IRCS echelle configurations

Unlike grism spectroscopy, observations at JHK with the echelle are detector-limited between OH lines Sky subtraction is therefore not an issue, and long exposures can be made with the target at a single slit position. See the guide to exposure times for more information.

The following table lists 5 sigma sensitivities per pixel assuming a single exposure of 3600 seconds using the 0.14'' slit. See the important notes below regarding the effects of seeing and slit widths on the actual sensitivities. If more than one exposure is taken, the signal-to-noise ratio will decrease by the square root of the number of exposures. Note also that it is necessary to take a dark frame of the appropriate length.

Band Coverage
(um) Spectral
resolution
(0.14'' slit) Dispersion
(A/pixel) Slit
length No. of exposures
to cover whole wavelength coverage Sensitivity
(mag/arcsec²)

Iz⁽¹⁾ 0.92-1.01 ~ 20 000 3.47'' 1

zJ 1.03-1.18 19 000 - 22 600 0.25 - 0.28 3.47'' 1 15.27

J 1.18-1.38 19 000 - 22 600 0.28 - 0.33 3.47'' 1 15.09

H 1.49-1.83 18 800 - 22 900 0.36 - 0.44 5.17'' 2 14.60

K 1.90-2.49 18 300 - 22 300 0.46 - 0.62 5.17'' 2 14.11

L 2.80-4.20 17 300 - 25 100 0.67 - 1.06 9.37'' 6 10.59

M 4.41-5.34 16 800 - 26 300 1.16 - 1.30 9.37'' 4 9.04

(1) Offered in "shared risk" mode.

Because the spectrum is properly sampled with the 0.14'' slit, spectra taken with wider slits can be binned in the dispersion direction to increase sensitivity by 1.25 log N magnitudes, where N is the number of pixels being binned (equal to 2 or 4 for the wider slits).

Important note: In general, the echelle does not provide continuous wavelength coverage, and more than one exposure is required to cover the entire wavelength range, as indicated in the above table. If you do not need complete wavelength coverage, you may be able to use fewer exposures; please see the details of the wavelength coverage of the echelle orders.

Corrections for finite seeing and slit width

Extended sources

Increasing the slit width will improve the sensitivity of an extended source, if its surface brightness remains constant. Corrections factors to be applied to the numbers in the table above are given here. Since observations at zJHK are detector-limited, increasing the slit width does not increase the noise of an observation. Sensitivity can be improved by binning in the spatial direction; again the improvement is 1.25 log N, for binning over N pixels.


	Correction (mag)
Band	0.14'' slit	0.27'' slit	0.54'' slit
zJHK	0.00	+0.75	+1.51
LM	0.00	+0.38	+0.75

Point sources

The sensitivity for point sources can be improved by extracting the spectrum along several spatial pixels. Apertures between 1 and 2 times the seeing FWHM are optimum. The following corrections should be added to the numbers in the above table to determine the point source sensitivity in the relevant seeing conditions.

Do not neglect the correction on the bottom line of the table for thermal-infrared observations!

Correction (mag)

Seeing 0.14'' slit 0.27'' slit 0.54'' slit

0.3'' +2.2 +3.2 +3.7

0.4'' +1.8 +2.8 +3.5

0.5'' +1.5 +2.5 +3.3

0.7'' +0.9 +2.0 +2.8

1.0'' +0.4 +1.4 +2.4

LM⁽¹⁾ -0.00 -0.38 -0.75

(1)Additional correction to be made for observations at L or M.