Auroras, which occur when charged solar particles pass through the thermosphere, are more than just polar eye candy — they offer tantalising clues about the inner workings of our planet’s relationship with its nearest star. Problem is, modern cameras can’t provide the data granularity needed to view them properly without be outfitted with specialised optical filters. Otherwise, it’s impossible to study specific ranges within the aurora’s visible spectrum.
So rather than manually flip between various filters while shooting, an international research team has designed and built a hyperspectral camera that captures the entire sky in over the entire range of visible light with every shot.
Initially developed in 2009, funded by the Norwegian Research Council, and dubbed the NORUSCA II (short for Norwegian and Russian Upper Atmosphere Co-Operation on Svalbard Part 2) project, this joint scientific venture aims to install an identical pair of NORUSCA II cameras, one installed at Kjell Henriksen Observatory (KHO) in Norway, the other going to the Auroral station in Barentsburg (BAB). Unlike conventional cameras that converge all wavelengths of incoming light into a single image (or specific wavelengths with the help of filters), the NORUSCA II camera captures 41 discrete optical bands that spans the entire visible spectrum from 400-720nm wavelengths. Each band has a 7nm wavelength range.
What’s more, the NORUSCA II captures all 41 images in just over 2 milliseconds, many times faster than what a conventional camera can achieve. “A standard filter wheel camera that typically uses six interference filters will not be able to spin the wheel fast enough compared to the NORUSCA II camera,” Fred Sigernes of the University Centre in Svalbard (UNIS), Norway explained. “This makes the new hyperspectral capability particularly useful for spectroscopy, because it can detect specific atmospheric constituents by their unique fingerprint, or wavelengths, in the light they emit.” This lightning-fast capture speed is made possible by the fact that the NORUSCA II has no moving parts, instead relying on its state-of-the-art Liquid Crystal Tunable Filters (LCTFs) to switch between bands every 50 microseconds.
According to the research team’s report, recently published in the journal Optics Express,
Spectral tuning is obtained by using liquid crystal variable retarders to a Lyot filter design (uses the effect of double refraction and polarization to separate wavelengths). A series of optical elements are bounded together with index-matching epoxy. Each element transmits light with transparency that varies sinusoidal as a function of wavelength. The transmitted light adds constructively in the desired bandwidth region and destructively everywhere else within the wavelength region of the filter.
By overlaying the the images of individual bands in various combinations, scientists can examine the numerous facets of this phenomenon in ways that a single amalgamation image could not and may eventually lead to new methods of identifying auroral forms. [Physorg – Optics Express – KHO 1, 2 – Wikipedia]
Image: Optics Express