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Although segmented mirror technology is essential for realizing large telescope, it has potential problems, which should be solved, on the difficulties for aligning independently-supported mirrors and the leakage of thermal radiation from the mirror gaps. To overcome them, we proposed Alignment-Free Gapless Segmented Mirror (AFGSM). AFGSM is an assembled mirror consisting of several mirrors that are mechanically connected to each other with metal bolts. With the tightly fixed mirrors and no gap between mirrors, both the alignment work and the thermal radiation can be reduced. Nakahori et al. (2018) fabricated a 300mm flat AFGSM with 6 ribbed fan-shaped pieces[7]. It uses cordierite CO720, developed by Kyocera. Cordierite is thought to be an alternative choice for developing segmented mirror because it has not only a low coefficient of thermal expansion (0 ± 0.02 ppm/K) but also various advantages on a very small aged deterioration in dimension, a high Young's modulus, and a high thermal conductivity compared to classical low expansion glasses such as ULE or Zerodur[1-6]. The first trial of AFGSM by Nakahori et al., however, could not pass environmental tests: the large surface deformation around the central hole (~ 7.12 um (PV)) and the misalignment between segmented mirrors (~ 0.5um) were occurred after a thermal cycle test and a vibration test[6]. Therefore, we newly proposed an improved AFGSM that has intentionally-produced gaps (< 0.3 mm) to prevent the surface deformation due to the thermal stress generated by the variation of the ambient temperature or the stress release among segmented mirrors by the vibration. The thermal radiation from the small gaps are negligible compared to that of classical segmented mirrors. In this paper, we present the design of improved AFGSM and report results of the optical performances before and after environmental tests.
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