The NIRC2 sensitivity values listed in the table below are distillations taken from measurements made in April 2004, just after optics on the AO bench were recoated. For comparison, please see the original commissioning measurements from June 2001.
The table below shows the zero point ZP, and sky brightness. SB through various NIRC2 filters combined with the narrow camera and the large hexagonal pupil (the largehex position of the pupil wheel).
The zero point is defined as
ZP[mag] = 2.5 log10 (DN/sec)for a Zeroth magnitude star.
Calculations assume a detector gain of 4 e-/DN corresponding to the original detector controller. The detector controller was replaced in November 2023. The new detector electronics has a gain of 8 e-/DN, but this should not change the values shown in the table below since the detector is still the original one.
The sky brightness is defined in magnitudes per square arcsec. Sky background in counts per second per pixel is given by:
Sky[counts/sec/pixel] = PS[arcsec/pixel]^2 * 10^0.4*(ZP[mag]-SB[mag/arcsec^2])where PS is the pixel scale in arcseconds per pixel, e.g. 0.040; for the wide camera, 0.010; for the narrow camera.
Tmax is the maximum exposure time without saturating the background in the narrow camera.
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(mag/arcsec^2) |
(sec) |
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For different pupil masks: Inscribed circle, subtract 0.3 from the zeropoints above, the sky backgrounds are nearly the same for the inscribed circle, incircle, and large hexagonal, largehex, pupil masks. The sky backgrounds are approximately 0.5 (up to 1.0 mag with Ms) higher with the open pupil mask.
The other pupil masks have not been measured, but they should lie between these two extremes.
For different cameras: Tmax should be divided by 4 for the medium camera, and 16 for the wide camera.
Grisms have been measured for H and K. A rough efficiency (not including slit losses) is 50% for the grism throughputs.