Form 8

Research Result Report
ICRR Inter-University Research Program 2023
Research Subject:
Research and Development of the high-performance segment mirrors and the camera
entrance windows for the CTA Large Size Telescopes
Principal Investigator:
Masahiro Teshima, Max Planck Institute for Physics
Participating Researchers:
Masahiro Teshima(a,b), Razmik Mirzoyan(a), Alessio Berti(a), Martin Will(a),Ryuji Takeishi(b), Hidetoshi Kubo(b), Shotaro Abe(b), Joshua Baxter(b)Koji Noda(c), Tokonatsu Yamamoto(d), Michiyuki Chikawa(d), Angelo Antonelli(e)a)

Max Planck Institute for Physics, Germanyb)

ICRR, The University of Tokyo, Japanc)School of Science, Chiba University, Japand) Department of S&T, Kinki University Japan,e) INAF Rome, Italy

Summary of Research Result：Introduction:

We designed LSTs not only with large reflectors but also with highlyreflective mirrors, UV transparent camera entrance windows, high Q.E. and highspeed photosensors, and high bandwidth readout electronics to achieve lower energy
thresholds. Thanks to the low energy threshold, the CTA-LSTs extend the gammaray observation horizon to the redshift z = 4 and empower observational studies ofhigh redshift AGNs and bursting sources Gamma-Ray bursts. In addition, CTA
assumes a long-term operation period of about 20 to 30 years, and all telescopeelements must maintain stable performance over a long period.
We understand that the degradation rate of mirror reflectivity is less than 1%/yr fromthe intensity of the muon Cherenkov light of LST1. We propose a new longer-life
mirror surface coating with a deterioration rate of 0.5%/yr or less, suppresses averagedeterioration to 10% or less over 20 years of operation, and reduces the frequency of
mirror replacements.
In addition, an anti-reflection coating is applied to the UV transmissive plastic dome
installed on the camera entrance windows to increase the transmittance to over 98%.
After investigation, we would like to provide an antistatic coating to minimize theadhesion of dust. Based on experience, the reflectance effect of dust is about 5% at
maximum on mirrors and entrance domes.

Form 8The mechanical structure of the mirrors supplied to LST1-4 consists of 2.7mm of glass+ 50mm of Al Honeycomb + 2.7mm of glass. The five layers of coating on the glasssurface, SiO2 + HfO2 + SiO2 + Al + Cr, are produced by sputtering. We obtained 92%
reflectance on average with this method.
Some research results:

The seven layers of coating, SiO2 + HfO2 + SiO2 + HfO2 +SiO2 + Al + Cr, are on the glass surface by the sputtering method will enhance thereflectivity and minimize the aging effect. Ten years ago, we tested these sevencoating layers and got an excellent result. We can also control the spectral responseof the reflectivity to fit better with future SiPM cameras.

Figure 1. The promising reflectance of seven layers with coating improves the peakreflectance to 97% with optics simulation. We estimate the lifetime becomes longer
than five layers of coating. The first trial in 2023 did not achieve such improvement,
but we will continue the effort.
For the entrance window we found the coating material which reduce the dust
attachment on the surface of the camera entrance window. Unfortunately it does not
improve optical transmission but keep the surface clean from dusts.

No.