Table B.2 below gives the AB magnitude of a star which, observed at an airmass 1.0 and with a wide slit, yields a count rate of 1 count/s/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: kU = 0.57, kB = 0.30, kV = 0.16, kR = 0.12, and kI = 0.08. Slit losses due to seeing can be found in Diego (1985, P.A.S.P., 97, 1209), and in Table 6.9. A 1" slit in 2" 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: AB6500 R + 0.2, AB5500 V, and AB4000 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 1.5 airmasses.
Table B.2: UCLES Countrates
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 ~70% 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 root N, 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 (see Section 6.1 of this manual for details on the CCD detectors &endash; in particular the section on the TEK CCD.). Sky brightnesses as a function of the phase of the moon are given in Table B.1.
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 count/s/Å, with the Tek CCD. For more practical numbers, column J gives the Johnson magnitude yielding 100 Hz per 0.01 Å at 1.5 airmasses in roughly 1.8" seeing, with the image slicer and Tek CCD. (Values for other detectors can be scaled by their relative quantum efficiencies.)
Table B.3: UHRF Countrates
More details on the use and sensitivity oif these instruments can be found in The UCL and UHRF Echelle Spectrographs, AAO User Manual 25.3.
This Page Last updated: Feb 21, 1996, by Chris Tinney.