Science with NIFS, Australia's First Gemini Instrument
Peter J. McGregor , Michael Dopita , Peter Wood , Michael G. Burton, PASA, 18 (1), in press.

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NIFS Performance

The expected performance of NIFS has been modeled in order to make a realistic assessment of the scientific capabilities of the instrument. The noise sources considered include read noise, dark current noise, cryostat thermal emission, thermal emission from the cryostat window, thermal emission from ALTAIR, thermal emission from the telescope mirrors, airglow line emission, airglow continuum emission, thermal emission from the sky, and scattered light within the cryostat. Signal photo-currents from point sources were modeled using a point spread function consisting of a diffraction-limited core and a seeing-limited halo modeled using a Moffat function with index of 11/6 (Racine et al. 1999). This point spread function is appropriate for partial AO correction. The details of these full simulations are described elsewhere⁴. A simpler web-based performance calculator is also available⁵and returns similar results for point sources.

These simulations predict that NIFS will be limited by read noise and dark current noise in the J and H bands between strong OH airglow emission lines and by thermal emission from ALTAIR in the K band. NIFS should achieve signal-to-noise ratios of 10 per spectral pixel in a

0.1'' x 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 on-source exposure on point sources with Z = 18.8, J = 18.4, H = 18.8, and K = 17.8 mag in the respective spectral bands. It is assumed that sky subtraction will be achieved using an off-source exposure of the same duration. However, it may be possible in practice to extract sky spectra for uncomplicated fields from field positions within the on-source exposure. NIFS should achieve signal-to-noise ratios of 10 per spectral pixel in a

0.1'' x 0.1'' aperture for single 1800 s on-source exposures on uniform, extended, continuum sources with surface brightnesses of Z = 15.4, J = 15.0, H = 14.8, and K = 13.5 mag arcsec^-2 in the respective spectral bands. Sky subtraction is again assumed to require an additional off-source exposure of the same duration. The sensitivity to extended line emission depends on both the line width and the background continuum surface brightness. The signal-to-noise ratio achieved in a

0.1'' x 0.1'' aperture on extended line emission can be estimated from the above results by comparing the emission line flux to the noise in the background continuum. The emission line surface brightnesses, , required to achieve a signal-to-noise ratio of 10 per spectral pixel on an extended emission line source with FWHM = 100 km s^-1 in a

0.1'' x 0.1'' aperture and 1800 s on-source integration time are listed in Table 1 for different background continuum surface brightnesses, . 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. The sensitivities in Table 1 assume that an additional 1800 s off-source exposure is used for sky subtraction. However, sky subtraction may not be necessary when working between strong OH airglow lines in the J and H bands where little sky emission is expected at the wavelengths of interest.

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Next Section: Guide Star Requirements
Title/Abstract Page: Science with NIFS, Australia's
Previous Section: Introduction
Contents Page: Volume 18, Number 1

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© Copyright Astronomical Society of Australia 1997