SPTpol First Light

A big update this week.  A ton has happened since my last post, and all of it very exciting.  I’ll do my best to touch on the best parts.

I left off last time mentioning how we had just closed up Black Cat for the third (and turned out to be final) time.  Since then the cryostat has gotten cold, we’ve done a lot of on-the-ground testing, and we hoisted the cryostat into the telescope boom on Wednesday January 25^th.  All of this culminated with first light late Thursday night, which was pretty exciting.  More has happened in the past couple weeks, but I’ll hit on all of that in a future post.

We had a few big concerns with how the receiver was going to function on this go around.  We really needed the base temperature of the camera to get colder.  Last time it got down to 363 mK, but we were hoping for something closer to 300 mK.  It doesn’t seem like a big difference but it’s the difference between our detectors operating really smoothly under varying conditions and being on the hairy edge of disaster.  I’m happy to say this time we got down to 288 mK, perfectly acceptable.  The extra filter we placed in front of the camera soaked up the extra light we didn’t want that was heating our camera.  A slightly different temperature stage went up a little bit in temperature as a result of this fix, but that stage has more cooling capacity so it can handle a bigger heat load.

Another problem we were concerned about was microphonic heating – shaking the camera and having bits of it vibrate and heat up.  We added bracing structures to the back side of the focal plane which made it rock solid, but when we cooled down we saw we still had heating when we banged the cryostat with a hammer.  By that point there weren’t many likely culprits, and we think we have it down to the filters lying directly on top of the feedhorns.  These things are just shy of a foot in diameter and they’re only clamped down on their edges.  Our current theory is that because of the center of the filters are just floating they can act like drum heads, vibrating and heating the stage as a result.  You can imagine this being a big deal.  The camera is installed in a 600,000 pound moving tower of steel, and that thing can shake.  I mean, we feel the control room shake when the telescope engages.  We’ll have to come up with a way of supporting the center of the filters without getting in the way of the pixels for next year’s deployment season.  I’ll let the cat out of the bag now, though, and say that installed in the telescope we see no obvious microphonic heating, so we’re REALLY happy with that.

After we did a round of ground testing, making sure our detectors could handle the kind of optical power they were going to see in the telescope without saturating and turning into useless chunks of metal, we needed to measure the pixel bandpasses.  A bandpass is just the specific range of light that is allowed to enter the feedhorns and couple to the pixels.  We’re just measuring the colors of light the detectors see.  We do this with a really nifty device called a Fourier Transform Spectrometer (FTS).  It takes light and splits it into two beams.  The beams travel down two paths but one path is a different length than the other.  On the other side of the FTS the light is combined into one beam again, but because the two beams traveled across different distances the light waves are out of phase and interfere with each other.  Sometimes they add together to get a brighter signal, and sometimes they cancel each other out and you get a dimmer signal.  Anyway, you send this light into the cryostat and let the pixels see it.  As you change the path length of one of the light beams, you look at the detector response, which turns out to be an oscillating signal of more power and less power.  This signal is called an interferogram (you’re measuring how the detectors respond to two light rays interfering with one another by different amounts). 

The FTS with its lid off. There are several mirrors and wire grids in there. At the bottom left is a hot light source, spitting out 1000 C radiation for our detectors to see.

This is getting a little complicated (sorry about that), but it turns out that the interferogram has two big features.  One is this oscillating signal I already mentioned.  The frequency of that oscillation gives us the center frequency or color of our bandpass.  The other feature is called the “white light fringe” and it’s another oscillating signal, but the peaks and troughs quickly grow until they hit a maximum, and then they die off just as quickly as you move away from the white light fringe.  This shape tells us how the bandpass “turns on” and “shutts off.” It gives us the shape of the low frequency and high frequency edges of the bandpass.

The measured bandpass for hte 150 GHz detectors. Plot is transmission versus frequency. The red lines are all the individual detectors, and the white line is the average for that particular wafer.

With the bandpasses measured it was time to hoist the cryostats into the receiver cabin, in the boom of the telescope.  This is a tricky and dangerous process.  I forget if I mentioned this before, but the cryostats are partially cooled by pulsing high-pressure helium in a device called a pulse tube cooler.  The helium comes from and returns to a compressor, being transported by 100 feet of high-pressure gas lines.  In order to hoist the cryostat we have to turn off the compressors and disconnect the gas lines.  But without the compressors turned on the cryostat rapidly begins to heat and as it heats the helium gas trapped inside expands more and more.  From the time we disconnect the pulse tube lines we have roughly 3 hours before the cryostat turns into a high-pressure helium bomb… not good.   If it were to blow a hole out of the cryostat the explosive re-compression of the camera would rip our detectors into millions of tiny fragments and probably seriously injure someone.  So, in that 3 hour window we have to hoist the cryostats, bolt them into place inside the telescope boom, get them reattached to pulse tube lines inside the telescope, and turn on the compressors.  Hoisting the cryostats is a well choreographed operation that needs to happen smoothly so we don’t damage the camera or ourselves.

The cryostats on the floor before hoisting, directly under the readout electronics racks.  Black Cat actually faces 180 degrees the other way in the telescope. (Photo by Cynthia Chiang)

Closeup of the receiver side of the cabin.  Installed, the readout boxes on Black Cat are about level with the horizontal steel bar right under the light.

Optics cryostat side of the receiver cabin.  The VW beetle-sized white cryostat fits inside and gets bolted to the giant metal support ring near the top of the photograph.

