Playing Catch Up

     We finally got Black Cat closed around 3:30 AM early Sunday morning. It got down to a low enough pressure Monday morning to start cooling down, and we expect to start the first cooling cycle to get down to operating temperatures around 1:00 PM Thursday, which means we'll be ready to start taking initial data around 11:00 PM or midnight the same day. But that means the next couple days aren't all that busy, so we mostly took Monday off. I slept a bunch, then took a 3 hour nap in the middle of the afternoon, so my sleep cycle is a little messed up. I can't sleep right now, so I thought I'd post about some miscellany that I've missed the last few weeks.

     When the Norwegian Prime Minister was here a few weeks ago we took a station photo with him at the geographic pole. Here it is. I'm directly under the Norwegian flag, three people below it.

The entire station took a group photo with the Norwegian Prime Minister.

     Christmas Eve was the Race Around the World. The race is held every year and starts and stops at the geographic pole. This year the race was only 2.3 miles long, but it was plenty difficult given the altitude and the crappy weather we had. Here's a few photos taken by Chris Kendall:

People are encouraged to wear costumes for the race. This photo was taken where the runway intersects the path to SPT, about 1/5 of the way to the telescope. You can see how crappy the weather was. Windchill was something like -40 F.

This woman is normally a fuely - one of the folks who refuels the Hercs - but during the race she was a joggler. She juggled those balls the entire time.

Proof I ran the race. We're maybe 1/4 of the way through the race at this point and I already have some decent frost building up.

Some of the folks from the IceCube neutrino experiment built a chariot and were pulled by a van for the length of the race. Most people run or walk the race, but a handful ski, bike, or come up with some other mode of transportation.

Taken from the same location as the above photos, but looking the other way toward SPT, which is barely visible in the background.

The winner passing the ceremonial pole and approaching the finish line at the geographic pole. He finished with a time of 19 minutes, 3 seconds. I finished at 31:10.

     Chris left this past Saturday, but the day before he left he snapped this photo of me holding up the "Big Top" mylar tent we eventually installed on the backside of the focal plane. I coated the backside of the blanket with kapton tape to keep the blanket from shorting the pins sticking out of the 90 GHz pixel circuit board. Kapton is sort of orange in color and he grabbed this when the light from the ceiling was reflecting onto my face off the kapton just right to make my face glow golden.

My face glowing from ceiling light reflecting off of kapton on the back of the mylar tent we made.

     Here are some photos from our last close up. (I hope it was our last…) We made a number of changes. I mentioned the extra rings and braces we added to keep the readout towers from shaking in the last post. Here are a couple more shots of all that. We also added an extra filter to the front of the camera to reduce the power coming in and heating it up.

Top-down view of the changes we made to the back of the focal plane. The top anti-shake ring is visible, along with the aluminum bars connecting the three center modules and the braces holding the two outer towers to the ring. Below it is the mylar blanket we made to cover the circuit board to protect it from radiation heating it up.

I like this shot looking along five of the nine readout towers. All the changes we made to the focal plane really make it look like a spaceship or something.  I have to say, it looks significantly cooler than it did before.

Oh, man… even cooler and more like a spaceship after we installed the heat sinking supports.

     I’ll finish with a few more shots of the camera while we closed up. Good luck, Black Cat! Here’s to collecting awesome data that will let us do awesome science (and let me write my thesis)!

I went downstairs to notify everyone that Abby was coming down with the focal plane. In those few moments I was gone Tyler and Jay thought it’d be fun to take this spoof shot of Jay pretending to destroy the camera with a soldering iron heater. Complete with nefarious looking black “Nighthawk” anti-static gloves. Awesome shot. Jay and Tyler were bummed when I found the photo just a few minutes later when I was looking for pictures of how we installed the camera last time so we could do it the same way. Sorry, guys!

The camera installed in Black Cat yet again. Compare this to a similar photo I took a month or so ago. A lot has changed…

This is what the camera looked like right before we started putting on the radiation shields. This should be the last time anyone looks at it until next winter.

One Last Time

It’s after 8:00 PM Friday January 13 and we’re all working hard to try and close up Black Cat one more time.  The second cool down only lasted a few days as we found some problems we wanted to address before leaving the cryostat cold for a year. So we opened up, tore the camera apart yet again, and started working on revisions and engineered some fixes for the problems we found.

