Abstract
The reflectivity of a telescope primary mirror is one of the fundamental parameters that determines the telescope performance. Due to a lack of suitable instruments, however, measuring the absolute value of the reflectivity, in particular wide spectral measurements in-situ, has been almost impossible. To solve this problem, we developed a portable spectrophotometer called the Subaru Portable Spectrophotometer (SPS). SPS covers a spectral range between 380 and 1000 nm with a resolution of 2 nm. Its dimension and weight enable in-situ measurement on the primary mirror. A modified V-N method is applied to SPS for obtaining the absolute reflectivity. A sequential measurement makes SPS compensate the instrumental drift. The great advantage of SPS is its capability of getting absolute spectral reflectivity in-situ, even after the primary mirror is mounted on a telescope. In the case of Subaru Telescope, SPS clarified the reflectivity of the primary mirror coated with aluminum 4 years ago. Periodic measurements have been on-going since the primary mirror recoating in 2017. It is now possible to study the telescope reflectivity degradation with SPS.
Highlights
Reflectivity degradation is one of the biggest problems for astronomical telescopes
We have developed a portable spectrophotometer that is capable of measuring the absolute spectral reflectivity of a telescope’s primary mirror in-situ
Available spectrophotometers generally use “V-N” method to obtain the absolute reflectivity of an object
Summary
Reflectivity degradation is one of the biggest problems for astronomical telescopes. Since the reflectivity degradation directly affects the efficiency of observations night by night, a longer and longer integration time is required to obtain data with the same quality at the time of the mirror coating. The reflectivity degradation at CFHT became 1.96% per year using this technique.[4] for a dirty mirror, especially the one whose reflectivity is below 85% to 86% at 670 nm, it is difficult to remove dust from the mirror.[6,8] CO2 cleaning is less effective and rather produces stripes on the mirror, which increases the scatter of the measurements.[6] The cost of CO2 cleaning is another factor of consideration. A witness sample is not suitable for evaluating the aging effect, such as the reflectivity degradation With these reasons, it has been difficult to obtain the absolute value, especially the wide range spectral reflectivity of an aged telescope’s primary mirror. We have developed a portable spectrophotometer that is capable of measuring the absolute spectral reflectivity of a telescope’s primary mirror in-situ.
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