|The 150-Foot Solar Tower
The dual-channel solar magnetograph was first developed by Horace W. Babcock and Harold D. Babcock at the Hale Solar Laboratory (in Pasadena, Calif.) in 1953. Four years later, a magnetograph was installed for full time use at the Mt. Wilson 150-foot solar tower — just in time for the International Geophysical Year. In the mid 1960's the magnetograph and data taking system were completely refurbished by Carnegie Institution staff member Robert F. Howard to allow the whole system to be put under digital control. The rebuild was essentially finished by July 1974. The magnetograph currently in use at the tower was constructed in 1982, and upgraded under the direction of Roger K. Ulrich in 1994 and 1996. From these numerous modifications, alterations, and advancements this magnetograph has remained a state-of-the-art instrument.
The image of the solar disk is projected onto a small table at ground level of the telescope. This table has a hole in its center to allow light from the solar disk to pass through. The beam then goes through a 4-position filter wheel, which is generally used in the "no filter" position. Next, a combination of a KD*P crystal (Pockels Cell) and a Glan-Thompson prism is used to act as a circular polarization analyzer. A discrete portion of the solar image is selected with a 20 or 12.5 arc second square aperture, which, in turn, is laid out by way of a Walraven image slicer onto the 203 micron entrance slit of the instrument. The light is then allowed into the spectrograph pit.
The spectrograph pit employs the use of a diffraction grating in a Littrow mounting. The Littrow lens is a 75-foot focal length air-spaced Brashear doublet with an aperture of 9 inches. The diffraction grating, produced by Milton-Roy in 1994, is 250 by 408 millimeters in size, ruled at 367.5 lines per millimeter, and is blazed at the ninth order green (60 degrees). This design allows for the simultaneous examination of the Fe I line at 5250.2Å (9th order), Na I D1 at 5895.9Å, Na D2 at 5890.0Å (8th order), Ni I at 6767.8Å (7th order), and Ca II K line at 3933.7Å (12th order), all within the three foot exit slit box. The different orders are separated by the use of narrow band-pass filters, located in front of the exit slits which follow.
In the current exit slit assembly, two separate stages maintain positions by servoing on absorption features in the solar spectrum. Generally, for the observations that are posted on the Web, the selected solar lines are Fe I at 5250.216Å and Na I at 5895.940Å. Each stage is equipped with a pair of fiber optic bundles that transfer light from the solar spectrum to 24 photomultiplier tubes (Hamamatsu R1477). The spectrum side of the fiber optic bundle is rectangular in shape, but through randomly rearranging the individual fiber optics, the PMT side of the bundle is circular. Each fiber optic bundle samples the light intensity at selected distances into the wing of the absorption line, and up to 10 points may be observed in the line profile. The stage maintains its position by incrementally servoing blueward or redward in the spectrum such that PMT intensities from both fiber optic bundles remain of approximately equal value. By this method, doppler shifts may be calculated. Zeeman line splitting is also observed and, by means of a 400Hz modulation of the KD*P crystal, magnetic field strengths and polarities are determined. Thus, both motion and magnetic fields on the sun may be measured with the use of this magnetograph. Two other absorption lines in the solar spectrum, Cr II at 5237.325Å and Ni I at 6767.782Å, are also observed. However, data from these lines are not posted on the 150-Foot Solar Tower Home Page, but are indeed archived for research purposes.
|Article by Tom Shieber and Larry Webster.
Images by Steve Padilla and Larry Webster.
Harold D. Babcock Bruce Medalist page.
Horace W. Babcock Bruce Medalist page.
Babcock, Horace W., The Solar Magnetograph, Astrophysical Journal, 118, 387 (1953)
Howard, Robert F., The Mount Wilson Solar Magnetograph: Scanning and Data System, Solar Physics, Vol. 48, June 1976.