Slit Spectroscopy

GNIRS has two basic slit spectroscopy 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)  blue and red cameras. The blue cameras are used in the XJHK bands, the red cameras in the LM bands. 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 depends is proportional to 1/(slit width) for sources that fill the width of the slit; the values of R in the table are for the 0.3" slit with the short cameras and for the 0.10" slit with the long cameras and are the highest ones achievable.

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 depends on 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).

Spectroscopic Modes

Long-slit

In the long-slit (single order) mode one band (e.g., X, J, H, K, L, or M) is observed at a time along a 99 arcsec (short camera) or 49 arcsec (long camera) slit. At the lowest resolution (31.7 l/mm grating and short cameras or 10.44 l/mm grating and long cameras), the usable wavelength coverage for a single order usually is limited by the order-blocking filter rather than by the size of the detector array. At higher resolution (e.g., 111 l/mm grating and either camera) only a (selectable) portion of the filter bandpass is observed. There is some inter-order contamination at the shortest wavelengths, as described below for the case of  R~1800.

The long cameras are generally used only if (1) high angular resolution provided by adaptive optics (AO) is a scientific requirement and/or if (2) the highest spectral resolution (provided by the 110.5 l/mm grating and a narrow slit) is a scientific requirement. The use of a narrow slit without AO (e.g., when observing very bright stars or observing in the thermal IR) results in large light loss.

Raw K band spectral image of a crowded field, obtained using the 32 l/mm grating, short blue camera, and 0."45 x 99" long slit. Sky emission lines are roughly horizontal. At least sixteen stellar spectra are apparent. Wavelength increases downward.

Cross-dispersed

In the cross-dispersed (multi-order) mode at the lowest spectral resolution (R~1800 for the 0.3 arcsec wide slit (short camera) and for the 0.1 arcsec wide slit (long camera)), the entire 0.85-2.5 µm region (orders 3-8) is observed with a single grating setting and without inter-order contamination. (Orders 9-11 also fall on the array but the transmittance of GNIRS drops steeply in these orders.) Two configurations can be used to achieve this wavelength coverage: the 32 l/mm grating and short blue camera with the short camera's cross-dispersing prism (SXD) and a 7" slit, or the 10 l/mm grating, long blue camera, long camera cross-dispersing prism (LXD) and 5" slit. Short slits must be used to prevent overlapping orders on the detector array. The 10 l/mm grating and long camera may also be used with the SXD prism, allowing a longer slit (7") but restricting the wavelength coverage to 1.2-2.5 µm (orders 3-5).

At intermediate resolution (111 l/mm grating and short camera or 32 l/mm grating and long camera) approximately one-third of each order is observed and 3-4 grating settings are required to obtain complete 0.85-2.5 µm coverage.The resolving power is ~5400 for the 0.3 arcsec wide slit (short camera) and for the 0.1 arcsec wide slit (long camera).

The figures below show examples of cross-dispersed data (obtained with the 32 l/mm grating and the short blue camera).

Fig 1: typical raw argon arc lamp image showing orders 3-9 (left to right, with wavelength increasing downwards); note the central wavelength is set at 1.65µm.	Fig 2: typical raw flat field image showing orders 3-8 (left to right, with wavelength increasing downwards; log flux scale)
Fig 3: typical raw image of a standard star visible in orders 3-9 (left to right), with wavelength increasing downwards; log flux scale)	Fig 4: typical raw image of a faint galaxy showing sky emission in orders 3-8 (left to right, with wavelength increasing downwards)

Grating/Camera configurations:	32 l/mm grating and short blue camera or 10 l/mm grating and long blue camera
Resolving power: (λ/Δλ)	1800 either with with short blue camera and 0.3 arcsec wide slit or with long blue camera and 0.1 arcsec wide slit. Lower resolving powers if wider slits are used.
Typical uses: (...and limitations)	Full coverage 0.85-2.5µm spectra in short slits: 7 arcsec long for the 32 l/mm grating, short blue camera and its (SXD) prism; 5 arcsec long for the 10 l/mm grating, long blue camera and its (LXD) prism. There is no inter-order contamination. One can also use the long blue camera with the SXD prism and the 7 arcsec slit, but then the wavelength coverage is 1.2-2.5µm. The orders are positioned on the array so as to avoid the patches of bad pixels. The short slits are sufficiently long that for point-like sources it is possible to nod the telescope while keeping the source in the slit and thus obtain sky subtracted-spectra for which the source is observed 100 percent of the time. For sources larger than a few arc-seconds it is necessary to nod the telescope to blank sky. Typical raw image of a faint galaxy showing sky emission in orders 3-8 (left to right, with wavelength increasing downwards) Reduced spectra of a standard star in orders 3 through 8 covering 0.85 to 2.5 microns are shown below. Click on each spectrum to enlarge.

Grating/Camera configurations:	110 l/mm grating with short blue camera or 32 l/mm grating with long blue camera
Resolving power: (λ/Δλ)	5900 with 0.3 arcsec wide slit and short blue camera or with 0.1 arcsec wide slit, long blue camera, and adaptive optics. Lower resolving powers if wider slits are used.
Typical uses: (...and limitations)	(1) Intermediate resolution spectra of selected portions of orders 3-8 (between 0.85µm and 2.5µm in short slits as described in XD R<1800 mode. Can be used for observing at high sensitivity between OH sky emission lines at 0.9-2.2µm or between sky absorption lines at 2.25-2.5µm.  The wavelength coverage is about one-third of that of the low resolution mode (e.g., short camera + 32 l/mm grating) and so three or more settings are required to obtain full wavelength coverage at 1-2.5µm. The positioning of the orders depends on the grating central wavelength, so in some grating settings one order may cross the small central patch of bad pixels. If this is likely to pose a problem the user should contact a member of the GNIRS science team. We have noticed contamination in observations that use the GNIRS configuration: Short Blue camera + 111 l/mm grating + SXD cross disperser. More information about this effect is available here. If this is likely to pose a problem the user should contact a member of the GNIRS science team.   Fig. 1: Cross-dispersed spectrum of the argon lamp obtained with the 111 l/mm grating set to 1.53µm, short blue camera, and 0.3 arcsec wide slit. Lines can be seen in orders 3-9 (left to right). Wavelength ranges covered are 1.95-2.13, 1.46-1.60, 1.17-1.28, 0.97-1.06, 0.83-0.91, 0.73-0.80, and 0.65-0.71 microns, respectively. Note how the orders are much more vertical on the array than when full spectra coverage is obtained in one setting (eg, using the 32 l/mm grating and the short blue camera).