March 29, 2020 – Liquid Telescopes

At first Hiram Bingham, generally credited as Machu Picchu’s discoverer, thought they were for grinding grain.  Later scientists discarded this hypothesis as the structures were not like others used in that time period.  The current consensus is they were used as astronomical devices.  Fill them with water and observe the stars reflections as they pass over.  But why two?  I wonder if using two somehow produced a wider view if the viewer was just the right distance looking down into them.

This type of observing device has limitations, including loss of light from the absorption of light by the water.  As fixed equipment the Inca astronomers were limited to objects directly or nearly overhead.  Who the heck wants that?  Well, apparently some astronomers today are turning to this principle and building telescopes with liquid mirrors.  Why?  The promise of big mirrors and low cost.

Today’s astronomers aren’t using water, they’re using mercury!  I remember playing with mercury.  Had a lot of fun with it.  Now, if a microliter gets loose in the classroom the school has to call hazmat, in their space suits and giant vacuum hoses.  So, what gives?  Isn’t it kind of dangerous to have a giant bowl of mercury in the room?

Isaac Newton first described the possibility of a mercury mirror when experiments showed liquid in a spinning container forms a parabola.  A parabola is just right for telescope mirrors.  Problem back then was he had no way to keep the container spinning at a constant speed.

After numerous fits and starts the first documented working mercury mirror telescope came in 1872 by H. Skey in Dunedin New Zealand.  The first astronomical observations came in 1908 in the U.S. by R. W. Wood.  These were small mirrors, between about 14” and 20”, but proved the concept.  Funding for projects involving mercury mirror telescopes is hard to get even though they are cheap compared with glass mirrors.  Why?  Same issue as the Inca had. They only work when pointing straight up or nearly so.  Only a narrow swath of sky can be observed, as the Earth rotates.  Few investors or universities seemed interested in this type of telescope.

But a few could see possibilities.  Very large mirrors (300” and greater) for less than 1% the cost of a conventional mirror.  With computer-controlled motors at 8 r.p.m., and special damping, these mirrors can have a figure as good as or better than glass.  Maintenance is minimal vs conventional observatories.  To date only three operate, two looking for space debris.  One in India is dedicated to astrophysical work.  It uses about 9 gallons of mercury.

Mercury is a health risk, so the telescope and operators are in separate buildings and the telescopes are in cold locations to minimize mercury evaporation.

What’s in the Sky?

March 28; dusk; west: A crescent Moon, Venus, and the Pleiades

April 3; 9pm; west:  Venus and the Pleiades kiss