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Long Slit Spectroscopy

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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 on the R~1800 page.

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.