Brad gets into position, standing on the readout racks to bolt his side of the optics cryostat into its support frame in the telescope boom. (Photo by Cynthia Chiang)

Brad was stuck up there until we were done hoisting the cryostats. I later spent five hours in the same spot in order to help connect all the readout cables to the cryostat.

Preparing to hoist the optics cryostat by hanging it on chain hoists and disconnecting it from its cart. (Photo by Cynthia Chiang)

Tyler and Abby secure the helium ballast tanks to the top of the optics cryostat so they don't get sheared off when the cryostat is hoisted into the cabin.  That'd be bad. (Photo by Cynthia Chiang)

Nick and I disconnected the pulse tube lines from the receiver while others disconnected lines from optics and the compressors so we could switch to lines in the telescope.  From this point on time is ticking... (Photo by Cynthia Chiang)

Before the cryostat got too high, Liz climbed on and rode the cryostat up while it was hoisted.  She needed to keep the cryostat from the edge of the cabin where things could get sheared off by pushing the cryostat away from the wall with her feet.  When the cryostat was fully lifted, Liz had barely enough room to connect pulse tube lines and bolt the cryostat into position.  Like Brad, she was stuck up there until we were done. (Photo by Cynthia Chiang)

Up it went with Liz riding it.

With the cryostats fully hoisted Bard bolts his side into the support brace.

Meanwhile Liz bolts her side in.  She's almost squished in there.  Not a place for people with claustrophobia.

With Brad's side bolted in, I rode the manlift up to deliver the pulse tube stepper motor.  Only with that installed and the pulse tube lines connected could we turn the compressors back on.

Everything installed and the pulse tubes turned back on, Liz climbs out by going around the optics cryostat to the receiver side of the cabin through a gap between the cryostat and the wall about 18 inches wide.  No, thanks.

Looking up at the cryostats installed in the boom.

We finished with 36 minutes to spare before we would have had to stop everything and hoist the cryostat back down to connect it to the compressor again and keep it from exploding.  PLENTY of of extra time!

Thankfully the hoisting went off without a hitch with plenty of time to spare before anything horrible would have happened.  With the receiver in the telescope, we then had to hoist all of the readout electronics into the telescope boom.  Then came all of the readout cabling.  Tyler, Abby, and myself were shut inside the telescope boom to plug in all of the cables, which have to be carefully tied down so they don’t rip out when the telescope moves.  Abby and Tyler were down on the cabin floor plugging the cables into all the readout boards while I was 8 feet higher than them on the same ledge Brad stood on to bolt his side of the receiver earlier in the day so I could plug the cables into Black Cat.  It was 5 hours of me on my knees in a place just big enough for me to stand in while I made a forest of zip tie brambles I was continuously cutting myself on.  Not the best experience ever, but I was literally INSIDE the telescope, which was pretty sweet.  Anyway, about 21 hours after we all met at the telescope to start the hoisting process we had everything installed and working again.

All the readout electronics were installed after the cryostats. All that was left then was cabling...

I'm standing where Brad was during cryostat hoisting, and this is what it looked like directly under Black Cat when Abby, Tyler, and I finished cabling.

All the read out cables installed.  They're zip tied to the horizontal bars to keep from flopping everywhere when the telescope moves.  Remember this is all in the boom, so this whole room tilts with the telescope.

Thursday was a big day: first light.  But it didn’t come easy.  We started the day turning our detectors on and looking at the sky, but they were totally saturated.  The detectors should see something like 10 pW of power from the sky (that’s 10 trillion times less power than a 100 watt lightbult puts out).  Instead they were seeing something like 50 pW of power, and they were being blown out of the water.  We all rushed out to the telescope to determine what was going on (the detectors had worked fine on the ground) only to discover we had mistakenly left a protective reflective sheet inside the telescope snout. It was the same thing as trying to take a picture and leaving the lens cap in your camera.  Whoops…

With the lens cap out our detectors were seeing normal amounts of power and we tried looking at a star-forming region in our galaxy called RCW38.  It’s a pretty bright object that the detectors should definitely be able to see.  But a lot of detectors were still acting funny and a lot of the readout electronics were really unhappy.  After a few hours of brainstorming and debugging we figured out what the problems were and tried observing RCW38 again.  We started flowing timestream data from our detectors – we were simply watching the raw signals being read out from the detectors with time.  As the telescope scans it moves left and right across RCW38, moves up in elevation, and scans left and right again, over and over until all the detectors on the focal plane had a chance to be pointed on the source.  ‘Lo and behold, as we watched the timestream we saw these sharp dips in the signal appearing, and the dips got bigger and bigger as the telescope moved higher in elevation and the pixels we were looking at were pointed more directly at RCW38.  It was 11:30 PM Thursday Januray 26^th, and it was the first time the SPTpol camera saw an astronomical signal.  Needless to say it was very exciting and champagne made an appearance to celebrate.

Me next to the first 150 GHz timestreams showing a signal from scanning over RCW38.  Break out the champagne!

150 GHz detector timestreams during first light.  The sharp dips occurred when the telescope scanned over RCW38 and those particular detectors saw it.  Awesome!

Now that we’ve achieved first light it’s on to calibrating and optimizing the camera, and in a couple weeks we’ll all be gone leaving our two winterovers to take care of the telescope over the long austral winter as we begin to take data and start mapping the CMB.  Science, here we come!