An Incomplete History of the Hindle Mount, Whiffletree, Swingletree, Swingles, and the Grubb Telescope

Mechanical supports for mirrors and other optical components and substrates to maintain their initial undeformed shape is a common engineering problem.   Ideally a mirror or similar substrate can be supported on three points if the mirror or substrate is stiff enough.  However in many cases, the deflections are too large and more support is required.  One of the earliest areas where this problem arose was for the mirrors in early telescopes.  Irishman Howard Grubb came up with a novel solution by supporting the mirror on a set of levers known as a whiffletree.  For a historical bio of Howard Grubb see Biographical Encyclopedia of Astronomers or the Museum Victoria (Australia) bio or a history of the Armagh Observatory and Grubb’s telescope.

Whiffletree mirror support developed by Howard Grubb in 1835.
Whiffletree mirror support developed by Howard Grubb in 1835.  This sort of mirror mount is also commonly known as a Hindle mount in some optomechanics literature.

Grubb’s mirror was made of a metal called speculum (wikipedia link), a mixture of tin and copper commonly used for telescopes at the time .

A whiffletree is a connection of levers or linkages which is used to distribute forces.  Many people are common with its use in areas such as connecting a group of horses or other farm animals when pulling equipment.  The word “whiffletree” is interchangeable with whippletree, swingletree, or simply swingle.  (see also the wikipedia links for whiffletree and swingletree).  Another familiar device where one may see a whiffletree is a windshield wiper or the artistic mobile that is made in simple form in elementary schools.  The whiffletree is also used in the suspension of some of NASA’s Mars rovers.

A popular set of texts Amateur Telescope Making (Volumes 1-3) edited by Albert G. Ingalls first published in 1926 contains a section in the 1945 edition by J.H. Hindle “Mechanical Flotation of Mirrors” which analyzed such a whiffletree support system.  Amazon.com link

Hindle mount from NASA Tech Briefs http://www.techbriefs.com/component/content/article/ntb/tech-briefs/mechanics-and-machinery/3102
Hindle mount  (nine point) by David W. Robinson of NASA http://www.techbriefs.com/component/content/article/ntb/tech-briefs/mechanics-and-machinery/3102

Although the idea for this type of support appears to have originated with Thomas Grubb, it is commonly referred to as a Hindle mount.  A thorough description of the history and physics is contained in the text Reflecting Telescope Optics II: Manufacture, Testing, Alignment, Modern Techniques by Raymond N. Wilson.  A brief history is also contained in the article “Mechanical principles of large mirror supports” by Hans J. Kärcher, Peter Eisenträger, and Martin Süss.

In addition to the article by D. Robinson in the figure above, Yoder and Vukobratovich’s text Opto-Mechanical Systems Design, Fourth Edition, Volume 2 and the article by P.K. Mehta “Flat circular optical elements on a 9-point Hindle-mount in a 1-g force field,” contain examples of Hindle mounts in use.   One amateur telescope maker even made a 54 point Hindle mount for a homemade 1100 mm telescope.

 

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