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Gemini Near-Infrared Integral-Field Spectrograph (NIFS) Sensitivity.

Gemini Near-Infrared Integral-Field Spectrograph (NIFS)

Research School of Astronomy and Astrophysics    AUSPACE Ltd. Institute for Astronomy

NIFS Sensitivity

In calculating the NIFS sensitivity, we adopt a read noise of 5 e-, a dark current of 0.01 e- s-1 pix-1, and a maximum exposure time of 3600 s.

Dominant Noise Sources in 3600 s

Grating RN2 Dark Current
(e/pix)		 Sky Thermal
(e/pix)		 OH Cont.
(e/pix)		 Tel. Thermal
(e/pix)		 ALTAIR Thermal
(e/pix)		 Cryostat Thermal
(e/pix)
Z253608000
J253609000
H2536014000
K2536417171160

The dominant noise sources in a 3600 s exposure are shown in the table above. NIFS will be limited by read noise and detector dark current in the Z, J, and H bands and by thermal emission from ALTAIR in the K band. When ALTAIR is not used, NIFS will be limited about equally by read noise, dark current noise, and background emission from the telescope and sky.

Detailed performance modeling suggests that NIFS should achieve signal-to-noise ratios of ~ 10 per spectral pixel in a 0.1"×0.1" aperture with median seeing and the expected Strehl ratios of 0.2 at J, 0.4 at H, and 0.6 at K in a single 1800 s exposure on point sources with J = 18.8, J = 18.4, H = 18.8, and K = 17.8 mag using the Z, J, H, and K gratings, respectively.

NIFS should achieve signal-to-noise ratios of ~ 10 per spectral pixel in a 0.1"×0.1" aperture for single 1800 s exposures on uniform, extended, continuum sources with surface brightnesses of µJ = 15.4, µJ = 15.0, µH = 14.8, and µK = 13.5 mag arcsec-2 using the Z, J, H, and K gratings, respectively.

The sensitivity to extended emission-line sources depends on the surface brightness of underlying continuum emission since the shot noise associated with this continuum emission can be the dominant noise source. The emission-line surface brightnesses required to make a 10sigma per spectral pixel measurement of an extended emission-line with FWHM=100 km s-1 in a 0.1"×0.1" aperture and 1800 s exposure time are listed in the table below. Such a spectrum would be suitable for measuring the profile of a 100 km s-1 wide emission-line at the full velocity resolution available with each grating.

Extended Emission-Line Source Sensitivities

Z grating J grating H grating K grating
µJ
(mag arcsec-2)		 R=5090
(W cm-2 arcsec-2)		 µJ
(mag arcsec-2)		 R=6100
(W cm-2 arcsec-2)		 µH
(mag arcsec-2)		 R=5340
(W cm-2 arcsec-2)		 µK
(mag arcsec-2)		 R=5340
(W cm-2 arcsec-2)
9.01.6×10-21 9.01.3×10-21 9.08.3×10-22 9.04.9×10-22
10.01.1×10-21 10.08.4×10-22 10.05.1×10-22 10.03.3×10-22
11.07.0×10-22 11.05.5×10-22 11.03.2×10-22 11.02.2×10-22
12.04.5×10-22 12.03.5×10-22 12.02.1×10-22 12.01.8×10-22
13.02.9×10-22 13.02.3×10-22 13.01.3×10-22 13.01.4×10-22
14.02.1×10-22 14.01.6×10-22 14.09.5×10-23 14.01.3×10-22
15.01.4×10-22 15.01.3×10-22 15.07.4×10-23 15.01.2×10-22

Atmospheric OH line emission is strong in the J and H bands. Consequently, limiting observations will be restricted to regions between these strong OH lines. The strongest OH emission is in the H band. Model NIFS spectra showing the expected OH emission-line photo-currents is shown below. These are still sufficiently low that individual integrations will be limited by uncertain factors such as the cosmic ray event rate and on-chip amplifier glow rather than detector saturation.

J Background H Background K background
[J Background] [H Background] [K Background]


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