In the mid-1970s, Lawrence Berkeley Laboratory (LBL) designed and built four identical instruments to measure circumsolar radiation. Each circumsolar telescope, as they were called, consisted of a "scanning telescope" mounted on a precision solar tracker. A digital electronics system provided control for the tracking and scanning mechanisms. The design has been described in more detail elsewhere.*
The telescope used as its basic optical element an off-axis mirror of 7.5 cm diameter and 1 m focal length. A fused silica window protected the mirror from the environment. The mirror formed an image of the sun and sky around it on a plate to the side of the telescope axis. A small hole in this plate, the detector aperture, defined the angular resolution (1/20 of the solar diameter), and the amount of light passing through the aperture into the detector assembly constituted the fundamental measurement. In the detector assembly the light was mechanically chopped, optically filtered, and focused onto a pyroelectric (thermal) detector. This type of detector was chosen for its uniform wavelength response in the 0.3 to 2.5 micrometer region and its wide dynamic range.
The telescope scanned through a 6 degree arc with the sun at the center and measured the intensity of the solar and circumsolar radiation as a function of angle. The instrument scanned in declination so that at sunrise and sunset it traveled nearly parallel to the horizon and at noon it moved in a vertical plane.
Each 6 degree scan required 1 minute of time. The intensity was digitized every 1.5' of arc. Within 0.5 degrees on either side of the sun an aperture of size 1.5' of arc was used, and outside this region the aperture was increased to 4.5' of arc. A set of measurements consisted of one scan at each of 10 filter positions: eight optical filters, one open (or clear) position, and one opaque position. The opaque position was used to measure the detector noise. The absolute determination of the normal incident flux (within 2.5 degrees of the sun center) was provided by an active cavity radiometer.** This device was self-calibrating and had an accuracy of 0.5%. This pyrheliometer was provided with a matched set of filters that rotated synchronously with those on the scanning telescope. Thus the telescopes produced an absolute measurement of the normally incident flux along with the detailed solar profile in eight wavelength bands. Two pyranometers were used, one mounted in the conventional horizontal position, and one tracking the sun.
The telescopes were capable of unattended operation for up to a week, although they typically received a daily inspection during the work week. During the night the solar trackers ran backwards and automatically initiated operation at the beginning of each day. The data were recorded on magnetic tape and processed at LBL's computer center.
During the late 1970s and early 1980s, the circumsolar telescopes were installed at various locations in the United States and collected a large amount of data, which LBL has archived.
*Donald Grether, Jerry Nelson, and Michael Wahlig, "Measurement of Circumsolar Radiation," Proceedings of the Society of Photo-optical Instrumentation Engineers, Volume 68, page 41, 1975.
**R.C. Willson, "Active Cavity Radiometer," Applied Optics, vol. 12, page 810, 1973.
Table of Contents