A&A Supplement series, Vol. 123, May II 1997, 179-182
Received May 14; accepted September 16, 1996
E. Oliva
, S. Gennari
, L. Vanzi
,
A. Caruso
, and
M. Ciofini
Send offprint request: E. Oliva
Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5,
I-50125 Firenze, Italy
Dipartimento di Astronomia e Scienza dell Spazio, Università di Firenze,
Largo E. Fermi 5, I-50125 Firenze, Italy
Istituto Nazionale di Ottica, Largo E. Fermi 6, I-50125 Firenze, Italy
The optical characteristics of birefringent materials transmitting
in the near IR (0.9-2.5 
m) are analyzed. Wollaston prisms
with large beam separation and virtually free
from lateral chromatism - e.g. with fields of view >1 arcmin
and image elongation 
-
could be manufactured using AgGaS
or LiYF
(YLF).
These materials are used for non linear
applications and may therefore find another interesting application
in IR focal plane instruments.
From the optical point of view
the best material is AgGaS
but YLF may be practically more convenient
because of its lower refractive index (
),
better thermo-mechanical properties and lower price.
New measurements of the transparency of Calcite at room
and cryogenic temperatures are also presented, these show
significant absorption (
) of the ordinary ray already
at 
m and
very strong bands (absorption coefficient 
cm
)
beyond 2.25 
m. The absorption does not decrease
significantly when the crystals is cooled,
CaCO
should not be therefore
used in instruments working beyond 2.0 
m.
keywords: instrumentation: miscellaneous -- instrumentation: polarimeters