CIRPASS Sensitivity

Sensitivity estimates are to be updated on or before 10 September 2002. Use the values here for guidance only.

The sensitivities presented are on a per fiber basis. The signal is spread over 2 pixels in the spatial direction and 2 pixels in the spectral direction. The following assumptions were made to estimate the sensitivity of CIRPASS on Gemini.

Assumptions for Sensitivity of CIRPASS on Gemini
Resolving power 3125
Sky Background b 16.0 mag arcsec-2 at 1.25µm
Throughput a 8%
Exposure Time 2 x 30 min (2 frames)
Detector Dark Current 0.3 e- s-1
Detector Read-Out Noise 20 e- rms per read
Instrument Thermal Background 0.15 e-/s/pix

Notes to table:
(a) Throughput is a conservative assumption pending verification.
(b) Sky background is a conservative, not the inter-OH sky brightness, pending on-telescope tests. 

Sensitivity estimates for the various spatial scales are given in the table below:

Sensitivities of CIRPASS: SNR=5 in 1 hour at 1.25µm
Configuration Point Sourcea Extended Sourceb
  mJy mag W/m2 mJy/arcsec2 mag/arcsec2 W/m2/arcsec2
Direct feed
0.33" 0.047 18.86 3.94E-20 0.119 17.84 1.00E-19
0.25" 0.063 18.54 5.29E-20 0.184 17.37 1.55-19

Notes to table:
(a) Flux density for point source assumed to be centred on the fiber. Nominal point source assumed to have expected median delivered image quality (50% EED) of 0.37 arcsec and median Mauna Kea water vapour column of 1.6mm (~0.45 arcsec at 1.25µm). Fraction of source flux at the 50% entering a single lenslet are: 0.44 and 0.29 for fiber diameters 0.33 and 0.25 arcsec, respectively. 
(b) Surface brightness sensitivity per fiber.

The CIRPASS pages at Cambridge provide an Exposure Time Calculator. If using this calculator, please ensure that you make the same throughput, background and noise assumptions as in the table above.

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Last update 26 August 2002; Phil Puxley