GONG Instrument

An instrument based on a Michelson interferometer called a Fourier Tachometer
(Beckers and Brown 1978)* was selected, and it will be supported by a highly
automated, portable installation, reminiscent of a spacecraft experiment in its design
philosophy.

The instrument consists of two mirrors tracking the Sun in elevation and cross-
elevation axes that feed light horizontally into a cargo container housing the rest of
the equipment. The optical system is sealed by a filtered window and has an
image-forming aperture of 6 cm. One of the mirrors is mounted on piezo actuators
to provide high-speed correction of tracking errors caused by the Earth's atmosphere.
A hybrid filter of 1 Å passband isolates the Ni I line at 6768 Å . This filter consists
of a 5 Å two-cavity interference filter followed by three birefringent elements. Two
of these birefringent elements combine calcite and ammonium dihydrogen phosphate
in appropriate thicknesses to produce a thermally-compensated passband. All of the
elements are mounted in an oven whose temperature is stabilized to the order of
0.00001 K.

                                 

The heart of the instrument is a polarizing Michelson interferometer having a path
difference of about 30,000 waves. This is constructed to have a wide angular field
and to be thermally stable. The cosine-squared transmission pattern produced by the
interferometer is scanned across the filtered spectrum of the Sun by a rotating wave
plate and, thus, modulation is produced by the presence of the Fraunhofer line. The
phase of the modulation is a good measure of Doppler shift. The image detector is a
modified commercial CCD camera giving a resolution of about 8 arcsecs per pixel.
Three images are obtained for each cycle of modulation to provide the phase,
amplitude, and brightness of each pixel. Sixty seconds of such images are integrated
and the result is recorded on helical-scan, digital tape.

                      

A 'breadboard' model, in operation since early 1988, produced the best solar
Dopplergrams yet obtained for this type of instrument. The breadboard instrument
was replaced by a full-scale prototype field station in early 1990. Final engineering
development of the prototype instrument and construction of the six network field
stations are underway. Science operations of the network are expected to begin in late
1994.

*Beckers, J.M., and Brown, T.M. 1978, Oss. Mem. d. Oss. Astrofis. d. Arcetri, 106,
189