LN2 above the cryostat window, etc. One of the outcomes of this discussion was a nice little mount based on a benchtop optical mount, a spare hexapod clevis, and a piece of scrap aluminum. This mount, an inverted altitude-azimuth mount destined for holding our millimeter-wave source atop the water tower, will allow us to map the beams of the telescope once the gondola gets here.

But the main advance of the day was helium, and lots of it. We finally got our gas regulator from Minnesota and could proceed with cooling down the cryostat with liquid helium. After pumping out most of the LN2 from the helium tank and then boiling off the little that remained with warm nitrogen gas, we then proceeded to transfer liquid helium out of one of the large 500L storage dewars we ordered. In the end we used about 250L of helium to cool and completely fill the EBEX cryostat's 130L helium tank. Because the optics box and focal planes inside are isolated from the 4K cryostat shell, they still need to cool down, and as they do, they boil off massive amounts of liquid helium. Thus, for the next few days, we'll be doing multiple fills a day, at all hours of the day or night -- Jeff and I are scheduled for a fill at about 3AM, and there will probably be another stupidly early/late fill tomorrow as well. Ah, the joys of having a big cryostat!

Pictures, again, finally:

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