Table 
gives the
AB
magnitude
of a star which, observed at
an airmass X=0 and with a wide slit, yields a count rate of 1 Hz Å
.
These conditions are not met in practice, and observers must allow for
atmospheric absorption at an airmass typical of their observations and
for slit losses at the slit width demanded for their resolving power and
anticipated seeing. Extinctions in magnitudes per airmass typical of Siding
Spring are: k
= 0.57, k
= 0.30, k
= 0.16, k
= 0.12, and
k
= 0.08. Slit losses due to seeing can be found in
Diego (1985, P.A.S.P., 97, 1209). A 1" slit in 
seeing
causes a loss of 1.1 mag for a perfectly guided star.
Furthermore, AB
magnitudes are not identical
to the Johnson magnitudes. Approximate offsets for A0 stars are:
AB
R + 0.2, AB
V, and
AB
B - 0.2.
The J100 figures are Johnson magnitudes yielding S/N=100 in 1 hour
with a 2.5 pixel slit in 1.8" seeing at X=1.5 airmasses.
Two additional points must be borne in mind. The tabulated
figures refer to blaze peak; at the edges of the free spectral range
(that is half a FSR off peak) the efficiency drops to 
of that at
blaze peak. Secondly, for many applications the quantity of interest is the
final signal-to-noise ratio of the spectra, rather than the number of counts
bin
. While for observations of bright objects the S/N approaches
, where N is the number of photons, in general it
may be necessary to take into account noise resulting
from (a) sky subtraction, (b) dark count, and (c) read-out noise of
CCDs.
At Siding Spring the dark sky has 
arcsec
.
As a rough approximation the sky is 
times brighter in full moon,
and even brighter at small angular distances from the moon. Dark count rates
and readout noise are given in Table . The
Tek CCD also has an XTRASLOW readout speed giving a readout noise of 2.3 e
RMS.
The throughput figures for UHRF are less reliable, since the image slicer
transmission depends critically on the seeing, how well the slicer aperture is
illuminated by the wavelength under study, and is highly sensitive to the
optical adjustment of the image slicer. The following figures should be
representative. As above, the AB
magnitude of a star is that which,
observed at an airmass X = 0 and with a wide slit, yields a count rate of 1 Hz
Å
with the Tek CCD. For more practical numbers, column J gives the
Johnson magnitude yielding 100 Hz per 0.01 Å at X = 1.5 in roughly 
seeing, with the image slicer and Tek CCD. (Values for other detectors can be
scaled by their relative quantum efficiencies.)
