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!

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.”

Photography by Nils Halverson

     For the last week or so my advisor, Nils Halverson, has been down at the Pole with the rest of the SPT crew.  He brought down his slick Nikon camera and an incredible wide-angle lens and has been snapping some truly impressive photos of all the things going on down here.  There’s a lot more happening than just putting the receiver together, (not to mention a huge crew doing all the work.  We currently have 18 people at Pole for SPT).  Nils’ pictures show that and he’s agreed to let me share some of his photography with you.  Just pictures and captions in this post (and a few pictures of me actually working and not just standing or posing!).  Enjoy! And please please please open up the full-sized pictures.  It's worth the extra few moments to download them.

Dale Li (right) came down with Nils.  Dale is the miracle worker that fabricated all of the NIST 150 GHz wafers.  He worked 18-hour days for months to finish all the wafers we needed and they’re just awesome.   I’m describing how we use a sheet metal reflector to do sky measurements without having the camera installed in the telescope.  The roof in the control room is open and the telescope is looming in the background facing away from us.

A shot of SPT facing away from the station with the wide-angle lens.  If all goes well in about a week we’ll be lifting the secondary and receiver cryostats into that boom on the far right to start on-sky testing using the telescope.

Another sundog behind SPT.

Abby and I opening up Black Cat after the first cooldown.

Obligatory shot of me in front of SPT with the new guard ring.

Brad working on the secondary cryostat. The spacey shiny stuff is aluminized mylar super-insulation.  It acts as a radiation blanket, absorbing radiation and re-radiating it in two directions, chopping the intensity of the re-radiated light roughly in half.  By having 20 or so layers of super-insulation you keep the hotter outer vacuum jackets from heating up the cooler inner jackets with their hot IR radiation.

Bill pointing out something screwy with the heat straps to the pulse tube refrigerator in the secondary cryostat.  There is another refrigerator just like this in Black Cat that gets the camera down to 4K.  Then we use another fridge that uses a combination of evaporating Helium 4 and Helium 3 to cool the camera to ~ 300 mK.

Bill and Brad working on fixing the heat strapping in the secondary cryostat.

Liz inspecting a metal mesh filter after being re-clamped.  The filters chop off high frequency light to reduce the amount of power the detectors see.  There are many filters in the system, each cutting off at a successively lower frequency.  The last set of filters defines the upper edges of our observing bands (the colors we look at).

Me working on one of the 150 GHz modules.  These are the modules I designed.  I’d make a lot of changes to them if I knew what I knew now when I started, but I suppose that’s the learning process, isn’t it?

From left: Nils, Tijmen, Brad, and Kyle right before mating the two halves of the optics cryostat back together.  Mating the halves is a tricky business.  There are many layers at different temperatures that aren’t that far apart so alignment is crucial… and it’s all done with four chain hoists.

One of our IR shaders (6 micron thick plastic discs stretched taut) was loose and wrinkly.  This is Bill holding up the shader after taking a heat gun to it like a hair dryer to winter window shrink wrap.  Nice and tight again!

Abby installing the SPTpol camera into Black Cat for the second cooldown.  She did the design work on the focal plane and all the heat sinking.  It’s a truly impressive camera!

Liz, myself, and Abby tightening the camera into Black Cat for the second cooldown. It’s probably about 10:00 PM on Monday January 2^nd in this shot.

Liz and Abby installing readout striplines and clamping them down to various heat sinking points.

Nils wearing the “tiara,” an aluminum and aluminized mylar shield he made to block stray light from filters and IR shaders in the secondary cryostat.

4:15 AM, Tuesday January 3^rd.  The whole crew prepares to mate Black Cat to the secondary cryostat for perhaps the last time.

Brad, myself, Dale, and Tijmen ride on a sled pulled by a snowmobile on our way out to DSL.

A breathtaking shot of the sky behind SPT beyond the new guard ring.

The photographer himself, Nils, working on the boom of the telescope.

Chris and Nils have been working this week to install a “snout” at the opening of the receiver cabin.  When we raise the camera into the boom, the window of the secondary cryostat will be where Chris’ head is.

Nils looking down the snout towards Chris.

Dr. “Ill” with his two Berkeley grad students Liz and Nick.

Jay looking at the sky with the sun backlighting.

The other night Kyle climbed up onto boom and then the mirror was tipped to scoop him up so he could climb around on the primary.  He then checked all the copper spacer tabs between each mirror panel to make sure none were popping out.

Liz in her winter gear.

Brad on top of the boom with the station a kilometer away in the far background.  The building between Brad and the station is MAPO, the site of DASI, the experiment Nils worked on for his thesis.  It was also the first experiment to measure the polarization of the CMB.

The SPT crew currently at Pole on the boom directly in front of the primary mirror.  (Back) Left to right: Chris, Clarence, Tijmen, Brad, me, Bill, Erik. (Middle) Left to right: Abby, Tyler, Liz, Stephen, Ryan, Kyle, Dale.  (Front) Left to right: Nils, Jay, Nick.

The same crew, but a shot farther back using the wide-angle lens getting more of the glory of SPT’s primary mirror.

John Kovac (the first author on the DASI paper that contained the first measurements of polarization in the CMB) took these group photos for us.  Thanks, John!

Thanks for the use of your pictures, Nils!