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: The thermal background level was recovered to low level one before 2005 (See Figure) since the image rotator was recoated and reinstalled in July, 2008. The sensitivities in tables are revised based on new low thermal background levels.]

[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 (um)	Spectral resolution (slit width)				Dispersion (A/pixel)	Sensitivity (mag/arcsec²)
Band	Coverage (um)	0.10''	0.15''	0.225''	0.45''	Dispersion (A/pixel)	Sensitivity (mag/arcsec²)
Iz	0.92-1.01	813	542	361	181	2.3	16.4
zJ	1.03-1.18	745	497	331	166	2.7	16.4
J	1.19-1.38	677	452	301	151	3.3	16.1
H	1.47-1.80	501	334	223	111	4.1	15.3
K	1.92-2.40	393	262	175	87	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 (um)	Spectral resolution (slit width)							Dispersion (A/pixel)	Sensitivity (mag/arcsec²)
Band	Coverage (um)	0.10''	0.15''	0.225''	0.30''	0.45''	0.60''	0.90''	Dispersion (A/pixel)	Sensitivity (mag/arcsec²)
Iz	0.92-1.01	1940	1294	862	647	431	323	216	2.7	17.3
zJ	1.03-1.18	1706	1137	758	569	379	284	190	3.1	17.3
J	1.18-1.38	1432	955	637	477	318	239	159	3.7	17.0
H	1.49-1.83	1146	764	509	382	255	191	127	4.7	16.1
K	1.93-2.48	869	579	386	290	193	145	97	6.1	15.6
L	2.84-4.16	331	220	147	110	73	55	37	15.9	10.8
JH	1.0-1.6	481	321	214	160	107	80	53	11.3	J=17.5 H=16.6
HK	1.4-2.5	442	294	196	147	98	74	49	12.2	H=16.7 K=16.0
Slit Length ('')		7	7	20	14	18	15	15

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.

Examples

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

[10^0.4(18-18.1) × (10/5)]² × 1 hour = 3.3 hours.

2. An observer wishes to map a 6'' × 6'' region of a nebula in the Br gamma and molecular hydrogen lines with 0.6'' resolution to a sensitivity of 5×10^-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 × 10^-21 W/m²/A, or K=16.9. This is K=15.3 mag/arcsec² 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

[10^{0.4(15.3-16.8)}]² × 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.