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Spectroscopy

GNIRS has two basic spectroscopic modes.


  • Long slit, 1.0-5.4 µm spectroscopy of any individual window (X, J, H, K, most of L, or M) at R up to ~1,800, or of any portion thereof at R up to ~18,000; either without adaptive optics or with adaptive optics (XJHKL only), using slit lengths of 50-100 arc-seconds.

  • Cross-dispersed, 0.85-2.5 µm short slit spectroscopy with full spectral coverage at R up to ~1,800 and partial coverage (disjoint 0.1-0.2 µm segments) at R up to ~6,000; with or without adaptive optics, using slit lengths of 5-7 arc-seconds.

Note: The sensitivity in a single order in cross-dispersed mode is reduced by about 10% compared to the sensitivity in long slit (single order) mode. Unless the science specifically involves only one order of GNIRS (e.g., J, H, or K) or the source size is more than 2-3 arc-seconds (requiring nodding to sky) it is advantageous to employ the cross-dispersed mode.

The GNIRS grating turret contains three gratings, each with an effective first order blaze wavelength of 6.6 µm. The wavelength diffracted with peak efficiencies then correspond fairly well to the atmospheric windows centered at 5, 3.5, 2.2, 1.65, and 1.25 µm (M, L, K, H, J) for orders 1 through 5 respectively. The blocking filters used for these orders cover most or all of the free spectral ranges of the individual orders. A filter for order 6 (1.1 µm, called X) is also available.

The coverages and resolving powers provided by the gratings are tabulated below for the short (0.15"/pix) and long (0.05"/pix) cameras. The resolving powers (λ / Δλ) in the table are for 2-pixel wide slits. Wider slits are available, see the list of slits. Note that the wavelength coverage within an order does not depend on the central wavelength setting, and that the resolving power within an order increases with wavelength. Note also that the resolving power decreases as 1/(slit width) for sources that fill the width of the slit.


Grating
(l/mm)
Order - Band Blocking Filter Range (microns) Short camera, 0.30" slit Long camera, 0.10" slit
Coverage (microns)(a) Resolving power
(2 pix wide slit)(b,c)
Coverage (microns)(a)
Resolving power
(2 pix wide slit)(b,c)
10.44 6 - X (1.10µm) 1.03-1.17 (d) (d) 0.332(f,g) ~2,100
10.44 5 - J (1.25µm) 1.17-1.37 (d) (d) 0.398(f,g) ~1,600
10.44 4 - H (1.65µm) 1.47-1.80 (d) (d) 0.497(f,g) ~1,700
10.44 3 - K (2.20µm) 1.91-2.49 (d) (d) 0.663(f,g) ~1,700
10.44 2 - L (3.50µm) 2.8-4.2 (d) (d) 0.995 ~1,800
10.44 1 - M (4.80µm) 4.4-6.0(i) (d) (d) 1.99(e) ~1,200
31.7 6 - X (1.10µm) 1.03-1.17 0.331(f,g) ~1,700(h) 0.110(i) ~5,100
31.7 5 - J (1.25µm) 1.17-1.37 0.397(f,g) ~1,600(h) 0.132(i) ~4,800
31.7 4 - H (1.65µm) 1.49-1.80 0.496(f,g) ~1,700(h) 0.166(i) ~5,100
31.7 3 - K (2.20µm) 1.91-2.49 0.661(f,g) ~1,700(h) 0.221(i) ~5,100
31.7 2 - L (3.50µm) 2.80-4.20 0.992 ~1,800 0.332 ~5,400
31.7 1 - M (4.80µm) 4.4-6.0(j) 1.98(e) ~1,240 0.660 ~3,700
110.5 6 - X (1.10µm) 1.03-1.17 0.094(i) ~6,600 0.0316 ~17,800
110.5 5 - J (1.25µm) 1.17-1.37 0.113(i) ~7,200 0.0380 ~17,000
110.5 4 - H (1.65µm) 1.49-1.80 0.142(i) 5,900 0.0475 ~17,800
110.5 3 - K (2.20µm) 1.91-2.49 0.189(i) ~5,900 0.0633 ~17,800
110.5 2 - L (3.50µm) 2.80-4.20 0.280 ~6,400 0.0944 ~19,000
110.5 1 - M (4.80µm) 4.4-6.0(j) 0.575 ~4,300 0.192 ~12,800(k)

Notes:
(a) Important: Wavelength coverages are accurate to +/-2 percent. Wavelength is linear with array pixel number. Actual wavelength settings (specified in the OT by the central wavelength) are accurate to better than 5 percent of the wavelength coverage. E.g., if the requested setting with the 110.5 l/mm grating and short camera has a central wavelength of 3.700 µm, the nominal spectral range delivered will be 3.560 - 3.840 µm, but could be shifted by as much as 0.014 µm either way. Should an observing program require higher accuracy than the above, a note should be added at phaseII.
(b) Values are for the wavelengths in column 2. Values are linear with wavelength within an order; for example, for the 31.7 l/mm grating and the short camera R~1550 at 3.0µm and R~2050 at 4.0µm.
(c) Resolving power decreases as 1/(slit width) for sources that fill the slit width; for example, the values of R for a 3 pixel-wide slit are 2/3 the values in the table.
(d) This mode potentially provides R=570 but offers no advantage over R=1700 and is not used in practice.
(e) Not recommended; the next higher resolution grating covers almost the entire accessible wavelength range in the M window and can be used with longer exposure times and significantly lower overheads.
(f) When used in single order (long slit) mode wavelength coverage is broader than bandpass of the blocking filter.
(g) When used with a cross-dispersing prism a complete 0.8-2.5µm spectrum is obtained across five orders (3-8).Efficiency in orders 7 and 8 is low.
(h) Since November 2012 and for the cross-dispersed mode with the 2 pix wide slit only resolving powers are somewhat lower, as follows: X-1400; J-1400, H-1400; K-1300. For cross-dispersion with other slit widths use the table and footnote c.
(i) When used with a cross-dispersing prism 0.1-0.2µm disjoint sections of the 0.9-2.5µm spectrum are obtained in the five orders (3-7). 3-4 wavelength settings are required to provide complete wavelength coverage
(j) Detector does not respond to light at wavelengths greater than 5.4 microns.
(k) Resolving power significantly lower than in other orders (because grating angles corresponding to M band in first order are significantly less than the angles of other bands in their orders).