Mobile Intensity Interferometer for Stellar Observations (MI$^2$SO)

TL;DR

The paper presents MI2SO, a mobile, low‑cost intensity interferometer using two 1 m Fresnel‑lens telescopes to test feasibility of dedicated stellar intensity interferometry. It grounds measurements in the Siegert relation and van Cittert‑Zernike framework and demonstrates data acquisition, noise handling, and angular‑diameter estimation for Arcturus at 655 nm, achieving results consistent with literature. The study shows that mobile, scalable baselines enable targeted sampling of the u‑v plane and discuss potential performance with many units and improved detectors. Limitations include environmental and electronic noise, but synchronization and processing advances could push precision toward percent‑level diameters for bright stars. Overall, MI2SO provides a proof‑of‑concept for portable, scalable SII instrumentation with significant potential for high angular resolution stellar studies.

Abstract

In recent years, intensity interferometry has seen renewed interest and successful application at Imaging Atmospheric Cherenkov Telescope arrays. These measurements are usually performed during bright moon periods while the instruments' primary purpose -- gamma-ray observations -- cannot be fulfilled. The Mobile Intensity Interferometer for Stellar Observations was designed as a proof of concept for a purpose-built intensity interferometer. Using acrylic Fresnel lenses 1 m in diameter with 1.2 m focal length, a compact, economical and lightweight design was realised. The detector fixture allows for translation in the z-axis to adjust for measurements at different wavelengths (and therefore focal points) and easy swapping of the detector in its entirety. Both mobility and scalability in quantity of this design allow for specific targeting of projected baselines and orientations based on the target. Particularly for potential binary systems, selective coverage of a target's u-v plane is essential to probing the characteristics accurately. A first campaign demonstrated the capability of these Fresnel lens telescopes by measuring the spatial coherence curve of Arcturus ($α$ Boo). In an observation time of less than 11 h, the angular diameter was measured with milliarcsecond precision, in agreement with the values in the literature.

Figures (12)

• Figure 1: The MI2SO telescopes in operation on the roof of the ECAP building during a night with bright moonlight.
• Figure 2: Projected baseline and optical path delay illustrated for off-zenith observations. Figure adapted from noauthor_basics_nodate.
• Figure 3: Schematic of a MI2SO telescope facing up with dimensions of primary components indicated. The main lens holding frame is made of aluminium rails with 3D-printed PLA plastic two-part lens holders at the center of the rails and 3D-printed PLA plastic corner sockets into which $18\,$mm diameter carbon fiber rods are glued.
• Figure 4: Predicted photon rates for Arcturus and Vega in the observation night of July 30th 2024. Colors of the curves were chosen to emphasize effective temperature and therefore peaking wavelength difference between Arcturus ($4286\,$K and $676\,$nm) and Vega ($9650\,$K and $300\,$nm). The simulation uses manufacturer values for lens transmission, filter transmission and PMT quantum efficiency as well as literature values for altitude and wavelength dependent atmospheric absorption, see stubbs_toward_2007.
• Figure 5: Average spatial coherence (color scale) expected for the target star Arcturus as a function of the position of the second telescope, over the course of the observation night of July 30th, 2024, with the first telescope in the centre. To exclude telescope orientations in which one telescope casts a shadow on the other, spatial coherence is set to zero for times during the night when the projected baseline is smaller than the telescope frame diameter. The grey area in the plot highlights orientations containing some fraction of observation time affected by this shadow. The telescope positions that were used are indicated in red.