A key diagnostic of the instrument’s optical performance during both the engineering and science- grade phases of the experiment was the near-field optical response of the instrument. While far-field beam maps are primarily sensitive to the amplitude distribution of the electric field in the focal plane, maps made in the aperture plane of the instrument are primarily sensitive to the phase of the electric field in the focal plane. As a result, near-field maps can serve as a probe of phase gradients within the phased-array antennas. Near-field maps can also serve as a probe of secondary reflections that focus near the aperture plane as well as vignetting within the telescope.
Near-field maps were made with an x y linear translation stage attached to a large blackened enclosure and placed directly above the window of the telescope. The source used for mapping was a ceramic heater covered with high-temperature silicone loaded with carbon lamp-black. The blackened surface was measured to be less than 3% reflective and highly absorptive. The source was chopped by a simple wheel chopper running near 18 Hz. A machined and blackened aperture plate with a roughly 1 cm diameter aperture was placed in front of the source and chopper, just a few finger-widths from the window. A large (⇠40 cm) slew made it possible to map the response over the entire telescope aperture area.
Near-field maps were acquired during two successive summer seasons at the South Pole. (Pictures of the mapper and the installation can be seen in Figure 3.13.) Maps were acquired in a step-dwell raster pattern at severalDK angles. Like the far-field beam maps, the data were acquired on the Al transition. Example maps are plotted in Figure 3.14 forA, B, and (A B). The sharp bright feature in the bottom right quadrant of the maps is a secondary reflection from the 4 K spectral filters refocused into the aperture plane. This spot contains less than 0.1% of the integrated power of the main beam. Moreover, since it forms a sharp focus in the aperture plane, it must be broadly
and diffusely coupled to the sky in the far field.
Near-field maps acquired both before and after deployment revealed two non-idealities in the optical performance of the instrument. The first is a “beam-steering” effect, where the main beam of the detectors appears steered into the aperture by 5 10 degrees, substantially more than is predicted by any physical optics model. This beam effect was described in detail in Aikin et al.
2010. This steering is readily apparent to the eye, and was, for a few pixels, so severe that the main beam was steered completely offof the aperture and into blackened surfaces. This impacts not only optical throughput, but also can potentially introduce beam distortion caused by the asymmetric and aggressive illumination of the Lyot stop. In general, this steering was not particularly well- matched between detectors in a polarized pair, resulting in mismatched beam shapes in the far field.
TheBicep2focal plane contains roughly 50 detector pairs that are steered at some level, while only roughly a dozen suffer from the severe steering illustrated in Figure 3.14.
Near field map, center pixel
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Figure 3.14: Left: Near-field map of a pixel near the center of one of the detector tiles inBicep2, with typical beam performance. Right: A highly steered detector (worst case) for a pixel near the edge of the detector tile. Plots are made on a linear color scale, both peak-normalized to unity.
Phase gradients in the phased-array antenna were demonstrated to be responsible for the steering.
Adapted from Aikin et al. 2010.
We found a great deal of tile-to-tile variation in the level of the beam steering. Additionally, the worst offenders were found to be consistently along the top and bottom tile edges. A large number of possible culprits for the steering were considered, including distortion from the detector plate holding the detectors in place and the optical backshort. Testing of devices fabricated after the Bicep2deployment demonstrated a dramatic sensitivity of the severity of the steering to the details of the device fabrication process. It is now believed that the device uniformity was being degraded toward the edge of the tile, leading to phase gradients in the microstrip summing tree. While this has been improved for later generations of detectors,Bicep2may require simply cutting the pixels that suffer most severely from this beam steering effect.
The second non-ideality revealed by the aperture plane maps is a near-constant A/B beam
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Figure 3.15: Measured beam mismatch in the near field. Left: A typical A polarized detector’s optical response mapped in the near field, normalized to unit amplitude. Center: A typical B polarized detector’s near-field response, also peak normalized to unity. Right: Difference ofA and B. The left-right splitting of the beam centroids was found to be caused by phase gradients in the summing tree. The bright spot in the bottom right quadrant is from secondary reflections within the telescope and is diffusely coupled to the sky.
centroid splitting. Beams measured in the near field show a consistent mismatch in the A and B beam centroids. Moreover, the centroid displacement is consistently (and suggestively) co-aligned with the polarization axes of each tile, and thus also the summing tree axes. The amplitude was measured to be constant across the focal plane, except for small subset of pixels suffering from the severe beam steering illustrated in Figure 3.14. Mismatch in the near-field centroids will not alone lead to any substantial far-field beam mismatch. While the beams may be displaced in the near field, the resulting angular displacement on the sky is negligible. It is true, however, that mechanisms that give rise to phase mismatch in the microstrip summing tree will also tend to give rise to amplitude mismatch, which translates directly to beam mismatch in the far field. Additionally, non-idealities in the optics of the instrument, such as spatially varying birefringence or an out-of-focus system, can complicate the situation, and phase mismatch in the object plane can result in mismatch in the far-field beam performance. In the case ofBicep2, it has not been demonstrated that this near-field mismatch is correlated or directly related to the far-field centroid mismatch described in Section 3.1.4.
Subsequent detector development efforts have reduced the near-field mismatch by adding a phase lag to the summing tree. The additional path length corrects the phase gradient difference between A andB detectors that results from interference within the summing tree. The efforts to improve
the matching of phased-array antenna beams in the aperture plane is described in detail in O’Brient et al. 2012.