Editing Mount Wilson Observatory (section)

== Solar telescopes ==
[[File:The 150-Foot Solar Tower Observatory on Mt. Wilson as seen from near the base.jpg|thumb|upright|At the base of the 150-foot Solar Tower]]

There are three [[solar telescope]]s at Mount Wilson Observatory. Just one of these telescopes, the {{convert|60|ft|m|adj=on}} Solar Tower, is still used for solar research.

===Snow Solar Telescope===

The Snow Solar Telescope was the first telescope installed at the fledgling Mount Wilson Solar Observatory. It was the world's first permanently mounted solar telescope. Solar telescopes had previously been portable so they could be taken to solar eclipses around the world. The telescope was donated to Yerkes Observatory by Helen Snow of Chicago. George Ellery Hale, then director of Yerkes, had the telescope brought to Mount Wilson to put it into service as a proper scientific instrument. Its {{convert|24|in|cm|adj=on}} primary mirror with a {{convert|60|ft|m|adj=on}} focal length, coupled with a spectrograph, did groundbreaking work on the spectra of sunspots, doppler shift of the rotating solar disc and daily solar images in several wavelengths. Stellar research soon followed as the brightest stars could have their spectra recorded with very long exposures on glass plates.<ref name="mtwilson.edu">{{Cite web |url=http://www.mtwilson.edu/Simmons2.html |title=Mount Wilson Observatory |access-date=2015-08-11 |archive-url=https://web.archive.org/web/20150712005138/http://www.mtwilson.edu/Simmons2.html |archive-date=2015-07-12 |url-status=dead }}</ref> The Snow solar telescope is mostly used by undergraduate students who get hands-on training in solar physics and spectroscopy.<ref>{{cite web |last=Turner |first=Paula C. |title=The CUREA Program at Mount Wilson |url=http://physics.kenyon.edu/people/turner/cureaweb/turner_aas07poster.pdf |publisher=[[Kenyon College]] |access-date=21 September 2020}}</ref>
It was also used publicly for the May 9, 2016 transit of Mercury across the face of the Sun.

[[File:The top of the 150-Foot Solar Tower Observatory on Mt. Wilson.jpg|thumb|left|Top of the Solar tower containing the mirrors]]

===60-foot Solar Tower===

The {{convert|60|ft|m|adj=on}} Solar Tower soon built on the work started at the Snow telescope. At its completion in 1908, the vertical tower design of the 60-foot focal length solar telescope allowed much higher resolution of the solar image and spectrum than the Snow telescope could achieve. The higher resolution came from situating the optics higher above the ground, thereby avoiding the distortion caused by the heating of the ground by the Sun. On June 25, 1908, Hale would record [[Zeeman splitting]] in the spectrum of a sunspot, showing for the first time that magnetic fields existed somewhere besides the Earth. A later discovery was of the reversed polarity in sunspots of the new solar cycle of 1912. The success of the 60-foot Tower prompted Hale to pursue yet another, taller tower telescope. In the 1960s, Robert Leighton discovered the Sun had a 5-minute oscillation and the field of helioseismology was born.<ref name="mtwilson.edu"/><ref>{{cite web |last1=Pinkerton |first1=Stephen |last2=Chen |first2=Casey |title=History of the 60 Foot Solar Tower |url=http://physics1.usc.edu/solar/history.html |publisher=[[University of Southern California]] |access-date=21 September 2020}}</ref> The 60-foot Tower is operated by the Department of Physics and Astronomy at [[University of Southern California]].

===150-foot Solar Tower===

The {{convert|150|ft|m|adj=on}} focal length solar tower expanded on the solar tower design with its tower-in-a-tower design. (The tower is actually {{convert|176|ft|m}} tall.) An inner tower supports the optics above, while an outer tower, which completely surrounds the inner tower, supports the dome and floors around the optics. This design allowed complete isolation of the optics from the effect of wind swaying the tower. Two mirrors feed sunlight to a {{convert|12|in|cm|adj=on}} lens which focuses light down at the ground floor. It was first completed in 1910, but unsatisfactory optics caused a two-year delay before a suitable doublet lens was installed. Research included solar rotation, sunspot polarities, daily [[sunspot drawing]]s, and many magnetic field studies. The solar telescope would be the world's largest for 50 years until the [[McMath–Pierce solar telescope]] was completed at [[Kitt Peak]] in Arizona in 1962. In 1985, UCLA took over operation of the solar tower from the Carnegie Observatories after they decided to stop funding the observatory.<ref>{{cite web |last=Gilman |first=Pam |title=The 150-Foot Solar Tower: History |url=http://obs.astro.ucla.edu/150_hist.html |publisher=[[University of California, Los Angeles]] |date=2003 |access-date=21 September 2020}}</ref>