For BICEP to detect < 0.1 μK-level B-mode polarization signals,
ground shields are required to effectively prevent the beam sidelobes from
picking up ground radiation. The ground shields must keep the polarized
beam spillover to be at least 10 orders of magnitude smaller in power
(-100 dB) than the main beam. They must work for telescope's
elevation range of 50°-90°.
BICEP will use 2 levels of shielding: an inner horn around the telescope aperture and a fixed outer screen around the dome. They are designed so that any radiation from the ground must diffract at least twice before entering the telescope's window (for elevations 50°-90°).
Inner shield
The inner shield will be a circular horn fixed to the cryostat at the window, therefore co-rotating with the detectors.
Since the inner shield will be fixed with respect to the detectors, its temperature will not affect the differential measurements. Thus, its inner surface can be coated with an absorptive material to suppress (polarized) reflections. The inner surface may be also corrugated (if possible while preventing snow and frost from accumulating) to prevent diffracted waves from traveling along the surface into the aperture.
The horn will have a base diameter at the window of at least 30 cm and a half angle of at least 14° to accommodate the 8°-radius field of view (6° clearance). Wider half angle allows the diffracted waves to enter farther away from the main beam. Measured/predicted beam pattern at the window will be used to choose a sufficiently wide half angle (after which the sidelobe response doesn't improve much).
The aperture was assumed to be 90 cm from the elevation axis.
For a given half angle, the inner shield's length is made long enough so that, even at the minimum telescope elevation of 50°, the radiation diffracted by the outer shield will never enter the window directly (equivalently, so that the window will never see the outer shield). However, a longer inner shield requires a taller outer shield for hiding the inner shield from the ground (when the telescope is vertical) and therefore a wider outer shield. For a given half angle of the inner shield, the above two conditions determined a unique inner shield length that minimized the outer shield's top diameter. With 15° half angle, the optimum length is ~80 cm from the window.
The edge of the horn will be rounded to reduce diffraction.
Outer shield
The outer screen will have a reflective (aluminum) surface to direct the beam spillovers from the inner shield to the cold sky instead of to the ground.
The outer screen height will be enough so that the ground radiation never hits the inner horn directly.
Its top radius will be wide enough to never be seen directly by the window even at the minimum elevation of 50°. For the inner shield half angle of 15°, the inner shield length of ~80 cm minimizes the outer shield top radius at 353 cm. Widening the half angle by 1° increases the inner horn length by ~1 cm and the outer shield top radius by ~10 cm.
inner horn inner horn outer screen half angle height height 14° 83 cm 338 cm 15° 84 cm 352 cm 16° 85 cm 368 cm 17° 86 cm 385 cm 18° 87 cm 403 cm 19° 89 cm 423 cm 20° 90 cm 445 cm
The bottom radius will be just enough (1.9 meter) to clear the square footprint of the boot-type mount (2.7 m on a side). This allows a gap for snow to fall and makes the shield slope ~50°, shallow enough to direct most radiation to the sky, steep enough to allow snow to slide down.
The edge of the outer screen is also rounded to reduce diffraction; radius ~5 cm [making larger improve its performance only slightly, based on GTD].