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Optical development of the BISOU breadboard
Morgane Loquet Le Gall, Creidhe O'Sullivan, Bruno Borgo, Clémence De Jabrun, Valentin Sauvage, Neil Trappe, Bruno Maffei
TL;DR
BISOU addresses the need to characterize optical systematics for precise CMB spectral-distortion measurements beyond current FIRAS limits. The paper presents a cryogenic breadboard based on a differential two-input polarizing FTS and develops its optical design through ray tracing, Gaussian beam, and physical-optics analyses. The resulting warm model is intended to validate the optical design and alignment before cold-model assembly, while telescope-path asymmetry and OPD accuracy remain important boundaries.
Problem
Limited FIRAS sensitivity has restricted CMB spectral-distortion measurements to upper limits, motivating characterization of systematics for more sensitive missions.
Method
The paper develops a reconfigurable cryogenic BISOU breadboard using a differential two-input polarizing FTS and complementary ray-tracing, Gaussian beam, and physical-optics analyses.
Results
The paper presents the BISOU breadboard optical design and cryogenic test facility, with the warm model intended to validate optical design and alignment before cold-model assembly.
Takeaways & Limitations
The breadboard provides a platform to characterize instrument systematics and technologies before their integration into BISOU and FOSSIL designs.
Takeaways & Limitations
The asymmetric telescope and reference paths can introduce telescope-specific cross-polarization and aberrations, while OPD errors can produce artificial spectral distortions.
Abstract
from arXiv · showhide
BISOU (Balloon Interferometer for Spectral Observations of the primordial Universe) is an astronomical balloon-borne pathfinder developed as part of a preparatory study for a future space mission aimed at measuring spectral distortions of the cosmic microwave background (CMB). A laboratory breadboard of the instrument is being developed at the Institut d'Astrophysique Spatiale (IAS), enabling the characterization of subsystems and instrument systematic effects, particularly in the optical system. The optical system is based on a differential polarizing Fourier Transform Spectrometer (FTS) that receives inputs from both a sky-facing telescope and an internal calibration source. The FTS focal planes include sub-K detectors coupled to multimode feed horns. The full spectral band, spanning between 90 and 1500 GHz, is sub-divided into two frequency sub-bands, thanks to the use of a dichroic. The optical analysis first relies on ray-tracing simulations to establish the overall configuration of the system, before proceeding to more advanced Gaussian beam and physical optics analyses.
1. INTRODUCTION
CMB spectral distortions encode information about the Universe’s thermal history, but FIRAS sensitivity has limited measurements to upper limits. BISOU develops and characterizes a breadboard to study systematics and validate the optical measurement concept for future distortion missions.
- CMB spectral distortions may arise from energy injection across cosmic history and have characteristic spectral shapes.
- FIRAS sensitivity has allowed only upper limits on CMB spectral distortions, despite their importance for probing the Universe’s thermal history.
- FOSSIL targets µ- and y-distortions at sensitivity far beyond FIRAS limits, motivating stringent control of instrument systematics.
- BISOU aims to study measurement systematics, measure the y-monopole distortion, and improve measurement of the Cosmic Infrared Background.
- The BISOU breadboard will characterize the optical design, cryostat-related effects, detectors, and dichroic technology before instrument integration.
2. THE BISOU BALLOON MISSION
BISOU compares the sky with a 2.7 K internal reference using a two-input polarizing FTS, adapted for balloon operation across a broad frequency range. Its optical design addresses cryogenic, atmospheric, path-asymmetry, and photon-noise challenges through a cryostat and dichroic splitting.
- Measurement principle: The two-input polarizing FTS compares sky emission with an internal blackbody reference and uses a scanning mirror to generate an interferogram.
- Balloon mission constraints: The BISOU design covers 90 GHz to 1.5 THz with 15 GHz spectral resolution and is adapted to balloon-borne constraints.
- Balloon mission constraints: A vacuum-tight cryostat maintains the instrument near 2.7 K while its approximately 270 K window can dominate photon noise.
- Balloon mission constraints: The telescope-facing and reference optical paths are asymmetric, so telescope-induced cross-polarization and aberrations require dedicated study.
- Balloon mission constraints: A dichroic near 300 GHz divides the band, reducing low-frequency detector power and photon noise to improve low-frequency sensitivity.
- The breadboard is intended to characterize these optical specificities and potential systematic effects.
3. BREADBOARD MODEL OPTICAL DESIGN
The breadboard is a reconfigurable cryogenic platform for characterizing BISOU components and optical systematics. Its design combines ray tracing, Gaussian beam analysis, and physical optics, with staged configurations and further studies planned for beam and OPD effects.
- Breadboard model: The reconfigurable breadboard is designed to characterize individual components and validate technologies before integration into BISOU and FOSSIL.
- Experimental facility: The cryogenic facility places the FTS on a 2 K cold plate and cools the focal plane to 100 mK or 50 mK, depending on detector technology.
- Experimental facility: The breadboard uses sky and 2.7 K reference beams, moving mirrors, polarizers, a dichroic, four focal planes, and multimode feedhorn-coupled detectors inside a cryostat.
- Optical design approach: Ray tracing selects mirror configurations, Gaussian optics optimizes beam dimensions and illumination, and physical optics captures diffraction and beam deformation.
- Optical schematic: The FTS uses five mirror pairs, with moving M3 mirrors, a dichroic in a collimated beam, and M6 mirrors focusing light onto co-located detection units.
- Optical schematic: The telescope follows the Mizuguchi-Dragone condition and is refined to meet a -20 dB edge-taper requirement on the 150 mm primary reflector.
- Optical schematic: The warm model uses two detector units instead of four to simplify optical characterization and alignment before testing the full configuration.
- Future work: Future work will model de-pointing, aberrations, spillover, multimode feedhorn beams, and OPD accuracy; OPD errors can create artificial spectral distortions.
4. CONCLUSION
The paper presents BISOU’s optical breadboard, differential two-input polarizing FTS concept, and cryogenic test facility. The warm model will validate optical design and alignment before assembly of the cold model.
- The work presents the BISOU breadboard optical design, its differential two-input polarizing Martin-Puplett FTS, and the cryogenic facility for operation-temperature characterization.
- The warm breadboard will validate the optical design and alignment procedures before the cold model is assembled.