A&A Supplement series, Vol. 128, March II 1998, 599-603
Received May 5; accepted July 22, 1997
E. Oliva and S. Gennari
Send offprint request: E. Oliva
Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5,
I-50125 Firenze, Italy
This paper investigates which normal Flint glasses
are best suited for the design of lens systems
working in the infrared up to about 1.7 
m, and possibly up to 2.5 m.
Between 0.9 and 2.5 m the best known achromatic pairs are BaF2-IRG2
and SrF2-IRG3 and, to a lesser extent, CaF2-IRG7 (Oliva & Gennari 1995).
Unfortunately, Schott will most probably stop the production of
these very little used and commercially uninteresting IRG glasses.
Here we show that equally good performances can be obtained coupling
BaF2 or SrF2 with standard SF glasses.
The pairs we analyze in details are BaF2-SF6 and BaF2-SF56A, and we
also present new measurements of transmission for several SF
glasses which are quite transparent
up to 1.65 m
and can be therefore
readily employed in fiber-fed spectrographs and other instruments which
do not extend beyond the H atmospheric window.
At longer wavelengths the use of SF glasses is limited by
strong water absorption
features, but these
could be eliminated by preparing the glass in vacuum environment.
Although this production is expensive and commercially unattractive,
we hope however that a large enough group of astronomers will support glass
manufacturers in the production of "IR-grade SF glasses''.
As a practical application we present representative results of the design of
F/2 (4 lenses) and F/1.4 (5 lenses) cameras for near infrared
(0.95-2.5 m) spectrometers.
keywords: instrumentation: miscellaneous -- instrumentation: spectrographs