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[Phys-l] Fraunhofer/Bessel Heliometer (was half lens)



Last night, when I posted a message about the split lens in a
heliometer, I did not have my copy of Hirshfeld's book, "Parallax: The
Race to Measure the Cosmos," so I made that post from memory. Now, in my
office, I have the book and I think list members might enjoy a quotation
from the book. This partly serves as an advertisement for the book
(because you'll note Hirshfeld's writing style) and also a nice
description of the double half-lens telescope (which precisely fits with
the original half-lens thread).

* * * Quoting from Hirshfeld, page 258, 259... * * *

The new heliometer looked much like a normal refractor telescope, except
that its six-inch-wide main lens had been sliced precisely in half with
a diamond cutter. The idea of cleaving such a flawless glass is enough
to make even a seasoned astronomer shudder. One slip, and the lens
might have cracked. But Fraunhofer and his workers rarely slipped.
Astronomer John Herschel, who had visited Fraunhofer's workshop when the
heliometer was being made, later wrote: "I well remember to have seen
this object-glass at Munich before it was cut, and to have been not a
little amazed at the boldness of the maker who would devote a glass,
which at the time would have been considered in England almost
invaluable, to so hazardous an operation."

Each of the semicircular lens pieces had been mounted in its own metal
frame and functioned like side-by-side telescopes, forming a pair of
images, each half as bright as that of a similar undivided lens. The
lens-halves could be slid laterally alongside one another by turning a
thumbscrew. A brass scale had been attached from which the relative
offset of the two lens-halves could be read. Here even the
precision-obsessed Bessel might have shaken his head in awe of the
master craftsmen in Fraunhofer's workshop; the divisions on the brass
scale were so close together that a microscope was included so Bessel
could distinguish them.

[snip]

As expected, when the two lens-halves were precisely fitted together,
the heliometer acted like a conventional telescope; the star images from
the lens-halves were merged. When Bessel offset the lens pieces
alongside one another, their respective images moved apart; the sky
appeared as to a drunkard, every star image doubled. Bessel planned to
work in reverse: force the images of two *different* stars to merge by
moving the lens-halves. The amount of offset between the lens-halves
would give the angle between the stars.

* * * end quote * * *

Observing the star "61 Cygni," Bessel made about a dozen measurements a
night and averaged them. He repeated this for over one-hundred nights
from October 1837 to October 1838. The apparent movement of 61 Cygni
compared to the much-more-distant comparison star was irrefutable.
Bessel had clearly demonstrated stellar parallax, and determined the
distance between Earth and 61 Cygni to within about 10%.

Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu