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RGO Spectrograph

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.

Table B.1: Countrates (photons/s/pixel) for the RGO Spectrograph with the TEK CCD
Setup O(BVRI=15)
Low Dispersion
(3.4Å/pix w/ 250B,
3.1Å/pix w/ 270R)
Object 30 24 20 9.3
Sky 1.2 1.4 1.5 2.0
Medium Dispersion
(1.6Å/pix for the 600V/R)
Object 15 13 7.8 4.3
Sky 0.61 0.70 0.59 0.93
High Dispersion
(~ 0.79Å/pix for the 1200B/V/R)
Object 6.2 4.9 2.9 1.5
Sky 0.25 0.28 0.23 0.32

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 count-rate,  
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).

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This Page Last updated: Feb 21, 1996, by Chris Tinney.
RGO+TEK Sebtivities from R.Stathakis, (1995), AAO Newsletter, 73, 11