NIRC2 sensitivity

NIRC2 Photometric Zeropoint
The following photometric calibration data were taken on April 1, 2004 after the recoating and realignment on the Keck 2 AO rotator. Three UKIRT standard stars were imaged using the NIRC2 narrow camera in photometric conditions at airmass < 1.3.

Filter Pupil Z (magnitudes) S (mag/arcsec^2)
J large hex 25.47 +/- 0.07 13.60 +/- 0.25
H large hex 25.51 +/- 0.07 13.24 +/- 0.13
K' large hex 24.84 +/- 0.07 12.66 +/- 0.12
K large hex 24.73 +/- 0.06 12.34 +/- 0.10
L' large hex 23.60 +/- 0.09 3.01 +/- 0.09
L' inscr circ 23.34 +/- 0.06 3.03 +/- 0.07
Ms large hex 21.42 +/- 0.29 0.23 +/- 0.30
Ms inscr cir 21.11 +/- 0.22 0.19 +/- 0.22

notes:
The photometric zeropoint Z is the magnitude of a star which would generate one count per second on the detector. The magnitude of an object observed with NIRC2 will therefore be given by

m = Z - kX - 2.5 log10[counts/second].
where k is the extinction coefficient in magnitudes per airmass and X is the airmass.

S is the sky brightness in magnitudes per square arcsecond. The sky background per pixel near zenith will be given by

counts/sec/pixel = (arcsec/pixel)^2 * 10^[0.4*(Z-S)]
where arcsec/pixel is the platescale.
NIRC2 Limiting magnitudes
The following are the NIRC2 limiting magnitudes for a 5-sigma detection of a point source after 1 hour (3600s) of integration with a given Strehl using the observing strategy indicated, large hexagonal pupil, and narrow camera.

Observing strategy Limiting magnitude
Filter Integ. (s) MCDS reads Strehl=0.10 Strehl=0.25 Strehl=0.50
J 300 64 24.45 25.44 26.21
H 300 64 23.99 24.99 25.77
K' 300 64 23.08 24.07 24.84
K 300 64 22.85 23.85 24.59
L' 0.20 2 16.94 17.94 18.70
Ms 0.05 2 14.20 14.89 15.12

Note that these limiting magnitudes are somewhat lower than those computed from NIRC2 commissioning observations in summer 2001.

. For more information please contact the support astronomers.

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