Echelle Gratings

Two echelle gratings are available. They both have the same blaze angle (63°) and therefore produce the same dispersion. The difference is in the number of lines/mm, which translates into a difference in which orders correspond to optical wavelengths and therefore in the free spectral range and the wavelength separation between order centers. The choices are:

31.6 lines/mm
Orders are narrow but closely spaced on CCD ==> better in red.
79.0 lines/mm
Orders are wide but far apart on CCD ==> better in blue. This grating has particular problems with ghosts, but still gives good results in most applications.

Wavelengths at Which Echelle Free Spectral Range Matches Detector Width Long/ Folded/ AirSch/ Tek2048 Tek1024 Loral3K 31.6 1/mm 13655 17590 32981 79.0 1/mm 5462 7036 13192
The above table gives lambda_full, in Angstroms, for available Camera/CCD combinations. The fraction of the detector filled by the free spectral range at any other wavelength is:

           fraction filled  =  lambda / lambda_full.

Figure 1. Ratio of grating efficiencies: 79 l/mm echelle divided by 31.6 l/mm echelle. This is the ratio of the counts per second per pixel at a given wavelength, measured while looking at a stable light source.

Long Table of Echelle Order Centers and Dispersions at Order Centers

These are calculated values of the central wavelength (A) and dispersion (A/mm) in each order between 3000 and 10000A, for each echelle grating and for the Short (Air Schidt and Folded Schmidt) and Long Cameras. The central wavelengths are only approximately correct. But the values are plenty good enough to help guide you around through the echelle spectrum.
31.6 l/mm 79.0 l/mm Order Center ShortCam LongCam Order Center ShortCam LongCam (Å) (Å/mm) (Å/mm) (Å) (Å/mm) (Å/mm) 55 10142.8 13.06 5.07 22 10227.0 13.04 5.06 56 9961.7 12.83 4.98 23 9782.3 12.47 4.84 57 9786.9 12.60 4.89 24 9374.8 11.95 4.64 58 9618.2 12.39 4.81 25 8999.8 11.47 4.45 59 9455.2 12.18 4.73 26 8653.6 11.03 4.28 60 9297.6 11.97 4.65 27 8333.1 10.62 4.12 61 9145.2 11.78 4.57 28 8035.5 10.24 3.98 62 8997.7 11.59 4.50 29 7758.4 9.89 3.84 63 8854.8 11.40 4.43 30 7499.8 9.56 3.71 64 8716.5 11.22 4.36 31 7257.9 9.25 3.59 65 8582.4 11.05 4.29 32 7031.1 8.96 3.48 66 8452.3 10.88 4.22 33 6818.0 8.69 3.37 67 8326.2 10.72 4.16 34 6617.5 8.44 3.27 68 8203.8 10.56 4.10 35 6428.4 8.20 3.18 69 8084.9 10.41 4.04 36 6249.8 7.97 3.09 70 7969.4 10.26 3.98 37 6080.9 7.75 3.01 71 7857.1 10.12 3.93 38 5920.9 7.55 2.93 72 7748.0 9.98 3.87 39 5769.1 7.35 2.85 73 7641.8 9.84 3.82 40 5624.9 7.17 2.78 74 7538.6 9.71 3.77 41 5487.7 7.00 2.72 75 7438.1 9.58 3.72 42 5357.0 6.83 2.65 76 7340.2 9.45 3.67 43 5232.4 6.67 2.59 77 7244.9 9.33 3.62 44 5113.5 6.52 2.53 78 7152.0 9.21 3.57 45 4999.9 6.37 2.47 79 7061.5 9.09 3.53 46 4891.2 