These days I work on an experiment called BICEP: the Badass Imager of Cosmic Extragalactic Polarization. You can read a full description elsewhere...or just stick around here for a five-second summary of the physics. BICEP is a telescope that will map the polarization of the cosmic microwave background (CMB) over a small patch of sky. We are looking for the presence of a "curl mode" in the polarization map -- a swirling or handedness in the polarization structure -- that is an imprint placed on the CMB by inflationary gravitational waves. If such a signature is detected, it will provide strong evidence for inflation and the existence of a gravitational wave background. The experiment is scheduled to deploy to Antarctica this winter; the telescope will be located at the South Pole Station on the roof of the Dark Sector Lab. Read onwards if you'd like to take a virtual tour of the instrument.

The tour starts with the outside of the instrument. To the left, you'll see a picture of the cryostat: the main housing for our optics, detectors, and all the guts of the instrument. It looks deceptively simple from the outside... Light enters through a window at the top of the cryostat (shown to the right). The window is a four-inch thick slab of zotefoam, a closed-cell foam that is virtually transparent to microwaves and can hold vacuum (much to my surprise...but it does look pretty scary).

The next objects in the path of the incoming light are a set of IR-blocking filters, one of which is shown in the image to the right. The filters are made of teflon, and are anti-reflection coated with a lower density version of the same material. The picture here was taken during one of our early cryostat runs, when we were still doing exhaustive thermal performance tests -- the filter is instrumented with temperature sensors, which are connected by 1 mil (0.001 inch!) wires to the outside world.

After the IR filters comes the refracting optics in the instrument, shown to the left. There are two lenses housed in a large aluminum cylinder: the top lens is visible in the image (along with an accompanying light baffle). The second lens is farther down the tube, and there are additional light baffles in between. The lovely black gunk on the uppermost light baffle is "Bock black," a mixture of stycast and carbon lampblack that's supposed to absorb any stray light. More optics photos are shown below... From left to right: (1) one lens mount, (2) the optics tube mounted inside the cryostat, and (3) some baffles after black painting.

We've finally arrived at the heart of the instrument -- the insert, shown in its assembled state to the left, and in its disassembled state to the right (along with its creator, Kiwon). This cylinder will be the home of our array of 49 pixels, where each pixel is a microwave feed horn assembly that funnels radiation into polarization-sensitive bolometric detectors (more about these later). Only a handful of pixels are visible in these photos, as we're still building up the focal plane. The insert also houses cold electronics for amplifying the bolometer signals, and a three-stage helium sorption refrigerator for cooling the focal plane to 270 mK. Starting with the top of the insert and working our way in...

The picture to the right shows the primary feed horns on the top of the insert. Although you can't see in the photo, the horns are corrugated on the inside to preserve the polarization of the incoming light. The gray cylindrical objects at the base of some of the feed horns are Faraday rotators -- essentially a solid state version of a waveplate -- that we use to quickly modulate the polarized signal. Note that there are two sizes of feed horns corresponding to the two frequency bands of our instrument (100 and 150 GHz).

The focal plane attaches to the top of the insert and can be extracted as one unit (shown to the left, upsidedown). The primary horns are joined to re-expanding horns on the opposite side of the insert top plate (shown in more detail in this photo). A third set of horns is used to couple the radiation to the polarization-sensitive bolometers (PSBs). This last set of horns and PSBs are thermally isolated from the rest of the focal plane via the trussed "wedding cake" structure visible in the photo. More pictures of the horns are shown below. From left to right: (1) primary horns on the insert top plate, (2) re-expanding horns on the opposite side of the primary horns, (3) PSB horns.

Photons that make it all the way through the feed horns end their journey in the PSBs -- our detectors that live in the little gold cylinders visible in the picture to the left. This picture is a bottom view of the focal plane, showing the large circuit board that services all the PSB modules. Each bolometer consists of a temperature-dependent resistor mounted on a metallized silicon-nitride web (a PSB used for the Boomerang experiment is shown to the right). Changes in temperature caused by absorbed photons correspond to changes in resistance, which are translated into voltage signals. The geometry of the web (parallel strands) makes a bolometer sensitive to polarization. Each bolometer module in BICEP contains two PSBs oriented perpendicular to each other.

Having made it to the innermost part of the instrument, we now begin our virtual journey back out. The bolometer signals leave the focal plane via very expensive cryogenic cables (shown to the right), and are immediately amplified by JFETs that live at 4K, inside the insert. The top of a JFET module is just barely visible through the insert window in the photo. The cables cross a temperature difference of a few Kelvin (270mK to 4K), and therefore need to be heat-sunk and tied to points at intermediate temperatures...hence all the teflon tape you see in the picture.

The picture on the left shows a better view of a JFET module (the tower towards the lower left; one of three that will be present in the final instrument configuration), and also shows the helium sorption refrigerator that's used to cool the focal plane. The fridge has three stages that operate at different temperatures, and massive copper bars connect the stages to various sections of the focal plane (heat straps not shown in the photo).

Jumping back to the world outside the instrument...the image on the left shows the assembled insert and optics tube, about to be mounted inside the cryostat. The cryostat is hanging from a crane in this picture -- one of the wheels and some of the cables can be seen dangling in the upper left. Once the insert is inside, the underside of the cryostat looks like the picture on the right. The white cables run from the insert out to room temperature (connectors with red covers on the outer rim of the cryostat).

The left photo shows the underside of the cryostat again, this time with a cover plate that connects to the liquid nitrogen bath. Note the kitty litter bucket in the reflection...we use it as a stand for the insert. Precision cosmology in action. The last cover plate, at room temperature, is shown in the image to the right.

...and that concludes the tour of the BICEP instrument. It probably took you only a few minutes to read the description...compare that to the 40 people-hours it typically takes to assemble the insert and close the cryostat (not to mention uncountable amounts of blood, sweat, and tears). Sometimes we celebrate cryostat closings and ends of runs with liquid nitrogen ice cream (right photo) -- one of the great benefits of working in cryogenics. The happier guy in the picture actually works on a different experiment on the opposite side of the lab, but we like him anyway.

So what exactly do you do with this behemoth named BICEP, you ask? If you happen to be reading this during one of our runs, check out cryostatus, our online temperature monitor. You might be lucky enough to catch something exciting: a liquid helium fill, a fridge cycle, or something catastrophically warming up.

Want to see more pictures and hear more stories? Read about BICEP in action!