Available Wavelength Ranges, Slits and Resolving Powers

You may examine data frames produced by NIRSPEC in its low spectral resolution and high spectral resolution modes in order to get a feel for what the instrument produces.

In the low resolution (grating) mode a single setting of its grating NIRSPEC covers a wavelength range of approximately 0.18 x lambda(central). For example, with lambda(central) set to 2.25Ám, the wavelength coverage is 2.05-2.45Ám, with wavelength increasing toward the top of the array.

The wavelength coverage of the echelle is more complicated. Schematic diagrams of the accessable wavelength range for any single echelle setting through each of the blocking filters are available. You also may download from Keck the echelle simulator to precisely quantify the wavelength coverage obtained in this configuration.

Note also that the wavelength coverage actually delivered may be limited by the blocking filter used (see table below). For example, one could set the grating to cover the wavelength range 1.53-1.83Ám, but there is no blocking filter that transmits that entire range. In specifying the wavelength range you also may wish to take into account the transmission of the earth's atmosphere.

In addition, as shown in the schematic diagrams, except at the shortest wavelengths one echelle setting will not provide complete coverage of the each order and additional echelle settings will be required to obtain continuous wavelength coverage across the 0.18 x lambda(central) range. The number of settings required for complete coverage increases with wavelength; the sensitivity table provides quantitative information about this.

Name N1 N2 N3 N4 N5 N6 N7 KL M-wide
Cut-on (Ám) 0.95 1.09 1.14 1.24 1.14 1.56 1.84 2.13 4.42
Cut-off (Ám) 1.12 1.29 1.37 1.59 1.80 2.31 2.63 4.23 5.53

Grating or
Slit width
Slit width
Slit length
Resolving Power
Grating 2 0.380 42 3,000 none
3 0.570 42 2,000 none
4 0.760 42 1,500 none
Echelle 1 0.144 12 75,000? none
2 0.288 12 37,000 none
3 0.432 12 25,000 none
4 0.576 12 19,000 none
5 0.720 12 15,000 none
3 0.432 24 25,000 >2.1Ám
4 0.576 24 19,000 >2.1Ám
5 0.720 24 15,000 >2.1Ám

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Last update December 3, 2000; Tom Geballe