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Sensitivity

The values in these tables are based on results from the FLAMINGOS-2 Integration Time Calculator and from the early commissioning runs. They assume 70-percentile (IQ70, ~average) image quality for all categories, photometric conditions (CC50), and an airmass of less than 1.2 for the sensitivites presented here. Water vapor has little impact on the near-IR imaging sensitivity, or on spectroscopic sensitivity in the middles of the J, H, and K windows, but does significantly effect the edges of the JHK windows where absorption by telluric water vapor is strong. The estimates also use the measured telescope, camera, and detector performances. Note that some sky conditions vary outside of those stated above. For example, OH emission, although scaled by airmass, can intrinsically vary by over a factor of two from night to night and over much shorter timescales - which can alter S/N and sensitivity by +/- sqrt2 in the J, H, and K bands. Thus observers should use both the table and the ITC with some caution and conservatism. These values are accurate to within 30% so please keep this in mind for SV proposals.

Regarding readout noise, note that JHKs imaging used CDS readout.

 

FLAMINGOS-2 IMAGING SENSITIVITY (S/N=5 IN 1 HOUR, N.I. OVERHEADS)
Filter Center wavelength (µm) (mag) Individual Exposure Time (sec)
Y 1.02 22.2 (in J) 300
J 1.25 23.5 36
H 1.65 22.7 15
Ks 2.20 22.8 90

Imaging Notes

  • For imaging, the ITC calculates the S/N in an aperture that maximizes S/N, given the predicted image quality for the observing conditions and wavelength requested. The sensitivity values in the table below use this optimum ITC aperture. For accurate photometry much larger apertures usually must be used, and sensitivities are reduced, typically by a factor of 2 or more.
  • Approximate total throughput values in the imaging table are as measured for the entire system, including the telescope, instrument, and detector.

 

FLAMINGOS-2 SPECTROSCOPIC SENSITIVITY FOR POINT SOURCES (S/N=5 IN 1 HOUR, N.I. OVERHEADS)
Grism Filter slit width (arcsec) (mag)
JH (1200)
JH 0.36 20.5 (J)
HK (1200) HK 0.36 20.5 (Ks)
R3000 J 0.36 20.3
H 0.36 20.2
Ks
0.36 19.8

Spectroscopy Notes

  • Estimated sensitivities are eyeball averages over the specified wavelength intervals. Within each IR band (and indeed within each of these narrow intervals) the sensitivity is a very strong and rapid function of wavelength. The ITC should be used to estimate the signal-to-noise ratio at the precise wavelengths of interest and/or over the entire band of interest.
  • The sensitivity per resolution element (which is 2 or more pixels depending on the grism and slit width) is better than the sensitivities in this table by the square root of the number of pixels per resolution element.
  • For point sources, the calculation assumes nodding along the slit and an optimized length along the slit (which gives the highest S/N in the extracted spectrum.
  • Sensitivities for slitless spectroscopy will be much (typically 10 times or more) worse than the values given in the table.