Filter Helps Predict Solar Flares

Richard Gaughan

Although it has been making tunable etalons since 1978, Queensgate Instruments Ltd. has outdone itself by developing a 150-mm etalon as part of a system that will enhance the US Air Force's ability to predict solar flares.

In helping the Air Force predict solar flares, the large diameter of Queensgate's etalon will enable high image quality while isolating the desired wavelength.

Queensgate has developed a tunable wavelength filter consisting of two Fabry-Perot etalons. The filter will be used in a prototype telescope that the Air Force is constructing at the National Solar Observatory in Sunspot, N.M., for its Improved Solar Optical Observing Network. It will enable measurement of fine structure in the solar magnetic field and, therefore, predict solar activity.

Etalons are typically suited to applications with a tight spectral bandwidth and a relatively low signal. The Air Force's telescope has plenty of signal, but the large-diameter etalon serves another purpose.

The large diameter lets us maintain 1-arc-sec resolution while still isolating the wavelength of interest -- even out to the near-IR," said Don Neidig, technical program manager for the network.

The 150-mm clear-aperture, high-resolution etalon provides a passband <0.01 nm wide. Another lower-resolution etalon prefilters the incident light so that only the wavelength of interest is passed from the high-resolution etalon.

Piezoelectric actuators and capacitive sensors control the cavity length. By varying the cavity separation, wavelengths of interest over the range of 600 to 1000 nm can be selected. Examination of solar images taken at very narrow wavelength windows indicates the magnetic field structure, which is then analyzed to predict solar activity.

Making such large-diameter etalons has its problems, most of which are related to the glass itself. The blanks must be homogeneous, and the relative surface flatness of the two cavity faces must be l/100 at 633 nm. The time required to produce such large, high-quality glass surfaces keeps costs up and limits the market.

Other applications

Large-diameter etalons typically find use in astronomical systems, while etalons with clear apertures as small as 0.5 mm function as wavelength filters in telecommunications. For filters with smaller diameters, Queensgate's Krzysztof Pietraszewski also envisions a large market in environmental applications.

"For example," he said, "supermarkets could monitor refrigerant leaks from their freezer cases by measuring absorption lines of the suspect compounds." If growing public concern results in legislation mandating more accurate monitoring of those compounds, the hot market for these etalons could get hotter.