Echelle Spectrograph Cross Dispersers
As built, the spectrograph used 165×220 mm cross-dispersing
gratings. These are designated G226-1, G226-2 and G226-3 in the
following tables. A later modification permits the use of the
203×254 mm gratings from the 4-m RC spectrograph. These are the
remaining gratings in the tables.
For a given grating, the wavelength coverage in the cross-dispersion
direction depends on the camera focal length and the detector
size. The following table lists the coverage for the standard
camera/CCD combinations. The order separation in arcsec at a given
wavelength depends on which echelle
grating (31.6 or 79 l/mm) is used.
Use the ECHMAP interactive program to
experiment with which cross disperser gives the best combination of
wavelength coverage and order separation.
GRATING PARAMETERS WAVELENGTH COVERAGE (Å)[1] ORDER SEPARATION [2]
Camera/CCD (arcsec, at 4000Å)
Long/ Folded/ AirSch/
Grating l/mm ord Blaze[3] Tek2048 Tek1024 Loral3K 31.6 l/mm 79 l/mm
G250 158 1 4000 4920 6689 4180 3.6 8.4
[4]G400 158 1 8000 4920 6689 4180 3.6 8.4
G226-2 226 1 8000 3440 4676 2923 4.8 12.0
G226-3 226 1 6300 3440 4676 2923 4.8 12.0
G510 300 1 10000 2591 3523 2202 6.4 15.9
KPGL2 316 1 4400 2460 3344 2090 6.7 16.8
G181 316 1 7500 2460 3344 2090 6.7 16.8
[5]G226-2 226 2 4000 1720 2338 1461 9.6 24.0
KPGL3 527 1 5500 1475 2005 1253 11.2 28.0
[4]G510 300 2 5000 1296 1761 1101 12.8 31.9
G181 316 2 3750 1230 1672 1045 13.4 33.6
KPGL1 632 1 4200 1230 1672 1045 13.4 33.6
Notes:
- The wavelength coverage given here is computed from nominal
focal lengths and detector sizes. The actual values are
probably slightly different.
- Order separation at other wavelengths scales as lambdaº
- Blaze wavelengths are given for the Littrow
configuration. Multiply by 0.92 to correct for the actual
configuration in the echelle spectrograph.
- Gratings G400 and G510 are silver-coated and thus do not reflect
much light below about 4000Å.
- G226-2 in second order is listed for reference only. It is
very inefficient. There are additional gratings, not listed
here, which are either of very low efficiency (100 l/mm, blazed
in red), or which are of higher dispersion (632 l/mm blazed in
red, >632 l/mm blazed at various wavelengths), and are
therefore not very interesting.
Echelle Cross Disperser
Relative Efficiencies
Grating: 250 400 226-3 226-2 510 181 KPGL2 KPGL3 420 KPGL1 KPGLF 450 KPGLG KPGLD 380
Lines/mm: 158 158 226 226 300 316 316 527 600 632 632 632 860 790 1200
Blaze: 4000 8000 6300 8000 10000 7500 4400 5500 8000 4200 8200 11000 11000 8500 8000
Wavelength: Blue-blazed gratings relative to grating 250
3250 1.00 1.13 0.77 3250
3500 1.00 0.04 1.14 0.38 0.76II 0.78 3500
3750 1.00 0.10 0.31II 0.64II 1.14 0.52 0.71II 0.83 3750
4000 1.00 0.26 0.50II 0.79II 1.17 0.70 0.73II 0.97 4000
4250 1.00 0.45 0.70II 0.83II 1.20 0.86 0.64II 1.04 0.73II 4250
4500 1.00 0.66 0.84II 0.81II 1.17 1.08 0.58II 1.11 0.71II 4500
4750 1.00 0.89 0.76II 1.25 1.29 0.43II 1.21 0.81II 4750
5000 1.00 1.16II 0.72II 1.24 1.42 0.34II 1.29 0.98II 0.67 5000
5250 1.00 1.40 0.62II 1.28 1.58 0.27II 1.34 5250
5500 1.00 1.64 0.66II 1.31 1.66 1.40 5500
5750 1.00 1.92 0.57II 1.25 1.71 1.36 5750
6000 1.00 2.14 0.58II 1.29 1.88 1.44 6000
6250 1.00 2.47 0.49II 1.18 2.02 1.47 6250
6500 1.00 2.65 0.36II 1.14 2.11 6500
6750 1.00 2.85 0.38II 1.12 2.15 6750
7000 1.00 3.18 0.35II 1.20 2.31 7000
Grating: 250 400 226-3 226-2 510 181 KPGL2 KPGL3 420 KPGL1 KPGLF 450 KPGLG KPGLD 380
II signifies second order.
Grating: 250 400 226-3 226-2 510 181 KPGL2 KPGL3 420 KPGL1 KPGLF 450 KPGLG KPGLD 380
Lines/mm: 158 158 226 226 300 316 316 527 600 632 632 632 860 790 1200
Blaze: 4000 8000 6300 8000 10000 7500 4400 5500 8000 4200 8200 11000 11000 8500 8000
Wavelength: Red-blazed gratings relative to grating 400
5000 0.86 1.00 0.62II 1.06 1.22 0.37II 1.11 5000
5250 0.71 1.00 0.45II 0.91 1.13 0.24II 0.96 5250
5500 0.61 1.00 0.40II 0.80 1.02 0.17II 0.86 5500
5750 0.52 1.00 0.30II 0.65 0.89 0.71 0.69II 5750
6000 0.47 1.00 0.27II 0.60 0.88 0.67 0.62II 6000
6250 0.41 1.00 0.20II 0.48 0.82 1.05 0.60 1.05 0.54II 0.74 6250
6500 0.38 1.00 0.14II 1.03 0.43 0.80 1.04 1.01 0.45II 0.75 0.46 6500
6750 0.35 1.00 0.14II 0.96 0.40 0.76 0.99 0.97 0.39II 0.77 0.50 6750
7000 0.32 1.00 0.11II 0.90 0.38 0.73 0.89 0.93 0.38II 0.77 0.48 7000
7250 0.30 1.00 0.94 0.66 0.91 0.91 0.35 0.70 1.01 0.90 0.89 7250
7500 0.27 1.00 0.91 0.70 1.00 0.90 0.32 0.67 1.04 0.86 0.90 7500
7750 0.27 1.00 0.92 0.75 1.00 0.86 0.32 0.69 1.01 0.84 0.95 7750
8000 0.26 1.00 0.96 0.79 1.05 0.89 0.67 1.00 0.82 1.02 8000
8250 0.24 1.00 0.86 0.75 1.14 0.90 0.64 1.02 0.81 0.46 1.09 8250
8500 0.26 1.00 0.85 0.76 1.13 0.90 1.03 0.81 0.54 8500
8750 0.23 1.00 0.83 0.78 1.14 0.84 0.60 8750
9000 0.25 1.00 0.79 0.79 0.99 0.92 9000
9250 0.27 1.00 0.98 1.08 9250
9500 1.00 1.09 1.11 9500
9750 1.00 1.20 1.16 9750
10000 1.00 10000
Grating: 250 400 226-3 226-2 510 181 KPGL2 KPGL3 420 KPGL1 KPGLF 450 KPGLG KPGLD 380
II signifies second order.