Two gratings are available with UCLES, one with 31.6 and the other with
79 lines mm
. Both produce the same dispersion.
The main difference between them is the
free spectral range (FSR) and order separation, which are 2.5 times larger
for the 79 lines mm
grating than for the 31.6 lines mm
grating. With small detectors (a few cm across), the 31.6 lines mm
grating allows full wavelength coverage shortward of about 6000 Å,
but with orders separated by only a
few arcsec. With the 79 lines mm
grating only a fraction of the free
spectral range can be recorded per setting but the order separation is greater,
allowing a 
long slit in the red and greater in the blue.
Thus, in general the 31.6 echelle gives more complete spectral coverage but at
the expense of good sky coverage. (The permissible slit length can be increased
by 37% by using the focal modifiers; see Section 
.)
Tables giving the wavelength coverage, free spectral range, dispersion and
order separation for each grating are presented in
Appendices
and .
These appendices also present a complete echellogram along
with the relative sizes of detectors.
UCLES gives images with FWHM 
20 
m over the 
mm
field of its 70 cm focal length camera.
The resolving power is therefore dictated by the slit width and pixel
size. As one example, a detector resolution element of 48 
m corresponds to
R = 50000, and projects to 
(or 
with the focal modifier) at the
slit. Existing CCDs yield maximum resolving powers of 50000 and 60000.
The IPCS provides resolving powers up to 115000 by reducing its pixel size
towards a limit of 10 
m. However, this requires a very
narrow slit, and spectral coverage is decreased since the IPCS scan length is
made shorter.
UHRF has three resolving powers of nominally 300000 (0.3M), 600000 (0.6M), and 1000000 (1.0M --- actually 940000). The lower resolving powers are obtained by inserting focal reducing lenses into the UHRF camera optical path, which also change the wavelength coverage. UHRF has a single echelle with a suite of cross dispersing gratings suitable for different wavelength ranges. The very small wavelength coverage of the detector (typically 2 -- 15 Å) dictates most setup parameters. Usually only a single order of spectrum is observed, widened in the spatial direction due to the necessity of using an image slicer to obtain adequate throughput with the small slit width. Two orders can be observed simultaneously at the lowest resolving power (300000), but the second order rarely contains a wavelength of interest.