Achromatic lens systems for near infrared instruments. II. Performances and limitations of standard Flint glasses
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 mu m, and possibly up to 2.5 mu m. Between 0.9 and 2.5 mu m the best known achromatic pairs are BaF_2-IRG2 and SrF_2-IRG3 and, to a lesser extent, CaF_2-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 BaF_2 or SrF_2 with standard SF glasses. The pairs we analyze in details are BaF_2-SF6 and BaF_2-SF56A, and we also present new measurements of transmission for several SF glasses which are quite transparent up to 1.65 mu 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 mu m) spectrometers.