AO4ELT5 Proceedings

Development of extreme narrow band filter for LGS daytime use

Hayano, Yutaka (National Astronomical Observatory of Japan), Chun, Mark (University of Hawaii), Packham, Chris (University of Texas), Honda, Mitsuhiko (Kurume University)

The use of laser guide star (LGS) in daytime expands the operational time of telescope for AO instruments, especially for thermal infrared wavelengths, because the sky brightness is not so different from night to daytime. However, the daytime sky background at 589nm is about 100 times brighter than a bright night. However, as the bandwidth of sodium D2 line is a few GHz, an extremely narrow bandpass filter at this wavelength makes it possible to use LGSs during the day. Begin with developing an interference filter, we found that the spectral bandwidth could not be achieved at the level of 1nm. Then we started to investigate etalon technology to achieve an extremely narrow band filter. We focused on two different technologies, one is an air-gap etalon and another is a thin etalon layer sandwiched by the high reflective coating. We present the prototype fabrication of etalons based on both technologies and report the results of measurement of the peak transmission and spectral bandwidth. The peak transmission of air-gap etalon reached around 80%, the spectral bandwidth is approximately 0.1nm and the free spectral range (FSR) is about 1.5 nm, which are well matched to the design specification. The next challenge of air-gap etalon is how to block the adjacent spectral transmission. For the thin etalons with high reflective coating, the peak transmission is distributed around 50% to 70%, the spectral bandwidth is about 0.15nm to 0.6nm. Thus, the designed specification and the measurement results have been not exactly matched, but we found some empirical relationship between these performance parameters. Also the central transmission wavelength varies across the etalon aperture. Finally, we propose the second prototype fabrication of thin etalon sandwiched by high reflective coating, including the experience and results of first prototype fabrication.

DOI: 10.26698/AO4ELT5.0155- Proceeding PDF

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