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The GONG 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 effective aperture of
2.8cm. Near the focus of the 1-m focal length objective lens is a box
that contains various optics that can be moved in and out of the
beam. These optics allow calibration of the response of the
instrument. A variable polarization retarder can be put into the beam
to allow the line-of-sight component of the solar magnetic field to be
imaged. All of these mechanisms are under computer control and
normally operate automatically.
The instrument is controlled by two computers and a precise clock. It
normally produces a data record every minute, day and night. During
night, only instrumental and environmental parameters are
recorded. When the program determines that the sun has risen, the
instrument front end is unstowed and pointed to the sun closely enough
so that a guider sensor can provide precise pointing information. If
it is cloudy, the computer estimates where the sun is and points to
that location. The first time enough sunlight is available on a day, a
calibration sequence is executed. Observations are made until near
sunset, at which time the instrument stows itself. The instrument
operates automatically for one week and the only user intervention
need is to change the data tapes or to perform maintenance.
This instrument is developed by the The Global Oscillation Network
Group (GONG), which is a community-based project to conduct a detailed
study of solar internal structure and dynamics using helioseismology.
In order to exploit this new technique, GONG has developed a
six-station network of extremely sensitive, and stable velocity
imagers located around the Earth to obtain nearly continuous
observations of the Sun's "five-minute" oscillations, or pulsations.
Additional information on the GONG Instrument available at
"http://www.gong.noao.edu/Instrument/instrument.html"
General information on the Global Oscillation Network Group available at
"http://www.gong.noao.edu/"
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