This paper describes a custom X‑ray telescope designed to collect X‑rays that would come from axions produced in the Sun. Axions are hypothetical lightweight particles that can convert into X‑ray photons in a strong magnetic field. The International Axion Observatory (IAXO) and its prototype BabyIAXO plan to search for those X‑rays. The authors present a tuned, cost‑aware optical design that covers the full 700‑mm diameter magnet bore while keeping mirror stress low.

The team built a “hybrid” mirror structure that combines two established glass techniques. The inner core uses thermally slumped glass optics (TGO) and the outer corona uses cold‑glass slumping optics (CGSO). Together the shells span radii from about 54 mm up to 350 mm. The inner core is set to a 5.6 m focal length and the outer corona to 5.0 m. The design uses many small, coated glass segments so the mirror can be both large and light. An external hexapod support with bellows feedthroughs is proposed to allow fine alignment without putting active mechanisms inside the vacuum tube.

The optic is tailored to the expected solar axion signal. It aims for a broad energy response from 0.03 to 15 keV and a very large collecting area. The calculated effective area exceeds 2400 cm2 near 1 keV (where one axion production channel peaks) and stays above 1700 cm2 around 3 keV (where another channel peaks). Effective area measures how many photons the telescope collects. The optical quality is quoted as a half‑power diameter (HPD) of roughly 46 arcseconds for an on‑axis point source and a focal‑spot HPD of about 120 arcseconds for the spread expected from axions coming from the roughly 3‑arcminute solar core. HPD is a simple way to describe how concentrated the focused light is on the detector.

Coatings were optimized to keep reflectivity high across the wide energy range and the range of grazing angles the mirrors see. The authors favor bilayer recipes that pair a high‑atomic‑number metal (for example platinum, iridium, or tungsten) with a light material such as carbon or boron carbide (B4C). That combination helps avoid a sharp reflectivity drop near the metal’s absorption edge. The paper explains that complicated multilayer coatings would take far more time and cost but would not give a clear advantage for this geometry.