One of our favorite things to do with BICEP is stick objects in front
of the window and look at the detector responses (a natural thing to
do with a telescope, eh?). Our first light came from a human hand;
since then we've gotten a little more sophisticated with our sources.
The image on the right shows our 90-Kelvin source: a custom-made foam
cup filled with liquid oxygen that's sitting on the cryostat window.
It took two oxygen gas cylinders and about 100 liters of liquid
nitrogen to condense this small amount of liquid! ...a great
demonstration of big differences in densities and heat capacites.
These two pictures show a dry ice source on the window, and Chao-Lin
and Darren in the process of loading up the dry ice. The easiest
thing to put in the cup, of course, is liquid nitrogen...but that's a
pretty boring example and I don't have any photos. :-) (Except for this one of the nitrogen being
dumped out after successful data collection.) We also use
non-beam-filling sources, e.g. a nitrogen bath stacked on top of a rotating
polarizing grid.
The image on the right shows a non-thermal source being used to
characterize BICEP: the upper object with two knobs is a Gunn
oscillator, a microwave source that's monochromatic and completely
polarized. The microwaves are beamed into the cryostat through
sections of straight waveguide and a standard-gain horn. The Gunn
assembly is mounted on a rotating stage, allowing us to probe
polarization properties of the instrument. Here's a closeup view looking
down along the waveguide.
Another thing we like to do with the cryostat is tip it over...since
the instrument will have to tip while scanning the sky. The left
photo shows the cryostat at an elevation of 8 degrees. Surprisingly,
the focal plane temperatures are relatively immune to this orientation
change. Tipping any lower, however, causes cryogens to pour out
(right photo).
The pictures on the left show a cute polarized Fourier transform
spectrometer (FTS) that we borrowed from Case Western to measure the
bandpass of our filters. A lens and two mirrors are temporarily
attached to the cryostat to direct the FTS beam into the window. The
mirror directly above the window can tilt and rotate. Note that we
had to resort to classy low-profile gas venting
fittings to allow free motion of the mirror mount. Space on top
of the cryostat is tight.
During the last weekend of July, we took BICEP on its first walk
outside to see first "real" light. The trip across the parking lot
was much more difficult than anticipated -- two of the four cryostat
legs broke off in transit, nearly resulting in destruction of the
instrument and loss of N theses. The photo on the right shows six of
us restraining The 1000-pound Beast. Things are actually fairly under
control in the picture...a few minutes before, the cryostat was
balanced haphazardly on piles of wood (while JPL security guards
obliviously circled around us twice).
One of the main reasons we took BICEP outside was to do preliminary
beam mapping. We built a giant XY translation stage in about 24 hours
(shown to the far left) for this purpose. The Gunn oscillator was
used as a source, and it's visible on the moving stage right next to
Yuki's head in this
picture. We put the XY stage on the roof of a building at JPL
(look carefully in the second image from the left), 60 meters away and
in the far field of BICEP. Setting things up took forever, and we
weren't ready to take data until the evening...
The two images here show us aiming the cryostat and trying to center
it on the rooftop source. Precisely moving a 1000-pound object isn't
an easy job...thankfully, we had an abundant supply of large C-clamps
and hefty extruded aluminum rails.
From left to right: (1) Yuki doing a raster scan, (2) John adjusting
the source shortly after sunrise, (3) the view from above...BICEP and
the source in the same picture. Note that the frame is tilted to be
approximately parallel to the cryostat window.
A special treat during BICEP's outdoor trip was seeing the full moon
on July 31 (a blue moon!). The left photo shows Darren carefully
adjusting the mirror, and the right photo shows the results of his
good aim. A beautiful detection of a celestial object.
So that's the status of my thesis experiment, folks. BICEP makes us all go a little crazy at times, but overall it's a fun toy to work on. More to come in the next few months before we deploy to the pole! In the meantime, feel free to troll through the photo repository...
