Here are some BICEP2 details to augment Chris White's answer:
BICEP2 accomplishes the task of measuring angular variations in polarization by converting those angular variations to a time domain signal. It does so by scanning its telescope across the sky at a constant rate.
Specifically, the telescope scans at a fixed rate of $2.8^\circ $/ second in azimuth (angle along the horizon), at constant declination. Because the telescope is aiming high in the sky (at its south pole location, average elevation = - average declination = $57.5^\circ$), the actual sky scan rate is approximately $2.8 * \cos(57.5^\circ) = 1.5^\circ /s$.
Therefore, a feature with angular size $\Delta \theta$ appears in the instrument data stream as a signal with time duration $\Delta t = \Delta \theta / 1.5$.
Since the $l$th multipole has $l$ nodes in $180^\circ$, the angular size of a "period" of this multipole (comprising two nodes) is approximately $180^\circ/(l/2) = (360/l)^\circ$, appearing in the data stream as a signal with period $T = (360/1.5)/l = 240/l$, or a frequency
$$f=1/T=(l/240) \, \, Hz$$
Thus, the target multipole range $l=20-240$ appear in the data as signal frequencies of approximately $0.083-1 \, Hz$, or time periods ranging from 12 to 1 seconds.
The telescope performs many scans, at varying declinations, with the data being combined to form the final polarization map. Each individual scan takes data over $56.4^\circ$ in azimuth, or approximately $30^\circ$ in the sky. An individual scan therefore takes $56.4/2.8= 20$ seconds, less than 2 periods worth of signal at $l=20$. Thus the scan size limits low-$l$ data collection.
References are the BICEP2 results (especially section III A) and experiment (section 12.2) papers.