We compare four types of instruments for recording monochromatic astronomical images: filters; slitless spectrographs; multislit spectrographs; and interferometers. Each of these instruments has been used on the sodium emission cloud of Io, with varying degrees of success. Multislit spectrographs and interferometers encode the signal, and it can be extracted with a noise level close to that for photon statistics. This is not normally the case for filters and slitless spectrographs. On balance, we find that a Mach-Zehnder interferometer provides the best system for imaging faint monochromatic signals on a brighter continuum background.
© 1978 Optical Society of America
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