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Throughput and limiting magnitude

We estimated total throughput under the following assumptions: a 1% efficiency loss at each coated air-glass interface, a 2% (or 8%) at each multilayer (or protected SiO Al) coated mirror reflection. Echelle efficiency has been estimated from similar R4 echelle gratings (e.g., UVES 31.6 gr/mm grating). Prism efficiency takes into account internal losses and air-glass interface losses. Total fiber efficiency includes entrance and exit losses, slit losses, fiber transmission. The calculations have been carried out at four different wavelengths (400, 450, 500, 700 nm). Results are shown in table 2.2.

Table 2.2: Total throughput for CAOS spectrograph, from telescope focal plane to detector.
  Total throughput
$\lambda$ (nm) 400 450 500 700
Fibers 0.80 0.80 0.80 0.80
Preslit optics 0.96 0.96 0.96 0.96
Mirrors 0.91 0.94 0.94 0.91
Echelles 0.70 0.70 0.70 0.70
Prism 0.72 0.88 0.92 0.92
Camera 0.90 0.90 0.90 0.90
CCD QE 0.75 0.82 0.86 0.78
Total 0.25 0.33 0.36 0.32

In standard observing conditions (seeing $\approx$ 2 arcsec, sky background V$\approx$20), limiting magnitudes at R=28000 for 1 hr exposure are V$\simeq$12.5 and 16.5 at signal-to-noise ratio 100 and 10 per resolution element, respectively.


next up previous contents index
Next: Observing modes Up: CAOS - The new Previous: High and Low Resolutions   Contents   Index
Innocenza Busa' 2005-11-14