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Michelle Gratings |
Michelle contains 5 gratings to provide various spectral resolutions
in the 10 and 20 micron atmospheric windows. Characteristics of the
gratings are shown in the table below. The resolving powers of the gratings
are proportional to wavelength across each window (i.e. the resolution is constant in microns), for each slit;
values listed are for 10um and 20um. The resolving power of the
echelle oscillates with wavelength, depending on the angle of the echelle.
Resolving powers vary inversely with the slit width (e.g., R with the 3-pw slit is 2/3 that with the 2-pw
slit). Because the echelle is in low order, resolution and efficiency vary rapidly with
wavelength. The echelle blaze angle corresponds to wavelengths of 7.4, 8.2, 9.3, 10.6, 12.3, 14.8, 18.5, and
24.7um. Close to these wavelengths the resolution and sensitivity are roughly 30,000 with the 2-pw slit and 20,000 with the 3-pw
slit. Midway between
these wavelengths efficiency is much lower and the resolving power can be 1.5 times higher or lower. Until additional
order sorting filters for parts of
the Q window are obtained, the echelle cannot be used beyond 22.5um or at the wavelength of the H2 0-0 S(1) line (17.04um)
Name | Usable wavelengths | Dispersion (microns/pixel) | Resolving power 10um, 2pw slit |
Resolving power 20um, 3pw slit |
Wavelength coverage for single grating setting (microns) |
LowN |
7-14um |
0.024 |
200 |
- |
7.7 |
LowQ |
16-26um |
0.031 |
- | 110 | 9.9 |
Medium (R~1000) |
7-26um |
0.0047 |
1000 |
1300 | 1.5 |
High (R~3000) |
7-26um |
0.0016 |
3000 |
4000 | 0.50 |
Echelle |
7-22.5um * |
~ 20,000-40,000 |
~ 13,500-27,000 |
1500 km/sec at blaze wavelength |
Last update September 28, 2005; R. S. Fisher & T. R. Geballe