A novel double-rim forebaffle design for centimeter to sub-millimeter astrophysical observations

TL;DR

The paper addresses stray radiation contamination in centimeter-to-submillimeter astronomy, focusing on CMB polarization B-modes and diffuse Galactic emission. It proposes a co-moving double-rim forebaffle design and provides conceptual guidelines, dimensioning prescriptions, and diffraction-based comparisons to quantify straylight suppression. The results suggest substantial reduction of diffracted sidelobes and improved observing efficiency, as demonstrated for configurations analogous to Cerro Toco and BICEP3-like forebaffle geometries. The design is simple, adaptable to various platforms, and could significantly enhance large-angular-scale measurements in both ground-based and space-based missions.

Abstract

Stray radiation of various origin is a major source of degradation of centimeter to sub-millimeter astronomical observations. This is particularly problematic for the detection of signals such as faint cosmic microwave background polarization B modes, or for mapping large-scale extragalactic or Galactic diffuse emission. In this paper, we propose a double-rim forebaffle design to reduce the impact of such stray radiation contamination. Using qualitative arguments and numerical simulations, we show that such a design has the potential to substantially improve the quality of future observations.

Figures (6)

• Figure 1: Sketch of the optics tube and forebaffle of a ground-based refractive small-aperture telescope designed for the detection of CMB polarization $B$ modes. Left: three-dimensional view, with a section of the forebaffle cut out; Right: cut through a plane of symmetry. The diameters of the entrance aperture and of the rim of the forebaffle are denoted $D_0$ and $D_1$ respectively, $F$ is the focal length and $H_1$ is the height of the forebaffle. These dimensions define the angles $\theta_0 = \arctan((D_1\!-\!D_0)/2H_1)$ and $\theta_1 = \arctan((D_1\!+\!D_0)/2H_1)$. Optical elements such as lenses and filters are not represented in this figure.
• Figure 2: Illustration of four mechanisms for straylight pickup by a telescope comprised of a refractive optics tube and a forebaffle. a) Reflections of radiation inside the forebaffle; b) Scattering by the inner surface of the forebaffle; c) Diffraction by the forebaffle rim; d) Thermal radiation from the inner forebaffle heated by the Sun.
• Figure 3: The double-rim forebaffle concept. The geometry defines three angles of interest: $\theta_0$ is the limit below which all light rays intercept the entrance window; $\theta_1$ is the limit above which no light ray reach the entrance window; $\theta_2$ is the limit above which no light ray reach the inner main forebaffle. The slanted rim shield intercepts light rays between ray 1 and ray 2, which otherwise would illuminate the inner forebaffle. When the solar aspect angle $\theta_{\rm Sun}$ satisfies $90^\circ > \theta_{\rm Sun} > \theta_2$, the Sun shines only on the inner surface of the rim shield, and no reflected, scattered, or re-emitted light ray points directly towards the entrance window. The yellow-shaded area illustrates the bundle of rays that illuminate the rim shield when the Sun is at an angle $\theta_{\rm Sun} = \theta_2$.
• Figure 4: Total double-rim forebaffle height and diameter, as a function of required rejection angle, for three different forebaffle designs as specified in table \ref{['tab:3-SAT-designs']}.
• Figure 5: Peak normalized beam profile of a center pixel (red), edge pixel (blue), and averaged over the focal-plane (black) for traditional forebaffle (dashed) and double-rim forebaffle (solid) designs. Note the double-rim forebaffle with a 60$^{\circ}$ rejection angle shows a strong suppression in sidelobe power at 60$^{\circ}$.