Computer Science – Performance
Scientific paper
Sep 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001rnaoj...5..107i&link_type=abstract
Report of the National Astronomical Observatory of Japan (ISSN 0915-6321), Vol. 5, No. 3, p. 107 - 115 (2001)
Computer Science
Performance
Mirrors, Coating
Scientific paper
We are promoting MIRA project (Mitaka optical and InfraRed Array) at the Mitaka campus of the National Astronomical Observatory of Japan. Since about 20 mirrors are used to lead the star light from the siderostat to the fringe detector for this interferometer, the small reflectivity decrease of each mirror will cause the large decrease of total reflectivity of all mirrors. It is preferable to use the silver coated mirror (silver mirror) instead of the aluminum coated mirror (aluminum mirror), because the silver mirror has higher reflectivity than the aluminum mirror at the wavelength longer than 380 nm. However, the degradation of silver coating is striking especially for the astronomical instruments in the open air, even with the protective over coating regularly applied to the silver coatings. The performance of over coating is not suitable for our use at present. We carried out the endurance tests of silver mirrors by using four sample silver mirrors supplied by four companies (hereafter called as A-mirror, B-mirror, C-mirror, and D-mirror, respectively) to know the usable duration of the silver mirror exposed to the open air. A- and B-mirrors kept reflectivity higher than that of the aluminum mirror after two months exposing test in the open air. After the five months exposure test, the reflectivity of four silver mirrors decreased lower than the one of aluminum mirror. The reflectivity of C-mirror decreased slowly, while the reflectivity at the wavelengths shorter than 900 nm is lower than the one of aluminum mirror before the exposing test. The reflectivity of D-mirror decreased most rapidly, although the reflectivity is high before the exposing test. We checked the decrease of visibility given by the degradation of silver mirrors with a test interferometer. The difference of visibility given by the degradation effect was not found among the four mirrors, the degraded A- and B-mirrors, the new aluminum mirror, and the degraded aluminum mirror.
Iwashita Hikaru
Kubo Koichi
Matsuda Ko
Torii Yasuo
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