Thermal Detector Studies the Sun

Richard Gaughan

Changes in the sun's luminosity may affect the global climate over a timescale of centuries. Scientists attempting to quantify these changes must cope with both a wide spectral range and the extreme spatial variation across the surface of the sun. Now a team at Cambridge Research and Instrumentation Inc. has developed an imager with a flat spectral response over the sun's wavelength range that may help researchers better understand solar variability.

The solar bolometric imager is built around NightSight, a ferroelectric array from Raytheon Commercial Infrared of Dallas that was developed for defense applications. Incident radiation in the 8- to 14-µm band increases the temperature of barium-strontium-titanate elements in the array. This changes their capacitance, which is interpreted as a thermal image. To convert this sensitivity range to meet solar wavelengths, where 96 percent of the light is between 0.28 and 2.6 µm, the scientists coated the detector with a 30-µm-thick layer of gold black.

To accurately measure the contribution from small facular structures in the photosphere, they integrated the detector with a 300-mm, f/12 Dall-Kirkham telescope with uncoated Pyrex mirrors from Takahashi Corp. The resulting system's angular resolution is 2.4 arc sec per pixel, and the diffraction-limited resolution within a 50-µm pixel is 2.1 arc sec at 2.6 µm.

Custom electronics improved the system's linearity, so that it was able to measure 15-arc-sec structures with a signal-to-noise ratio of 5:1. Some nonlinearities were present when the system observed highly nonuniform illumination near the solar limb, but the team expects to reduce this in the next version of the solar bolometric imager.

Explaining variability

A more accurate system will enable the researchers to determine whether photospheric structures such as faculae and sunspots are responsible for all the variation in the sun's brightness, said Peter Foukal, a scientist at the company and coauthor of a paper describing the work in the March 1 issue of Applied Optics. Other, unidentified mechanisms may contribute to variations in the sun's total output.

The first flight version of the imager will be deployed on a balloon, and a later orbital flight on the Solar Dynamics Observatory is possible. Foukal's main interest is applying technology to explain variation in solar luminosity, but he said that he is also excited to see that thermal arrays, a technology developed for the military, can be used to study global climate change.