One major aspect of the revisions is swapping out some old 90 GHz pixels for some new ones, which is labor intensive and really tricky.  That’s why I’m writing right now, actually – we’re all ready to go to rebuild the focal plane and the folks doing the 90-rework are finishing up.  Another major problem was that we had microphonics pickup problems.  Essentially, as the receiver was bumped or moved, parts of the camera would start vibrating like tuning forks at particular frequencies.  This vibrational energy has to get dumped somewhere, and it gets deposited on our cold plate, which heats it up.  We actually ran some experiments where we blasted the receiver with sound waves at different frequencies and watched the temperature of the cold plate in real time.  Sure enough, there were particular resonance frequencies that made the cold plate rise almost instantly by 10 mK or so.  That doesn’t sound like a lot, but it eats into our pixel sensitivity pretty hard so we needed to remedy the situation. 

To fix the vibrating parts, (the readout towers for the 150 modules and the 90 GHz pixels), I designed a set of solid rings that the towers can screw into, essentially making all the readout towers one solid piece.  The more rigid the part, the higher the resonance frequency, and all we want to do is push the resonance frequency above vibrations from moving the telescope and the like so parts in the camera won't shake and heat everything up.  So these rings should do the trick.

It’ll probably take 4 hours or so to reassemble the camera, so we’re planning on getting that far, maybe bolting the camera into the cryostat , and then calling it a night.  We’ll attach readout thermometry, readout cables, and close up the cryostat and start pumping down tomorrow morning.  It’s getting pretty late in the season, and we still have to measure the bandpasses of our detectors, how efficient they are, and their polarization angles before we start the process of handing the reins over to the two winter over’s for SPT this year.  It’s a lot to do, so this will be our last close up for the season barring a major catastrophe– the cryostat will remain cold until next fall.  So, we all have to be super careful over the next 20 hours or so…  Here goes!

Edit:  It's now nearly 5:30 AM on Saturday.  Didn't quite make it as far as we wanted.  Got the detectors installed with the new rings that make everything more rigid (they work amazingly well, by the way), and we covered the green circuit board for the 90 GHz detectors with a blanket of kapton-coated aluminized mylar to reflect light away from the board - otherwise the board would absorb it and heat up. But the back side of the camera is ready to go and other folks will go out in a few hours to keep going as the rest of us sleep to meet them after lunch and actually close the cryostat.  Exciting!

Before I head to bed, here are some pictures of the new additions to the back side of the receiver.  The rings and braces holding all of the readout towers up make them as solid as a rock. Before, you could flick the towers and feel them vibrating with your finger.  Now you flick them and they just take it, no questions asked.  I'm really pleased with how well they turned out.  And the mylar blanket Liz and I made turned out really awesome too, though Liz made it work as well as it did - I just coated the back side with kapton.  We affectionately call it the "Big Top" tent.  I'll have a new post in a few days after we close up and the 18 hour work days stop.  Almost there...

The readout towers shake around a lot and heat up the focal plane, which is bad. Jay had the idea to connect all the towers by a solid ring to make them act like one big piece. From that concept, I designed a set of rings and braces that hold the towers together. Here’s the lower ring installed. Two aluminum bars hold the center tower to the two towers next to it. There are two more towers, not installed, that attach to the rings by those spaceship-looking braces screwed onto the outside of the ring.

The green circuit board in the above picture absorbs stray light really well, which may have also been heating our camera. To keep the circuit board from absorbing the extra light, we covered it in the “Big Top Tent,” a mylar blanket that reflects light away from the circuit board. The back side is coated in two layers of kapton tape so the aluminum coated mylar doesn’t short out all the pins going to the 90 GHz pixels. The Big Top rests on the lower tower support ring in the above picture to keep it floating above all those nasty pins. Once the tent was installed, we placed the second bracing ring on and attached everything to the two outer readout towers. It works great! The readout towers are made of “LC” boards, (an inductor [L] and capacitor [C] in series with a resistor has a particular resonance frequency, and we use these different frequencies to bias up many different detectors with the same set of wires). I like to call this new configuration with the rings and everything “LC Henge.”