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GRO Instrumentation -
The Solar Spectroscope
The sun is first imaged with a small telescope and is then passed through a 2X Barlow and focused on a 50 micron air slit. The slit image is collimated using a 150mm fl spherical mirror and projected on to a 1200lpm ruled grating blazed at 500nm and working in the second order. The instrument is quite compact being just under 10” long, 4.0” wide and ~ 3.0” high and it too is painted gloss white.
A micrometer is utilized to rotate the grating allowing the entire solar spectrum to be observed with hundreds of Fraunhoffer absorption lines including the elusive Nickel line located between D1 and D2.
The spectroscope has been equipped with two mounting options, the first is a simple 2.0” adapter allowing the instrument to mount up to any telescope with a 2.0” focuser such as the GRO 80mm Vixen. However by installing the long UNITRON draw tube seen in the photos the spectroscope can be mounted to any of the smaller GRO UNITRON telescopes but specifically either the 60mm X 500mm f/8 or the 60mm X 700mm f/11.
With the aperture(s) stopped down to ~ 25mm and the suns image positioned and focused on the 50μ air slit (via a little fun addition I will describe next) the entire solar spectrum can be observed as the micrometer rotates the grating. By utilizing a small group of narrow band filters the first and third spectral overlap due to using the grating in the second order can be all but eliminated.
A 10μ H-alpha filter is used at the red end and a deep violet filter enhances the visibility of the Calcium lines (H & K).
The “little addition” I mentioned is a small “trap door” installed on the spectroscope lid and centered over the air slit holder. Once the instrument is installed on the telescope the solar image can be viewed with through the trap door aperture and precisely centered on the slit aperture.
When the spectroscope is mounted up it provides a complete solar observing system allowing the visible spectrum, the chromosphere and the photosphere to all be viewed simultaneously.
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