6.24 2.42 80 6973.2 8.98 3.49 47 4787.1 6.10 2.37 81 6887.1 8.87 3.44 48 4687.4 5.98 2.32 82 6803.1 8.76 3.40 49 4591.7 5.85 2.27 83 6721.1 8.65 3.36 50 4499.9 5.74 2.23 84 6641.1 8.55 3.32 51 4411.6 5.62 2.18 85 6563.0 8.45 3.28 52 4326.8 5.52 2.14 86 6486.7 8.35 3.24 53 4245.2 5.41 2.10 87 6412.1 8.26 3.20 54 4166.6 5.31 2.06 88 6339.3 8.16 3.17 55 4090.8 5.22 2.02 89 6268.0 8.07 3.13 56 4017.8 5.12 1.99 90 6198.4 7.98 3.10 57 3947.3 5.03 1.95 91 6130.3 7.89 3.06 58 3879.2 4.95 1.92 92 6063.6 7.81 3.03 59 3813.5 4.86 1.89 93 5998.4 7.72 3.00 60 3749.9 4.78 1.86 94 5934.6 7.64 2.97 61 3688.4 4.70 1.83 95 5872.2 7.56 2.93 62 3628.9 4.63 1.80 96 5811.0 7.48 2.90 63 3571.3 4.55 1.77 97 5751.1 7.41 2.87 64 3515.5 4.48 1.74 98 5692.4 7.33 2.85 65 3461.4 4.41 1.71 99 5634.9 7.26 2.82 66 3409.0 4.35 1.69 100 5578.6 7.18 2.79 67 3358.1 4.28 1.66 101 5523.3 7.11 2.76 68 3308.7 4.22 1.64 102 5469.2 7.04 2.73 69 3260.8 4.16 1.61 103 5416.1 6.97 2.71 70 3214.2 4.10 1.59 104 5364.0 6.91 2.68 71 3168.9 4.04 1.57 105 5312.9 6.84 2.66 72 3124.9 3.98 1.55 106 5262.8 6.78 2.63 73 3082.1 3.93 1.53 107 5213.6 6.71 2.61 74 3040.5 3.88 1.50 108 5165.3 6.65 2.58 75 2999.9 3.82 1.48 109 5117.9 6.59 2.56 110 5071.4 6.53 2.53 111 5025.7 6.47 2.51 112 4980.8 6.41 2.49 113 4936.8 6.36 2.47 114 4893.5 6.30 2.45 115 4850.9 6.25 2.42 116 4809.1 6.19 2.40 117 4768.0 6.14 2.38 118 4727.6 6.09 2.36 119 4687.9 6.04 2.34 120 4648.8 5.99 2.32 121 4610.4 5.94 2.30 122 4572.6 5.89 2.29 123 4535.4 5.84 2.27 124 4498.8 5.79 2.25 125 4462.8 5.75 2.23 126 4427.4 5.70 2.21 127 4392.6 5.66 2.20 128 4358.2 5.61 2.18 129 4324.5 5.57 2.16 130 4291.2 5.53 2.14 131 4258.4 5.48 2.13 132 4226.2 5.44 2.11 133 4194.4 5.40 2.10 134 4163.1 5.36 2.08 135 4132.3 5.32 2.07 136 4101.9 5.28 2.05 137 4071.9 5.24 2.04 138 4042.4 5.21 2.02 139 4013.3 5.17 2.01 140 3984.7 5.13 1.99 141 3956.4 5.09 1.98 142 3928.6 5.06 1.96 143 3901.1 5.02 1.95 144 3874.0 4.99 1.94 145 3847.3 4.95 1.92 146 3820.9 4.92 1.91 147 3794.9 4.89 1.90 148 3769.3 4.85 1.88 149 3744.0 4.82 1.87 150 3719.0 4.79 1.86 151 3694.4 4.76 1.85 152 3670.1 4.73 1.83 153 3646.1 4.70 1.82 154 3622.4 4.66 1.81 155 3599.1 4.63 1.80 156 3576.0 4.60 1.79 157 3553.2 4.58 1.78 158 3530.7 4.55 1.76 159 3508.5 4.52 1.75 160 3486.6 4.49 1.74 161 3464.9 4.46 1.73 162 3443.5 4.43 1.72 163 3422.4 4.41 1.71 164 3401.6 4.38 1.70 165 3380.9 4.35 1.69 166 3360.6 4.33 1.68 167 3340.4 4.30 1.67 168 3320.6 4.28 1.66 169 3300.9 4.25 1.65 170 3281.5 4.23 1.64 171 3262.3 4.20 1.63 172 3243.3 4.18 1.62 173 3224.6 4.15 1.61 174 3206.1 4.13 1.60 175 3187.7 4.10 1.59 176 3169.6 4.08 1.58 177 3151.7 4.06 1.58 178 3134.0 4.04 1.57 179 3116.5 4.01 1.56 180 3099.2 3.99 1.55 181 3082.1 3.97 1.54 182 3065.1 3.95 1.53 183 3048.4 3.93 1.52 184 3031.8 3.90 1.52 185 3015.4 3.88 1.51 186 2999.2 3.86 1.50