Gratings

The grating unit holds a mirror for imaging as well as three gratings for spectroscopy. Both LUCIs contain G210, G200, and G040 gratings. The G040 was recently installed in LUCI1 and is pending checkout and commissioning. The gratings are named for their groove density, for example the G210 grating has 210 grooves per millimeter.

 

LUCI	Grating	Band/Filter	Order	λc [µm]	Usable λrange [µm]	50% Cut on/off [µm]	Resolution	Free Spectral Range [µm]
LUCI1/2	G210	K	2	2.20	2.02-2.40	2.02-3.18	5000	0.328
		H	3	1.65	1.50-1.75	1.41-1.90	5900	0.202
		J	4	1.25	1.15-1.35	1.09-1.41	5800	0.150
		z	5	0.97	0.85-1.02	0.89-1.11	5400	0.124
LUCI1/2	G200	HKspec	1	1.93	1.50-2.40	1.32-2.40	1900/2600	0.880
		zJspec	2	1.17	0.9-1.20	0.90-1.25	2100/2400	0.440
LUCI1/2	G040	K	2	2.20	2.02-2.40
		H	3	1.65	1.50-1.75
		J	4	1.25	1.15-1.35
		z	5	0.97	0.85-1.02

While any combination of grating and camera can be used, the above table below calculates the basic characteristics for seeing-limited observations assuming the N1.8 camera is used with a 2-pixel (0.5″) wide slit.

On the left side of the table are listed the orders, central wavelengths, and grating efficiency ranges (wavelengths where the grating is above 50% of its peak efficiency in that order) for each viable grating/band combination. On the right side are listed the effective resolution at the central wavelength, the free spectral range (width of the wavelength range on the detector for that central wavelength), and dispersion for these default configurations. The quoted spectral resolution includes the contribution from the instrumental resolution.

For AO spectroscopy even the gratings must be diffraction-limited. The G210 in LUCI2 and both G040 gratings are the only diffraction limited gratings available. The consequence of using one of the non-diffraction limited gratings for diffraction limited spectroscopy is that the PSF at the detector will be distorted by the aberrations in the grating. This is not recommended.

Grating Efficiencies

Below are plots of the grating efficiency vs wavelength, in Littrow configuration, for several LUCI1 gratings.