URSIES: an Ultravariable Resolution Single Interferometer Echelle Scanner

A Fabry-Perot interferometer in a Ramsay mount is used in tandem with an echelle Hilger monochromator with pinholes instead of slits. The instrument, URSIES, is enclosed within a pressure chamber filled with Freon. Photoelectric pulse counting techniques and pressure scanning are used to record the spectrum. This design has four basic advantages: (1) The resolution of the scanner is variable from 5.0 A to 0.005 A, and the effective wavelength range is quite broad, from 3500 A to 13,000 A. (2) A light gain of ten over conventional grating scanners at resolutions of 0.1 A or better is achieved. (3) Very low levels of light from outside the wavelength passband reach the detector; for a resolution of 0.03 A at 6000 A this level is 5%. (4) There are very low levels of scattered light from the pinholes when extended sources are observed. The scattered light from the instrument at the edge of the solar disk is found to be less than 1% from 4000 A to 11,000 A. Measurements demonstrating these advantages are discussed.