High-speed cameras and solar observing


The National Solar Observatory operates two facilities with demanding needs for rapid image collection (i.e. > television frame rates). The first is GONG, a global network of six identical small telescopes devoted to nearly continuous observations of the sun’s surface vibrations in order to study its internal properties by helioseismology. The second, SOLIS, is a suite of three instruments that collects images and spectra of the sun needed to study the behavior of solar activity on time scales of minutes to decades. Five different types of cameras are installed in these instruments. High speed, high sensitivity, large dynamic range, and good photometric performance are key factors for cameras used to make measurements of subtle solar signals that pass through the noisy terrestrial atmosphere. A camera that combines all these characteristics is elusive. The combination of high speed and good photometric performance, when observing small intensity changes, is particularly hard to get in practice. High speed in large format CCD and hybrid FPA cameras is achieved by dividing the array into multiple channels that are read simultaneously. An unwanted result of this technique is cross talk between signal channels. It is of order 1 percent in the case of Silicon Mountain Design 1M60_20 cameras (1k x 1k, 60 fps) and Rockwell Scientific Company HyViSI-1024 cameras (1k x 256, 92 fps). Cross talk (and also successive-frame image retention) are particularly hard to deal with since they may exhibit non-linear characteristics that depend on illumination light level. We describe these and other phenomena, attempts to mitigate the effects, and results from solar observations.