Astronomical Society of Coonabarabran

The 150' Mt Wilson solar telescope (a "visit" with Tom Cragg) - Part 1

by Harry Roberts, with thanks to Tom Cragg.

When I first stumbled across the Mt Wilson 150' website it was a revelation. Here at last was an instrument that showed sunspots as they looked in the eyepiece: huge masses of penumbra dotted with umbrae big and small, often stretching across 15º of solar longitude; awesome giants. The "150'" images were quite unlike the tiny blurs that pass for sunspots on other Sun patrol sites.

The 150' solar tower is located on the pine-clad summit of Mount Wilson in southern California. The instrument that makes these high resolution images is the Babcock Magnetograph that sits at the focal plane of the Mt Wilson 150' solar telescope. Built by Hale in 1912; it's a venerable instrument, and one that has not (yet) been superseded by digital technology. Indeed, the output of the telescope is pencil drawings made to a resolution of better than an arc second! It allows the observer to directly view and record the polarity of individual umbrae within a sunspot, and so determine its Hale polarity class, its magnetic "power" level and to map zones of contrasting polarity. The instrument is unique in this capability and is the only site that makes and posts daily magnetic sunspot drawings. Although closed in 2004, to howls of protest after ninety years of operation (lack of funds), the 'scope is operational again and ready for Solar Max C24.

By chance I learnt that the Coonabarabran Astronomical Society had a member named Tom Cragg, and I wondered if this was the T. Cragg mentioned often in Taylor's "Observing the Sun" and in Zirin's "Astrophysics"? A phone call proved that it WAS, and moreover that Tom was happy to describe the incredible Mt Wilson solar telescope with which he observed for twenty-four years from 1952 to 1976. In '76 Tom signed on at the AAT as Chief Night Assistant, where he worked until 1993: a career in astronomy spanning forty years.

Before I describe (with Tom's help) how the 150' telescope works I must explain why amateurs with an interest in sunspots need to consult the daily magnetographs.

The Sun is a magnetic body, and little happens there that is not shaped by magnetic fields. Hale discovered that the Sun has magnetic cycles and that its N-S polarity reverses every 22 years. He also found that sunspot groups obeyed polarity laws, from which he derived the current Hale Polarity Classes. And as sunspots develop through the Hale classes to the Beta-Gamma-Delta class they begin flaring. Where do the flares arise? Often it's along the boundaries between zones of opposite polarity. So an amateur with access to both white light and H-alpha bands can consult the Mt Wilson magnetographs to determine the chance of flares and the host spot's magnetic structure. The magnetographs provide this crucial high resolution data, and the facility has made ~25,000 drawings since 1917, a treasure trove for solar researchers.

George Ellery Hale (1868 - 1938), creator of Mt Wilson and the 150' telescope, discovered in 1912 the magnetic fields of sunspots by the Zeeman effect in their spectra, and in 1919 Hale et al showed that spot groups in a given hemisphere had one polarity preceding (westwards, or p) and the opposite following (f).

In Part 2 of this article I will describe how the 150' telescope works (from my communications with Tom) and explain why the "flare-hunter" needs to monitor the daily magnetograph drawings.

Until then, keep a close watch on the Sun.