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The sensitivities reported for NIFS reflect the magnitude of an object that would provide a signal-to-noise ratio of 1 for an hour of integration time (SN1 hr). To calculate the signal-to-noise ratio, the signal is determined from telluric standard stars observed with requested photometric (CC50) IQ20 to IQ70 conditions, while the noise is determined from a subtracted pair of long-sky frames. The sky frames used have an integration time (T) > 500 seconds, observed within temporal proximity of the telluric standard, and are observed under the same photometric conditions.
The signal is found by taking the mean over the telluric's spectrum (S) with a circular aperture area defined by the guiding source. For AO and P2 guiding, apertures are fixed at 0.3'' and 2.5'', respectively.
With a subtracted pair of long-sky frames, on an area of sky generally free of sky lines and artifacts, noise (N) is determined by multiplying the average standard deviation in the background counts by the square root of the aperture area used to find the signal.
|SN1 hr =|| S * (3600 / Ttelluric)
N * (3600 / (2 * Tsky))1/2
To find the sensitivity, the signal-to-noise ratio (SN1 hr) is converted to a magnitude and added to the Simbad magnitude (MSimbad), in the corresponding waveband, of the standard star:
|Sensitivity = 2.5 * log(SN1 hr) + MSimbad .|
Sensitivity measurements for telluric standard star observations, taken at the central wavelength of each band, are reported below. The Gemini Integration Time Calculator (ITC) predictions for IQ20 and IQ70 (CC50) conditions are drawn in the sensitivity plots. For NIFS-P2 the ITC calculations do not differ for IQ20 and IQ70 conditions, and only one dashed line is shown. The NIFS-AO predicted sensitivities do exhibit noticeable differences between IQ20 and IQ70 conditions. For NIFS-AO observations, measurements made with the field lens OUT are represented by a larger symbol than those made with the field lens IN, as the field lens will contribute to signal being lost.
|NIFS Sensitivity||AO Guiding||P2 Guiding