Astronomical Society of Coonabarabran

The 150' Mt Wilson solar telescope, (a "visit" with Tom Cragg) - Part 2

by Harry Roberts, with acknowledgement and thanks to Tom Cragg.

In this article I describe how the 150' Mt Wilson solar telescope works.

The 'scope consists of a 180' high tower supporting a coelostat that directs sunlight down into an apochromatic lens of 150' (46m) focal length. It is known that daytime seeing improves with height above the ground, and that long focal lengths equate with high resolution, hence the tall tower design.

At the observing room, located at ground level (Fig 1), the lens forms a 17" (430mm) diameter image of the Sun onto (yes) a piece of paper about two feet across on the observing table. There the observer traces in pencil the outlines of all the sunspots visible.

Now for the interesting bit: in the centre of the table is a spectroscope slit, and the observer, using the mirror controls, steers a selected spot umbra onto the slit. The light from the spot passes down a shaft below the table where, 80' below, it strikes a reflection mirror and a diffraction grating. From there it returns to the underside of the table where it forms a high dispersion solar spectrum (11mm per Angstrom!).

The observer now views the spectrum of the spot (through relay lenses) centred on the green absorption line Fe1 at 5250Å. The line needs to be sharp and narrow with no close neighbours. Earlier researchers used an orange line at 6173Å, but the Fe1 line gives a better photomultiplier response. This device is the "tipping plate micrometer" that permits the observer to measure the displacement of the line from its neutral position under the effect of powerful sunspot fields. This measure gives the magnetic field strength of the spot, and the direction of displacement (towards red or violet) gives its polarity. The displacement of a spectral line in the presence of a magnetic field is termed the Zeeman effect; and the 150' telescope is, in fact, a giant spectroscope.

The resulting drawings show the size and position of all sunspots together with their polarities and field strengths in hundreds of Gauss (only the significant digits are shown). The most powerful spots have fields in the range 20 - 40; and a field-strength of 40 (i.e. 4000 gauss) is rare.

The Sun drawings are big so I reproduce only a small part of one drawing (Fig 2) made by Tom in September 1957 showing a group of large active regions. Note the numerous spots, and the mixture of red (R) and violet (V) polarities, and also the strong fields in the larger spot umbrae, e.g. the R31 level in the leading spot of region 12614. The check marks N10 W36 give the position of the spot group on the solar disc. This drawing was made very close to the highest solar maximum on record, Cycle 19, when sunspot numbers reached an historic peak, and field strengths were very high, much higher than current Cycle 23. And note the group just to the west, region 12623, it's unusual in being large and complex, but has only RED polarities, something the writer has not come across before!

Tom stressed the distinction between the two kinds of field measurement made with the 150' telescope. The Babcock magnetograph measures the very strong fields found in sunspots using the tipping plate micrometer. This uses the Zeeman effect to measure fields of thousands of gauss. The other magnetogram measures the much weaker fields encountered at the sun's higher latitudes, using sensitive photoelectric equipment, where field strength are in tens of gauss.

Workers at Mt Wilson are slowly digitising the whole 25,000 page archive for on-line access, but it will take a while. Meantime, take a look at current and historic drawings, just Google "Mt Wilson 150' solar telescope" and the "drawing" link will appear. If you want to know what a big sunspot looks like, go to the drawings for the first week of April 1947, and gaze at the biggest sunspot on record, over 6000 area units in size with fields reaching V38 (3800 gauss). Now that's a big sunspot!

My deep thanks go to Tom for his detailed description of this historic instrument. As far as I know the 150' is the only 'scope of its kind anywhere, and it would be hard to overstate the importance of its solar field measurements. How are the anomalously strong fields of the 1950's to be explained? Time will tell. But without the 150' we would be largely ignorant of the long term magnetic variability of our star, which is of crucial importance to us Earthlings who live so close nearby.

Enjoy the nearest star.

(All illustrations (c) The Regents of the University of California.)

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