Optimization procedure of the baffle of the GroundBIRD Telescope to mitigate stray light

Abstract

We presented the optimization procedures of the baffle mounted on the GroundBIRD telescope for measuring the polarization of the Cosmic Microwave Background~(CMB). The telescope employs dual mirror reflective telescopes installed in a cryostat. The primary objectives were to minimize stray light contamination, maintain the integrity of the main beam, and ensure that thermal loading from the baffle remains significantly below that from the atmosphere. Using quasi-optical simulations, we have optimized the baffle's aperture angle to suppress stray light without degrading the main beam quality. We confirmed through Moon observations that the optimized baffle design works to eliminate the contamination of the stray light as expected. Furthermore, no measurable degradation in the noise equivalent temperature~(NET) was detected, indicating minimal thermal impact. These results show that our baffle optimization strategy effectively reduces systematic errors while maintaining observational sensitivity, providing valuable insights for future CMB experiments with similar optical architectures.

Figures (12)

• Figure 1: A photograph of the GroundBIRD telescope installation captured by a monitoring camera. The arrangement of the ground shield, dome, baffle, and cryostat is shown.
• Figure 2: Primary stray light paths in the GroundBIRD optical system, including key optical components, are illustrated. The main beam is drawn by a solid line. Two types of stray light paths are illustrated: the "spillover" component (dashed line) and the "direct" component (dotted line).
• Figure 3: A photograph of the detector array mounted on the focal plane. The pixels for which we show the results of our analysis are assigned numbers. Pixels numbered from 1 through 8 are selected because their thermal and optical characteristics are most affected by baffle design.
• Figure 4: (a) The simulation setup for the quasi-optical simulations with GRASP. The white holes in the gray areas represent the optical components. The top circular area represents the entrance aperture at the top of the baffle. The entrance window from the cryostat and the aperture of the cold stop, which defines the entrance pupil, are shown by another two circular areas. (b) The definition of the parameters that define the principal geometry of the baffle.
• Figure 5: A photograph of the wide baffle mounted on the GroundBIRD. The baffle length is 84 cm and the aperture angle $\Theta =27^{\circ}$. (a) External view. (b) Internal view showing the radio-absorptive material (ECCOSORB AN-72) covering the inner wall. The absorber is bonded to the aluminum surfaces using Stycast 2850FTJ adhesive.