Brief History of the Solar Observatory

The McMath-Hulbert Observatory (MHO), was founded in 1930 by three amateur astronomers, Francis McMath, his son Robert McMath, and Henry Hulbert, with the goal of enhancing their clever development of time lapse astrophotography, for outreach purposes. It soon garnered the interests of the University of Michigan and evolved into a world-class solar observatory, operated by the U of M through 1979. It is now in private ownership, nonfunctional, but largely intact.

General Motors Astronomy Club Visits MHO

On Saturday, July 28th, 2018, nine members of our club toured all three towers of the historic observatory, and learned a great deal about the history and technical capability of the facility.  We would like to extend a special thanks to Jim Shedlowsky for donating his time and sharing his knowledge.

Since solar observatories are such complex systems, I had a few gaps in understanding after the tour, so I did a some follow-up research and found many published papers referencing the McMath-Hulbert Observatory, and was able to answer some of the questions that stumped us on the tour.  The information below is a mixture of what I learned on the tour and what I gleaned from the published papers.

The Oldest Telescope

The tour began in Tower 1, the smallest of the observatory buildings and the first one  built, more than 85 years ago, in 1930.  It was in this building, in 1934, where the very first “movies” of a solar prominence was filmed, using a 10.5-inch equatorial refractor, married to a custom-made motion picture camera, called a spectroheliokinematograph.  Much of the original equipment here is still intact, although somewhat deteriorated by the passage of time.

Our group was also led to the basement of Tower 1, where we saw the electrical control room, which included a number of devices to convert the DC power available at the time, into the AC power used to supply the observatory.

Inside a Tower Observatory

Upon entering Tower 2 of the observatory, our group was amazed at the sheer size and complexity of the equipment we found, complete with 1940’s era wiring and gauges and a steel staircase,

winding 50 feet up to the observatory dome, reminiscent of both an underwater submarine and Frankenstein’s laboratory, at the same time.  After puzzling at what all the dials, switches, and gauges were for, I jokingly asked, “Where is the instruction manual?”

Tower 3, also known as the McGregor Building, was the last one built and also the largest at the observatory.  It came complete with 2-story offices, laboratory, and machine shop.  In this last part of the tour, our group enjoyed inspecting the darkroom facilities, old photographic glass plates, mirror plating vacuum chamber, and peering up into the 70-foot steel tower.   Throughout the 1940’s and 50’s, light from the sun was collected from this tower and redirected into an giant vacuum spectrograph, used by scientists to further our understanding of our closest star.

A bit puzzling, was a sign on a hallway door stating “DANGER 50,000 ANGSTROMS”, which left us wondering what was so dangerous about angstroms.  Honestly, I didn’t know what an angstrom was, until I looked it up later and found that it is a unit of distance, equal to one-tenth of a nanometer.  Although not obvious to the casual 21st-century observer, it seems this sign is warning of the dangers of infrared light, which has a wavelength of 50,000 angstroms, but it is still not clear what the danger is, exactly.

Coelestat vs. Heliostat

One of the unanswered questions that came up on the tour, was regarding the operation of the mirror systems at the observatory and how they are able to track the sun.    It turns out, this system of moving mirrors is called a “coelostat,” which is distinctly different from a “heliostat.”

Diagram of the mirror system in Tower 3 of the McMath-Hulbert Observatory, which reflects and diffracts sunlight, for the purpose of imaging solar phenomena in various wavelengths of light.

A heliostat is a moving mirror that reflects the suns light toward a fixed target, such as an imaging device or an energy absorbing device, in the case a of solar-thermal farm.  The problem with using a heliostat in a solar observatory, is that the image of the sun produced by the device rotates at a rate of one revolution per day, due to the rotation of the earth.  The solution to this problem, is the coelostat, which is a system of two mirrors (like the ones we inspected in the top of Tower 2) that move in unison to produce a stationary image.  I imagine this would be very useful when attempting to precisely measure the velocity of a solar prominence, for example.

Both Tower 2 and Tower 3 of the McMath-Hulbert observatory are fitted with a coelostat system of mirrors.

Further Reading and Published Works

As a result of my follow-up research, I have compiled 11 published works (which I found online) into a “book” attached here:  McMath-Hulbert Technical Papers_GoodallCompilation2018

This document has very detailed information on the operation of most parts of the facility, and some of the scientific discoveries resulting from the work of Robert McMath and his associates.

A printed copy of this book will be provided to the McMath-Hulbert Astronomical Society to be used for reference in future work, and as a token of gratitude for reaching out to our club.  Anyone wishing to find out more about the observatory and future tours, can visit the McMath-Hulbert Observatory website at http://www.mcmathhulbert.org/.

Clear Skies,

Jim Goodall
President, GMAC