IRCS grisms and exposure time calculation

This page lists the grisms which are currently installed in the IRCS camera section. λ 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.

BandCoverageComparison
lines
Spectral resolution (slit width) Dispersion
(A/pixel)
Sensitivity
(mag/arcsec2)
0.10'' 0.15'' 0.225'' 0.45''
zJ 1.03-1.18 [Ar-lines] 745λ 497λ 331λ 166λ 2.7 16.4
J 1.19-1.38 [Ar-lines] 677λ 452λ 301λ 151λ 3.3 16.1
H 1.47-1.80 [Ar-lines] 501λ 334λ 223λ 111λ 4.1 15.3
K 1.92-2.40 [Ar-lines] 393λ 262λ 175λ 87λ 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''. The lower limit of wavelength range, 0.95 micron, is due to AO188 beam spliter.

Band Coverage
(um)
Comparison
lines
Spectral resolution (slit width) Dispersion
(A/pixel)
Sensitivity
(mag/arcsec2)
0.10'' 0.15'' 0.225'' 0.30'' 0.45'' 0.60'' 0.90''
zJ 1.03-1.18 [Ar-lines] 1706λ 1137λ 758λ 569λ 379λ 284λ 190λ 3.1 17.3
J 1.18-1.38 [Ar-lines] 1432λ 955λ 637λ 477λ 318λ 239λ 159λ 3.7 17.0
H 1.49-1.83 [Ar-lines] 1146λ 764λ 509λ 382λ 255λ 191λ 127λ 4.7 16.1
K 1.93-2.48 [Ar-lines] 869λ 579λ 386λ 290λ 193λ 145λ 97λ 6.1 15.6
L 2.84-4.16 [Ar-lines][R1200_atm] 331λ 220λ 147λ 110λ 73λ 55λ 37λ 15.9 10.8
zJH(1) 0.95-1.5 [Ar-lines] 705λ 470λ 313λ 235λ 157λ 118λ 78λ 8.5 J=17.5(TBD) H=16.6(TBD)
HK 1.4-2.5 [Ar-lines] 442λ 294λ 196λ 147λ 98λ 74λ 49λ 12.2 H=16.7 K=16.0
Slit Length ('') 7 7 20 14 18 15 15

(1)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 from 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.

20mas

Seeing Correction (slit width)
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

52mas

Seeing Correction (slit width)
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 of exposure time estimation


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/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

[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.

Notes for exposure time


Even with the 0.15'' slit, the spectral resolution of the IRCS grisms is not high enough to work effectively between the OH lines. Good removal of these lines is an essential part of obtaining high quality scientific data, and this is usually performed by nodding the target along the slit on a timescale no longer than the typical variability timescale of the OH lines.

There is no autoguider available for the instruments on Infrared-Nasmyth focus of Subaru telescope. IRCS, one of IR-Nasmith instruments of Subaru telescope, relies on AO188 for guiding.Therefore, targets without suitable AO188 guide stars, either, NGS, or TTGS, relies on the telescope tracking only. Any exposure longer 300 seconds on targets without suitable AO guide stars should be avoided.




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