The combination of large telescopes and IR arrays with small pixels
requires that
IR spectrometers with a reasonably high throughput (i.e. with few
pixels sampling a 
slit) must have very fast cameras.
For example, an instrument equipped with the Rockwell 10242 array
(18.5 
m pixel size) and intended to work on a 8m class telescope
with a slit sampled by 4 pixels must have a F/2 camera, while a
much more extreme F/1.4 system is necessary to obtain 3 pixels/arcsec.
The results presented here indicate that these types of cameras are
relatively easy to design when BaF2-SF pairs are adopted.
Figure 3:
Layout and ray tracing of F/2 and F/1.4 near-IR cameras.
Spot diagrams and encircled energies are displayed in Figs. 4, 5
while the details of the design are listed in Table 2
Figure 4:
Monochromatic spot diagrams (neglecting lateral chromatism)
at several wavelengths and distances from the array center for the BaF2-SF6
cameras (Fig. 3 (click here)), the array is a 10242 with
pixels of 18.5 m. The size of one pixel is indicated
by the thick squares, while the larger squares are 2 pixels
(37 m)
Figure 5:
Diameters of 80% and 90% encircled energies (in m) for the F/2
and F/1.4 cameras. The value is the maximum image size at the wavelengths
of Fig. 4, for SF56A the curve at 2.5 m is shown separately.
Note that BaF2-SF6 is excellent over the full range (and is in practice
equivalent to BaF2-IRG2). The other pair is
slightly better at the shortest 
's and is achromatic
up to about 2.2 m, but rapidly deteriorates beyond
Representative solutions for F/2 and F/1.4 systems are shown
in Fig. 3 (click here), the spot diagrams and encircled energies are displayed
in Figs. 4 (click here), 5 (click here) while the details of the lenses are listed in Table 2.
In these designs we
assumed a "typical'' 
100 mm collimated (parallel) beam
allowing for about 200 mm free space between the pupil image
and the first optical surface.
Note that the cameras were not optimized for lateral chromatism because this
aberration does not influence the spectrograph performances.
The performances of the BaF2-SF6 cameras are remarkably good and virtually
identical to those obtained using BaF2-IRG2. The BaF2-SF56A combination
is also very good for 
m but not at longer wavelengths
where their chromatisms decouple, but this limitation is not important
for applications to instruments not extending into the K atmospheric window.
As already mentioned in Sect. 2, the image quality is not influenced by
the exact behaviour of 
with wavelength, and any reasonable
uncertainty on this parameter can be recovered by simply refocusing the array.
The design of the cameras was somewhat simplified by the use
of one aspheric surface on BaF2, but this should not be a problem because
this material can be quite easily diamond turned.
Another potential limitation is the
size of the BaF2 elements, especially in the F/1.4 camera
which requires lenses with 
mm useful diameters. However,
these are not much larger than the 160 mm BaF2 aspheric
collimator mounted in ISAAC, the ESO infrared instrument for the ESO-VLT
8m telescope (Delabre 1993).
We believe therefore that the designs presented here are within the
present possibilities of lens manufacturers.
C. 