NIRSPEC Sensitivity

The tables below rely on our limited personal experience with NIRSPEC at 1-2.5 microns and projections of its performance longward of 2.5 microns based on the performance of other spectrometers. (Analysis of the data obtained in May 2000 show that the estimated sensitivities at JHK in the original table were within a few tenths of a magnitude in most cases.) The first table gives 3 sigma detections in one resolution element after 30 minutes for a pointlike source observed in 0.5" IR seeing using a 3-pixel wide (0.57") slit for the grating (R~2,000) or a 3-pixel wide slit (0.43") for the echelle (R~25,000), and summing the signal over three rows of the array (0.57" with the grating or 0.75" with the echelle). The second table gives rough corrections for other slit widths. The tables will be updated as more information becomes available.

Band Lambda
(Ám)
Grating (3-pixel wide slit) Echelle (3-pixel wide slit)
continuum
magnitude
line flux
(W/m2)
continuum
magnitude
line flux
(W/m2)
no. settings for
complete spectrum
Z 1.0 20.3 2.3E-20 17.3 2.7E-20 1
J 1.25 19.5 3.3E-20 16.5 4.1E-20 2
H 1.65 18.5 2.4E-20 16.0 3.0E-20 2
K 2.2 18.3 2.2E-20 15.7 2.6E-20 3
L 3.4 12.8 9.5E-19 11.1 3.7E-19 4
L' 3.8 12.6 9.0E-19 10.9 3.5E-19 4
M 4.7 11.0 2.1E-18 9.3 4.0E-19 5

Slit
Width
(pixels)
Continuum
Correction
(mag)
Line Flux
Correction
Factor
1 (ech. only) -1.5 1.7-4.0
2 -0.4 1.3-1.5
3 0.0 1.0
4 +0.3 1.0-0.8
5 (ech. only) +0.4 1.0-0.6

Notes to tables:
(a) The correction magnitudes in the lower table are to be added to the continuum sensitivities in the upper table. The line flux sensitivities should be multiplied by the line flux correction factors. Ranges are given for the line flux correction factors; the first value is for an unresolved line, the second for a line that is fully resolved in the 3-pixel wide slits.
(b) To convert to surface brightness per square arcsec (for extended sources) when using the 3-pixel wide slit, subtract 0.2 mag from the point source continuum sensitivities and multiply the point source line fluxes by 1.2. I.e., at K with the grating the sensitivities are 18.1 mag/arcsec2 and 2.6E-20 W/m2/arcsec2.
(c)For extended sources sensitivities scale roughly as the square root of the slit width
(d) If it is necessary to nod to sky rather than along the slit, subtract 0.4 mag from the continuum sensitivities and multiply line fluxes by 1.4.
(e) To calculate integration times with the echelle, multiply the time obtained for one setting by the number of echelle settings required. E/g/. If a continuous spectrum in the K band is required, multiply the time by 3.
(f) Sensitivities are averages over the better portions of each band - they will be worse at wavelengths of poor transmission and/or strong sky emission. Applicants seeking measurements at very specific wavelengths are referred to plots of sky transmission and to plots of OH and O2 sky emission.

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Last update September 1, 2001; Tom Geballe