Roughness tolerances for Cherenkov telescope mirrors

Abstract

The Cherenkov Telescope Array (CTA) is a forthcoming international ground-based observatory for very highenergy gamma rays. Its goal is to reach sensitivity five to ten times better than existing Cherenkov telescopes such as VERITAS, H.E.S.S. or MAGIC and extend the range of observation to energies down to few tens of GeV and beyond 100 TeV. To achieve this goal, an array of about 100 telescopes is required, meaning a total reflective surface of several thousands of square meters. Thence, the optimal technology used for CTA mirrors' manufacture should be both low-cost (∼1000 euros/m 2 ) and allow high optical performances over the 300-550 nm wavelength range. More exactly, a reflectivity higher than 85% and a PSF (Point Spread Function) diameter smaller than 1 mrad. Surface roughness can significantly contribute to PSF broadening and limit telescope performances. Fortunately, manufacturing techniques for mirrors are now available to keep the optical scattering well below the geometrically-predictable effect of figure errors. This paper determines first order surface finish tolerances based on a surface microroughness characterization campaign, using Phase Shift Interferometry. That allows us to compute the roughness contribution to Cherenkov telescope PSF. This study is performed for diverse mirror candidates (MAGIC-I and II, ASTRI, MST) varying in manufacture technologies, selected coating materials and taking into account the degradation over time due to environmental hazards.

Absolute reflectance of a concave mirror used for astro-particle physics experiments

“…While the measurement presented here is based on an absolute calibration w.r.t. the measured light flux of a star incident to the IACT reflector, including also the reflectivity of the mirror surface material, Tayabaly et al [23] do not include the latter in their calculations, considering only the surface geometry of the mirror. In addition, the authors assume an incident power of one, thus putting no limiting assumptions on the size of the detector and therefore accounting for the whole scattering.…”

“…By contrast, the absolute reflectance measurements we present here were only measured up to a certain radius (see Figure 13). Finally, for our measurements, a blue bandpass filter was centered on 450 nm was used, while Tayabaly et al [23] present encircled energy profiles that were calculated for 300 and 550 nm. Since both, the surface reflectivity and the scattering due to micro-roughness are wavelength-dependent, it is thus not straightforward to perform a detailed comparison between these results.…”

Mirror production for the Cherenkov telescopes of the ASTRI mini-array and the MST project for the Cherenkov Telescope Array

“…The segments obtained are lightweight (about 10 kg∕m 2 , excluding the three supporting pads, made in stainless steel, that increase the mass of less than two additional kilograms) and with high surface accuracy, which is measured with two different parameters, the microroughness and the residual shape error, both contributing in worsening the point spread function (PSF). Both parameters are expressed as the RMS deviation of the reflecting surface from its ideal shape in the normal direction: but in the case of the microroughness, the deviation is measured on microscopic spatial scales (with spatial wavelengths from a few micrometers up to 1 mm) at high spatial frequencies, 40 while in the case of the shape error the deviation is measured at longer spatial scales. Surface microroughness affects the optical performance by degrading the PSF and reducing the image contrast, 41 while the residual shape error causes deviation from the ideal focusing directions due to the geometrical imperfections.…”

Using Muon Rings for the Calibration of the Cherenkov Telescope Array: A Systematic Review of the Method and Its Potential Accuracy

“…The angle φ denotes inclination with respect to on-axis. We have introduced here also a common spread of ∆ 0 subsuming the intrinsic optical quality of the individual mirror facets (mainly due to form deviations from ideal mirror shapes and to surface roughness; Tayabaly et al (2015)), the aberrations caused by the fact that most facets are inclined relative to the optical Telescope Requirement Application Range Type on θ80 (from Camera Center) Table 5. Requirements for the spot size encircling 80% of the light reflected on to the camera (θ80).…”