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GNIRS Grating Usage: Short Camera and 32 l/mm Grating

Home » Sciops » Instruments » GNIRS » Long-Slit, XD & IFU Spectroscopy » Gratings/Cameras

Resolving power:
(lambda / delta lambda)

1700 with 0.3 arcsec (2 pixel) slit

Typical uses:
(...and limitations)

(1) Spectrum of all or most of an entire atmospheric window at one grating setting with 99 arcsec long slit

Minimum usable wavelength is ~1.15um observed in 5th order: at shorter wavelengths there is severe inter-order contamination e.g. 6th order is contaminated at the blue end by light from 5th order and at the red end from 7th order. Use the cross-dispersing prism for shorter wavelengths.

There is also some contamination (a) at the red end of 5th order from shorter wavelength 6th order light and (b) at the red end of 4th order from shorter wavelength 5th order light, but these fall in the regions between atmospheric windows. This does mean that spectral features between the windows will be unreliable. Use the cross-dispersing prism if these regions are important.

The maximum exposure time at 5um is 0.2 seconds with the 0.3arcsec slit and the deep-well readout mode. When possible, the 110 l/mm grating is recommended at thermal wavelengths to reduce background.

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), 2nd order (L, 3.8um) and 6th order (1.10um) [the latter is shown for completeness but should not be used, as described above].


(2) Cross-dispersed spectrum of 0.9-2.5um simultaneously with 6 arcsec slit

Cross-dispersion removes the inter-order contamination discussed above. The prism (SF57 glass) has excellent transmission across almost the whole wavelength range but attenuates slightly at the red end of the K-window. There is a roll-off in transmission shortwards of 1.2um due to the broadband blocking filter; this filter is due to be replaced in late 2004. (The ITC includes the prism and filter properties). 

Examples shown below: (a) cross-dispersed spectra (orders 3-8) of an A-type star, (b) grating transmission response in the various orders (as modelled in the ITC), (c) typical raw flat field image showing orders 3-8, (d) typical raw image of a standard star tracing the same orders, (e) typical raw image of a faint galaxy showing sky emission 
 

Example spectra and other figures:

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)

Fig 1e: long-slit spectrum in 2nd order (L-window, 3.8um central wavelength)

Fig 1f: long-slit spectrum in 6th order (J-window, 1.10um central wavelength). Note the contamination at the blue end from 5th order (~1.1um) and at the red end from 7th order (~1um). For these reasons this order is not recommended. Use the cross-dispersing option for wavelengths shorter than 1.15um.

Fig 2a: cross-dispersed spectra in orders 3 (top right) - 8 (bottom left)

Fig 2b: ITC model of the 32 l/mm grating transmission for orders 3-8

Fig 2c: typical raw flat field image showing orders 3-8 (left to right, with wavelength decreasing upwards; log flux scale)

Fig 2d: typical raw image of a standard star tracing orders 3-8 (left to right, with wavelength decreasing upwards; log flux scale)

Fig 2e: typical raw image of a faint galaxy showing sky emission in orders 3-8 (left to right, with wavelength decreasing upwards)

Last update February 24, 2004; Phil Puxley