Designing ultra-narrowband interference filters

摘要

To understand the factors involved in the cost and risk of manufacture of ultra-narrowband imaging filters in the near IR, we have carried out detailed design work for a range of different filter specifications. This work was essential to the success of the DAZLE near-IR imaging spectrograph, a project led by the Institute of Astronomy at the University of Cambridge. The necessity for very narrow passbands (approximately 1 nm), for observing in the windows between the OH emission lines in the near-IR bands, requires coatings of more than 200 layers and greater than 30 μm thickness. Broadband filters to block OH and thermal emission over wide near-IR spectral ranges (1.4-2.8 μm) also require many thin film layers in the filter coatings. In most cases, the large number of layers is the most significant factor in determining manufacturing cost, as such coatings require long periods of time for deposition (days to weeks), with a resultant high risk of failure during manufacture.