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Long Slit Spectra at Low Resolution (R<1800)

Grating/Camera configurations:

32 l/mm grating and short cameras or 10 l/mm grating and long cameras

Resolving power:
(λ/Δλ)

1800 either with short cameras and 0.3 arcsec wide slit or with long cameras and 0.1 arcsec wide slit. Lower resolving powers if wider slits are used.

Typical uses:
(...and limitations)

Spectrum of all or most of an atmospheric window or filter passband as defined by the blocking filter (X, J, H, K, L, or M) at one grating setting with 99 arcsec long slit

The minimum usable wavelength for long slit spectroscopy is ~1.03um observed in 6th order. Use the cross-dispersing prism for wavelengths shorter than 1.03 microns. In addition, the wavelength ranges 1.37-1.47, 1.82-1.91, and 2.49-2.56 microns are not covered by the blocking filters; they can only be observed in cross-dispersed mode.

When viewing an X band spectrum, signal at apparent wavelengths shorter than 1.00 um is from 5th order at the long wavelength end of the X filter; likewise signal at wavelengths greater than 1.18 microns is the 7th order spectrum at the short wavelength end of the X filter.

Likewise, in the J band (5th order) signal at apparent wavelengths below 1.15 microns is actually 4th order light near 1.4 microns; likewise the spectrum apparently beyond 1.40 microns is 6th order light near 1.2 microns.

In the H band (order 4) the spectrum beyond an apparent wavelength of 1.85 microns is the 5th order spectrum at the short wavelength end of the H filter.

Note also that the spectral coverage in the L window is "only" 0.99 microns, and thus the full window cannot be covered in one grating setting. For example, two setting would be required to cover both the water ice band near 3.0 microns and the Br alpha line at 4.05 microns.

Large observing overheads mean that the 32 l/mm grating and short red camera should generally not be used for M band spectroscopy. Use the 111 l/mm grating instead.

See the example spectra shown below of an A-type star: 5th order (J, 1.25um), 4th order (H), 3rd order (K), 2nd order (L, 3.4um), and 2nd order (L, 3.8um)

Example spectra:

Fig 1a: long-slit spectrum in 5th order (J-window, 1.25um central wavelength).  Note the contamination at the red end from 6th order (~1.2um) light that is passed by the order-sorting filter.

Fig 1b: long-slit spectrum in 4th order (H-window). Note the contamination at the red end from 5th order (~1.5um) light that is passed by the order-sorting filter.

Fig 1c: long-slit spectrum in 3rd order (K-window)

Fig 1d: long-slit spectrum in 2nd order (L-window, 3.4um central wavelength); note that the full L band is not covered.

Fig 1e: long-slit spectrum in 2nd order (L-window, 3.8um central wavelength); note that the full L band is not covered.