An instrument based on a Michelson interferometer called a Fourier Tachometer (Beckers and Brown 1978)* was selected, and is supported by a highly automated, portable installation, reminiscent of a spacecraft experiment in its design philosophy
Optical Table Showing Layout
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| 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 effective aperture of 2.8cm. | ||
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 Calibration wheel schematic |
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. | |
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| 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. | ||
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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. |
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| The image detector is a CCD camera which has 2.5 arc second square pixels giving a resolution of 5 arc seco nds, which matches the diffraction limit of the optical system. 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. | ||
 Camera, wave plate and rotator assembly |
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The shelter also houses the electronics rack, a data
acquisition computer and various other support electronics to
make it a self sustaining instrument capable of being operated
in remote locations. |
Electronics Rack Assembly: |
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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. |
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Instrument Performance Information | Maintenance Data Index
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All six shelters at integration site prior to shipment in November 1994
*Beckers, J.M., and Brown, T.M. 1978, Oss. Mem. d. Oss. Astrofis.
d. Arcetri, 106, 189
For more information contact
Jack Harvey,
| Dave Armet | |
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