Today had an unexpected hiccup–we found a devil amongst the details. In the morning, Jane, Alan, and Zach continued with the final harnessing. Steve completed the battery boxes. I worked on a bit of thermal analysis before installing the rest of the shield pieces with Alan.
After lunch, though, Jane was taking a look at the shields and the cryostat-side harness to make sure everything was fitting right. When she checked with a multimeter, though, she found an electrical connection between the cryostat and the gondola! To minimize electrical noise, the detector-cable-card cage chain is electrically isolated from nonessential parts of the gondola, typically with insulating spacers. Since there is precious little play in the shields or the detectors, we were fretting about how to find and remove the electrical contact.
By this point the whole group was taking a look at things, and it got worse–the card cages themselves weren’t isolated from the gondola! So, potentially we had problems on both ends of the chain. On closer inspection, we found that the metal card cages were directly touching to the metal rails holding them. We had to interpose an insulating layer to give us the isolation we needed. After brainstorming about cutting mylar film or using Kapton tape to insulate the rails, we asked ourselves why we didn’t have this problem in the New Mexico flight–the isolation check was part of our preflight checklist. Eventually, we figured out that we had omitted half of the fiberglass washers surrounding the bolts which attach the rails to the card cages! I had simply misremembered how they were mounted.
Fixing that oversight took a couple of hours and involved removing all the card cages from the ebay (again!). The good news was that adding the washers isolated the cryostat from the shields as well, so the shields don’t have to be moved. We’re back up and running as we expected again. As our old family saying goes, if you don’t have it in your head, you’ve got it in your feet! It does underline the importance of regular, step-by-step testing.