The observing strategy forBicep2follows directly upon the successful strategy pursued byBicep1.
The sky is mapped in an azimuth-elevation raster at a slew rate of 2.8 /s. The slew rate has the effect of modulating the science angular scales at a few Hz (at this rate, ` = 300is modulated at 5 Hz). The entire observing field is covered at four discreetDK angles, oriented at +45, +180, and +225 from the first DK angle. The center of the azimuth scan is held fixed with respect to the ground for 53 minutes at a time. By holding the scan center fixed instead of continuously tracking the field, it is possible to construct and remove a ground-fixed template.
Bicep2 maximizes on-target time by minimizing cryogenic operations, turnarounds, and time spent on calibrations. In this regard,Bicep2has been enormously successful, achieving a 79% duty cycle during normal operations.
9http://www.deltatau.com
10http://www.aetechron.com
11http://www.sekisuivoltek.com/
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2.8.1 Observing site
Bicep2is located in the Dark Sector Laboratory (DSL) at the Amundsen-Scott South Pole Station, at a latitude and longitude12 of (89.99 S, 44.65 W). The mount sits on a raised platform on the second floor of DSL, observing the Antarctic sky through an extrusion cut into the roof of the building. The instrument itself is in a short-sleeve lab environment, with only the instrument window and forebaffles exposed to the harsh climate. The telescope is enclosed with an accordion-style vinyl
“boot” that moves with the telescope.
The South Pole has long been the preferred observing site for observational cosmologists. A number of other highly successful CMB experiments have been sited at the South Pole, including Bicep1, QUaD, Dasi, Acbar, Python, SPT, and others. It is worth noting that, as of this writing, South Pole experiments can lay claim to i) the most sensitive T T small angular scale measurements to date (SPT, Reichardt et al. 2012), ii) the most sensitive measurements of the EE spectrum to date (QUaD, Brown et al. 2009 andBicep1, Chiang et al. 2010), and iii) the strongest upper limits on theB-mode amplitude of the CMB to date (Bicep1, Chiang et al. 2010).
This highly successful program has been the product of excellent observing conditions and superb logistic support.
With a pressure altitude of ⇠11,000 ft and an average temperature of 65C, the atmosphere above the Antarctic plateau contains minimal precipital water vapor (PWV) and mild, near constant winds. The lack of a diurnal cycle results in long, extended periods with stable observing conditions.
The unique geographic location of the instrument also lends itself to extremely favorable obser- vations of the night sky. Since the observing target never sets below the horizon, observing schedules can be run with an extremely high duty-cycle, interrupted only by cryogenic operations and routine calibrations.
2.8.2 Observing target
The Bicep2observing target is the Southern Hole, a ⇠ 800 degree2 patch of sky accessible from the Southern Hemisphere centered at RA= 0hr, dec= 57.5 degrees. The observing target covers latitudes distant from the galactic plane, making it exceptionally free from galactic foregrounds.
This particular region is the cleanest of its size, with average dust emission averaging 1/100th of the sky median. By sheer coincidence, it is also at a near ideal declination when observed from the South Pole.
Bicep2’s observing band at 150 GHz is chosen for three considerations: i) 150 GHz corresponds to the predicted minimum of the sum of galactic synchrotron and dust emission within the observing region (assuming a synchrotron spectral indexk= 3), ii) as a thermal blackbody at 2.7 K, the CMB
12As of 2005. Because of the 10 m/year motion of the polar ice cap at the South Pole, the coordinates of the lab move with respect to the geographic pole.
6 h 18 h 0 h
12 h
1 3 10 30 100 300 1000 µKCMB
Figure 2.16: TheBicep2observing field overplotted onto the Finkbeiner, Davis, and Schlegel dust emission model (Finkbeiner et al. 1999). Average dust emission in theBicep2target region is over 100 times lower than the sky median. Figure is adapted from Chiang et al. 2010.
peaks at⇠150 GHz, and iii) there is a convenient atmospheric window centered at 150 GHz, between oxygen and water lines at 118 and 183 GHz, respectively. As a single-color instrument, Bicep2is incapable of distinguishing primordialB-modes from galactic foregrounds, and, as a result, can only place upper limits on theB-mode amplitude of the CMB. In the scenario of a detection ofB-mode polarization, further multi-frequency followup will be necessary to distinguish a primordial signature from a foreground.
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Chapter 3
Characterization of the Bicep2 telescope
As instrument sensitivity has improved over successive generations of CMB polarization experiments, the requirements for polarization systematics have become increasingly stringent. The threat of in- strumental systematics looms large when attempting to measure theB-modes, in part because the analysis relies so heavily on component separation. Small errors in instrument calibration or match- ing between polarized detectors can leak the (comparatively) very bright temperature fluctuations into polarization. Polarization systematics can be identified both in data analysis and through instrument characterization.
The principal goals of the characterization of the optical, thermal, and magnetic performance ofBicep2 are to: i) characterize the optical and polarization response of the telescope for faithful map reconstruction, and ii) assess and isolate potential sources of instrumental polarization. The latter may be accomplished by generating simulated data with instrument parameters captured from calibration data as inputs. The process of taking measured instrument parameters to constraints on falseB-mode polarization will be detailed in Chapter 4.
In this chapter, we summarize our effort to characterize Bicep2. As in the previous chapter, particular attention will be paid to potentially dominant sources of systematics, and to efforts in which I played a substantial role. A non-trivial fraction of my graduate career has been devoted to characterizing the optical response of the telescope. This effort has included three trips to the South Pole, hundreds of hours of data-taking, and thousands of lines of code. Collaborators can view much of the analysis code used to produce the results below in my online working notes, available
athttp://bicep.caltech.edu/~rwa/rwa_working_notes/index.html. In addition to the optical
calibration efforts, this chapter will also summarize characterization of the polarization, thermal, and magnetic response of the instrument. We will conclude the chapter with characterization of the instrument performance and noise properties.