Evolution of the Astronomical Eyepiece - Brayebrook Observatory

EVOLUTION of the ASTRONOMICAL EYEPIECE
VARIANTS OF SOLID EYEPIECES:
No single element eyepiece, no matter
what the form or figure on its surface
can be corrected for both spherical and
c h romatic a berration. Following the
manufacture of Chester Moor Hall’s
prototype achromatic doublet by the
jobbing optician George Bass, and the
work of Leonhard Euler an d S.
K l i n g e n s t i e rna on the geometrical
method of simultaneously correcting
both spherical and chromatic aberration
in object-glasses, John Dollond
assembled the first achromatic lens. It
had plano-concave flint and equi-con -
vex crown componen ts, with focal
length in the ratio of their dispersive
powers. Dollond manufactured doublets
in small sizes from about 1759,
and employed them in microscopes
and telescopes as objectives and as an
inverting eyepiece.
THE ACHROMATIC DOUBLET:
Possesses a much wider f ield with
superior edge definition to a simple
Keplerian type, and was well suited to
the shorter focal l en gth achro m a t i c
refractors of the late C18th. It shares
the same eye relief of about 0.9Fe.
Lenses of this form were scarce and
expensive to produce because of the
difficulty in pouring bubble and striae
free flint glass. It was not until 1828
when H. Guinand founded an opticalglass
works at Choisy-le-Roi, on the
principles of the research carried out
by his father, Pierre Louis Guinand, a
Swiss horologist of Les Brenets, for
Joseph Fraunhofer, that this technological
deficiency was rectified. During
the French revolution of 1848, George
Bontemps, an employee of Guinand,
left for England bringing with him a
number of French and Belgium sheetgla
ssma kers. Bon tem ps joi ned the
newly f oun ded Birmi ngham f irm ,
Chance Brothers, and imparted his
techni ca l knowledge. Within a few
years, English optical-glass production
surpassed tha t established on the
Continent.
It was only with the development of
optica l-gla ss production techno logy,
and the availability of a variety of
cr own and flint glasses having differing
refracting and dispersive powers. that
i t be came feasib le to de sign a nd
m a n u f a c t u r e ach ro mat ic lenses in
a ny qua nti ty.
20