Reflectivity of the Mirrors

Reflectivity of the Primary Mirror

The Subaru Telescope primary mirror was recoated with Aluminum on October 20, 2017. This was the eighth coating work from its arrival at Maunakea, Hawaii in 1998, and was about four years from the previous recoating on August 15, 2013. After the recoating, we measured the reflectivity outside of the vacuum chamber using a portable measuring instrument named "Subaru Portable Spectrophotomer (SPS)" on October 23, 2017. The reflectivity of the primary mirror immidiate after the eighth coating is indicated in the following figure.

The reflectivity of the primary mirror has been measured in-situ on the telescope since October 10, 2017, by using SPS. The following figures show the reflectivity before and after of the recoating, as well as the reflectivity degradation of the primary mirror.

Plain text data are available as below.

Witness Sample Reflectivity

A number of microscope slide glasses were placed in the vacuum chamber and coated with Aluminum together with the primary mirror to make "witness samples." The reflectivity of a witness sample was measured with a Hitachi U-4001 spectrophotometer at the Base Facility in Hilo on December 26, 2017. The following figurere shows the reflectivity of the witness sample.

Plain text data is also available as below.

Subaru Portable Spectrophotometer (SPS)

Subaru Portable Spectrophotometer (SPS) is a compact, portable, and powerful reflectivity measuring instrument developed by Subaru Telescope in 2017. SPS can measure the absolute spectroscopic reflectivity of the primary mirror in-situ on the telescope. Its small dimension (450 mm x 310 mm x 370 mm) and light weight (5.9 kg) enable in-situ measurement on the primary mirror. SPS covers the spectral range from 380 nm to 1000 nm with 2 nm resolution. The incident angle to the measuring surface is 12 degrees. The measurement beam size is about 12 mm in diameter. The statistic error is less than 0.2 % in rms between 450 nm and 900 nm, and there are less than 0.6 % in rms at 380 - 450 nm and 900 - 1000 nm ranges. The systematic error is expected to be less than 0.2 %.

We wish you will make a same type of measuring instrument and measure the absolute spectroscopic reflectivity of telescopes at your observatory. For your convenience, we will open detailed information of SPS.


We are now planning to upgrade SPS with eXtra Wide Wavelength (XWW) capability. Reflectivity at near infrared will be available in near future.

Further information

This page was last updated on July 2, 2018. Questions regarding this page should be directed to Hirofumi Okita ( ).

Old Information

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Reflectivity of the Primary Mirror

The re-aluminization of the 8.3 meter primary mirror was conducted in August, 2010. This was the sixth re-coating after the arrival of the mirror at the summit of Mauna Kea in 1998.

A number of microscope slides are placed in the coating chamber when the primary mirror is aluminized. These slides are then brought down to the Base Facility in Hilo, where their reflectivity is measured in the laboratory by a spectrophotometer.

The reflectivity of such samples coated in August 2010 is indicated in the following figure. They should be indicative of the reflectivity of the primary mirror. Degradation of the primary mirror's performance is kept to a minimum by regular CO2 snow cleaning.

Plain text data is also available as M1-2010s.txt.

Current Reflectivity and Scattering of Primary Mirror

The reflectivity and scattering archived data of the primary mirror is monitored with micro-Scan (TMA Technologies, Inc.). The reflectivity and scattering are fairly stable at about 82 % and 80-100 A, respectively, after 400 days from re-coating.

Reflectivity of IR Secondary Mirror

Infrared (IR) secondary mirror was re-coated with only silver in April, 2003. Reflectivity of silver coated the infrared secondary mirror at the wavelength from 300 to 2,500 nm (Kurakami et al. 2004).

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