Science Operations

Gratings

The grating unit holds a mirror for imaging as well as three gratings for spectroscopy. The grating complements differ between LUCI1 and LUCI2. Both contain G210 and G200 gratings. Although both G210 gratings were made with the same master, only the one in LUCI2 is truly diffraction-limited. LUCI1 has a G150 grating and LUCI2 a G040 grating. 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 G150 Ks 2 2.15 1.95-2.40 1.81-2.40 4150 0.533
LUCI2 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. At this time, the G210 and G040 in LUCI2 are the only diffraction limited gratings available. As we gain experience with AO spectroscopy, the plan is to obtain at least one diffraction-grating for LUCI1. 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.