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Cross-dispersed (XD) Spectroscopy

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 flat field image showing orders 3-8 (left to right, with wavelength increasing downwards; log flux scale)

Fig 2: typical raw image of a standard star visible in orders 3-9 (left to right), with wavelength increasing downwards; log flux scale)

Fig 3: typical raw image of a faint galaxy showing sky emission in orders 3-8 (left to right, with wavelength increasing downwards)