IRCS grisms 

This page lists the grisms which are currently installed in the IRCS camera section. lambda indicates the wavelength in microns. All sensitivites are limits for 5 sigma detections per pixel in 1 hour of on-source integration (usually assumed to comprise six 600 second integrations), using the 0.15'' slit. See the important notes below regarding the effects of seeing and slit widths on the actual sensitivities. Because the spectrum is properly sampled with the 0.15'' 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, 4, or 6 for the wider slits).

[Note: IRCS has a low-resolution prism,  which has the wavelength coverage of 0.8 - 5.0 um @ 52 mas (1.8 - 4.1 um @ 20 mas). If you are interested in it although there are several problems which should be solved yet , please contact to Dr. Hiroshi Terada.]

Grisms for use with the 20 mas pixel scale 

There is a single JHK grism for the 20 mas pixel scale, but only a single spectroscopic window can be observed at a time. The short wavelength cut-off is due to the order sorting filter and is listed at the approximately 10% transmission level. The long wavelength cut-off occurs at the end of the array when the transmission is still high. 

Band Coverage Comparison
Spectral resolution (slit width) Dispersion
0.10'' 0.15'' 0.225'' 0.45''
Iz 0.95-1.01 [Ar-lines] 813lambda 542lambda 361lambda 181lambda 2.3 16.4
zJ 1.03-1.18 [Ar-lines] 745lambda 497lambda 331lambda 166lambda 2.7 16.4
J 1.19-1.38 [Ar-lines] 677lambda 452lambda 301lambda 151lambda 3.3 16.1
H 1.47-1.80 [Ar-lines] 501lambda 334lambda 223lambda 111lambda 4.1 15.3
K 1.92-2.40 [Ar-lines] 393lambda 262lambda 175lambda 87lambda 5.4 14.9
Slit Length ('') 7.0 6.5 6.5 18.0

Grisms for use with the 52 mas pixel scale 

These grisms provide the slit length of a maximum 20''. 

Band Coverage
Spectral resolution (slit width) Dispersion
0.10'' 0.15'' 0.225'' 0.30'' 0.45'' 0.60'' 0.90''
Iz 0.95-1.01 [Ar-lines] 1940lambda 1294lambda 862lambda 647lambda 431lambda 323lambda 216lambda 2.7 17.3
zJ 1.03-1.18 [Ar-lines] 1706lambda 1137lambda 758lambda 569lambda 379lambda 284lambda 190lambda 3.1 17.3
J 1.18-1.38 [Ar-lines] 1432lambda 955lambda 637lambda 477lambda 318lambda 239lambda 159lambda 3.7 17.0
H 1.49-1.83 [Ar-lines] 1146lambda 764lambda 509lambda 382lambda 255lambda 191lambda 127lambda 4.7 16.1
K 1.93-2.48 [Ar-lines] 869lambda 579lambda 386lambda 290lambda 193lambda 145lambda 97lambda 6.1 15.6
L 2.84-4.16 [Ar-lines][R1200_atm] 331lambda 220lambda 147lambda 110lambda 73lambda 55lambda 37lambda 15.9 10.8
zJH 0.95-1.5* [Ar-lines]JH 705lambda 470lambda 313lambda 235lambda 157lambda 118lambda 78lambda 8.5 J=17.5(TBD) H=16.6(TBD)
HK 1.4-2.5 [Ar-lines] 442lambda 294lambda 196lambda 147lambda 98lambda 74lambda 49lambda 12.2 H=16.7 K=16.0
Slit Length ('')  7 7 20 14 18 15 15
*The lower limit of wavelength range, 0.95 micron, is due to AO188 beam spliter. It is 0.90 micron without AO188 system.
*New zJH grism has 1.5 times higher wavelength resolution than previous JH grism.

Comparison lines data

For wavelength calibration, please use Ar_0.79-4.05.dat which contains Ar lines from 0.79 to 4.05 micron. The Ar lines in short wavelength region (0.79-1.10 micron) are added into argon.dat of IRAF.

Corrections for finite seeing and slit width 

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 tables to determine the point source sensitivity in the relevant seeing conditions. Note that the first two rows represent image quality only attainable with Adaptive Optics. Please ensure you use the correct table for your choice of grism. 

20 mas Correction (slit width)
Seeing 0.10'' 0.15'' 0.30'' 0.60'' 0.90''
0.06'' +4.8 +5.0 +5.1 +4.9 +4.7
0.2'' +3.5 +3.9 +4.3 +4.3 +4.1
0.5'' +2.0 +2.5 +3.2 +3.6 +3.6
0.7'' +1.5 +2.0 +2.7 +3.2 +3.3
1.0'' +1.0 +1.5 +2.2 +2.7 +2.9

52 mas Correction (slit width)
Seeing 0.15'' 0.30'' 0.60'' 0.90''
0.06'' +4.2 +4.2 +4.1 +3.9
0.2'' +3.2 +3.4 +3.6 +3.4
0.5'' +1.8 +2.4 +2.9 +2.9
0.7'' +1.5 +2.1 +2.5 +2.6
1.0'' +0.9 +1.6 +2.0 +2.1

Extended sources 

Increasing the slit width will improve the sensitivity of an extended source, if its surface brightness remains constant. Corrections to be applied to the numbers in the table above are given here. Sensitivity can be improved by binning in the spatial direction; again the improvement is 1.25 log N, for binning over N pixels. 

Slit width 0.15'' 0.30'' 0.60'' 0.90''
Correction (mag) 0.00 +0.38 +0.75 +0.97

If the source is too extended to be nodded along the 20'' slit, remember to double your integration time to allow for observations of blank sky. 


Both examples are for the 52 mas grisms. 

1. An observer wishes to obtain S/N=10 per pixel in a properly-sampled spectrum of a K=18 quasar with 500 km/s resolution.

The 52 mas K grism has a baseline sensitivity from the above table of 15.3 mag/arcsec^2. The desired resolution requires the use of the 0.30'' slit, which provides a correction of 2.4 mag from the above table on point source corrections, assuming typical seeing of 0.5''. Since the spectrum will be oversampled, we can bin into groups of 2 pixels for a gain of 1.25 log 2 = 0.4 mag. The 5-sigma, 1-hour sensitivity is therefore K=18.1 for a point source. The required integration time is therefore 

[100.4(18-18.1) x (10/5)]2 x 1 hour = 3.3 hours. 

2. An observer wishes to map a 6'' x 6'' region of a nebula in the Br gamma and molecular hydrogen lines with 0.6'' resolution to a sensitivity of 5 x 10-19 W/m2 (5 sigma) per spatial element.

With the 0.6'' slit, the spectral resolution will be ~320, so the lines will be unresolved. Each resolution element will cover approximately 22000(A) /320 = 69A, and therefore the emission line sensitivity corresponds to a continuum sensitivity of approximately 7.2 x 10-21 W/m2/A, or K=16.9. This is K=15.3 mag/arcsec when the seeing is  0.5" (spatial element = 0.5" x 0.5").

According to the above tables, the extended source sensitivity for the 0.6'' slit is K=16.05 mag/arcsec2, to which a further 0.75 mag can be added to account for 4-pixel binning in the dispersion direction. The exposure time in each slit position is therefore 

[100.4(15.3-16.8) x (10/5)]2 x 1 hour = 15 minutes 

and 10 separate exposures must be taken to cover the desired region. In addition, sky exposures will be required, so the total exposure time will be 300 minutes. 

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