The table below shows the zero-point, Z, and sky brightness, S, through various NIRC2 filters, combined with the narrow camera and the large hexagonal pupil (the "largehex" position of the pupil wheel).
Scaling rules for other pupil masks are given in the following notes.
Z is defined as 2.5 log10 (DN/sec) for a Zeroth magnitude star.
The gain, G, is 4.0 electrons/count.
S is the sky brightness in magnitudes per square arcsec. The sky background in counts per pixel is given by:
counts/sec/pixel = (arcsec/pixel)^2 * 10^[0.4*(Z-S)]
where (arcsec/pixel) is the pixel scale, e.g. 0.040" for the wide camera,
0.010" for the narrow camera.
T(max) is the maximum exposure time without saturating the background
in the narrow camera.
Zero Points and Sky Magnitudes with "Large Hex" Pupil Mask |
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Filter |
(magnitudes) |
(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:
T(max) should be divided by 4 for the medium camera, and 16 for the wide camera.
Note:
The grisms have been measured for H and K. A rough efficiency (not including slit losses) is 50% for the grism throughputs.