Science1 publisher2 min readPublished
A UAB team solved the muon-ring shadow geometry of CTAO's dual-mirror telescopes
Simplified treatments of the secondary mirror's shadow can misstate how much of a muon's Cherenkov light reaches the camera by up to 40 percent, ten times the calibration precision CTAO requires of itself.
The Scientist · Science desk
What happened
- A physics team at the Universitat Autonoma de Barcelona has published an analytical description of muon-based calibration for dual-mirror Cherenkov telescopes in The Astrophysical Journal Supplement Series.
- Working out precisely how much of a muon's Cherenkov light reaches the camera, once the shadows of the secondary mirror and the camera are counted, had stayed unsolved for more than 30 years.
- The formalism calculates explicitly, for each individual muon trajectory, the fraction of Cherenkov light blocked by each of the telescope's own components.
- In some telescope geometries the predicted light reaching the camera differs by as much as 40 percent from the approximations used until now.
- CTAO's own specification calls for a calibration method with precision better than 4 percent.
Compiled by The ScientistSomething wrong?How this is made
Why it matters
- cost Shadow accuracy no longer depends on computing time: a closed-form expression can be evaluated per muon in a calibration pipeline, where the alternative was continuous and costly simulation.
- capability Telescopes exposed to the weather can have their optical throughput followed using light that arrives during ordinary science observing, on time already booked for science.
- constraint The improvement is on paper until it is measured on a built dual-mirror camera, so any throughput number quoted for the Small-Sized Telescopes still comes out of a geometric model.
A muon crossing the atmosphere faster than light travels in air emits Cherenkov radiation, and one that passes close to a telescope draws a ring on the camera [5]. In a dual-mirror telescope, the secondary mirror, its supports and light deflectors, the camera and the hole in the middle of the primary can each intercept part of that light [10]. How much they intercept depends on the muon's inclination and where it passed, and it varies from one point on the ring to the next [10]. The ring is useful in the first place because the emitted light can be calculated in advance with great precision, so the brightness is known before the camera measures it [6].
The gap opens where the older approximation of the secondary's shadow is weakest: inclined muons, and secondaries wrapped in protective structures [13]. Divide the two figures and the disagreement is ten times CTAO's calibration budget [15]. Gamma-ray energy reconstruction and the systematic uncertainties around it both rest on that calibration [9].
The 40 percent compares two calculations of the same quantity. The phys.org report describes an analytical result and does not mention a comparison with muon rings recorded on a built dual-mirror telescope [19]. Data for that comparison would come from the Small-Sized Telescopes and the other Schwarzschild-Couder-inspired designs that account for more than half of CTAO's planned array [18]. That array is split between Roque de los Muchachos in La Palma and the Atacama Desert in Chile [17].
The study also includes two results for conventional single-mirror telescopes. One is a first-order correction to the maximum altitude from which a muon's Cherenkov light can still reach the reflector, which follows from the primary mirror's curvature. The other is an analytical description of how coma aberration deforms muon rings in parabolic reflectors [16].
The work began as Victor Giraldez-Segalas's bachelor's thesis at the Universitat Autonoma de Barcelona, under Markus Gaug's supervision. The stated aim was to extend single-mirror muon calibration methods to CTAO's dual-mirror telescopes [4]. Gaug is a lecturer in the UAB physics department and a researcher at CERES-IEEC, and the third author is doctoral student Fiona Redmen [3].
What to watch
- A comparison of the analytic formalism against muon rings recorded by a dual-mirror Small-Sized Telescope, and the residual it leaves against CTAO's stated precision requirement.
- Whether CTAO adopts the closed-form shadowing in its production calibration chain or keeps ray-tracing simulation as the reference.
- Whether the mirror-curvature and coma corrections change published throughput histories for existing single-mirror Cherenkov telescopes.