The following sensitivity estimates are for the 25cm camera of the RGO spectrograph, with the TEK CCD running at 170K ("cold"). The observations assume a grey night, 30° zenith distance, a 2" slit, and 2" seeing. The effects of vignetting in the 25cm camera are included. The estimates presented in the Table below are for "low dispersion" (~3.4Å/pix for the 250B grating, or 3.1Å/pix for the 270R), "medium dispersion" (~1.6Å/pix for the 600V or 600R gratings), and "high dispersion" (~ 0.79Å/pix for the 1200B, 1200V or 1200R gratings). The estimates have also been made at the mean wavelengths of the BVRI passbands, assuming a star of magnitude 15 in the relevant passband.
To convert these rates to those expect for different seeing and/or slit sizes see the RGO Spectrograph Manual, or Table 6.9 of this manual.
(3.4Å/pix w/ 250B,
3.1Å/pix w/ 270R)
(1.6Å/pix for the 600V/R)
(~ 0.79Å/pix for the 1200B/V/R)
Then the signal-to-noise ratio (SNR) can be estimated as,
SNR = Ot / Sqrt(Ot+St+(DN**2)
SNR = O Sqrt(t) / Sqrt(O+S+(DN**2/t)
O is the object
S is the sky count rate,
t is the exposure time, and ,
DN is the detector noise (the read-noise in the absence of dark current) per pixel, times the number of spatial pixels to be extracted (4 for the above examples, 6 for AUTOFIB).
The read-out noise for the TEK chip depends on the speed used :- 2.3e- for XTRASLOW, 3.6e- for SLOW, 4.8e- for NORMAL and 7.2e- for FAST. Note that XTRASLOW takes about 3 minutes to read out the full slit.
When illuminating the slit of the RGO with the AAO fibre-optic feeds(both FOCAP and AUTOFIB 320um bundles), the above count-rates are reduced by a factor of 2.7, due to the transmission of the fibre bundles and to the smaller fraction of a 2" wide seeing disk accepted by the circular aperture of a fibre relative to a long slit. Vignetting causes additional losses of up to 30% for fibres near the edges of the slit (low and high fibre numbers). Further information on the transmission of different fibre bundles as a function of wavelength is given in the Users' Guide to the AAO FOCAP System (AAO UM 18).
This Page Last updated: Feb 21, 1996, by Chris Tinney.