Field Lens And

Fig. 4.20 Optical design of LAMOST spectrographs for R = 2,000. This uses aspherized reflective gratings of 200 mm groove length and an aperture ratio of f/1.5 in this direction. A dichroic provides the two spectral ranges XX [370-590nm] and XX [570-900 nm]. The CCD detectors have 2048 x 2048 pixels of 24 x 24 ^m. The mounting angle between the collimator and camera axes is 80 = 35°. The normal diffraction angle, fi ~ 0, allows a design with axisymmetric aspherized gratings (Zhu and Lemaitre [97])

adding central obstruction. For the second-generation spectrographs of the giant Schmidt Lamost (cf. Sect. 4.3.4), the spectral resolution will be set two-times larger, i.e. at R = 2,000 for both blue and red channels (instead of R = 1,000). For such instruments, the throughput will be optimal by use of axisymmetric surface relief gratings working with diffraction angles close to fi0 = 0 (Fig. 4.20 and Table 4.5). As for the previous examples, the aspherization of such reflective gratings provides an achromatic correction of the spherical aberration and then the largest possible spectral range.

Other advantages with aspherized reflective gratings are to avoid the difficult task of making huge aspherical refractive Schmidt plates and then to cancel its two associate ghost images. In addition, one should mention that the two ghost images generated by the glass substrates of volume phase transmission gratings (generally sandwiched between two plates) are also cancelled by use of reflective gratings. The optical performance of quasi-all-reflective designs - only including a field-flattener cryostat-window lens as refractive element - are similar for both blue and red channel (Fig. 4.21).

The output of the optics fiber array - spectrograph slit - is a parabolic line in a X, Z plane. At Xo, the lateral shift of the spectra shows a sag of Ay ~ 0.495 mm over the field; the red channel shows a quite similar lateral sag Ay ~ 0.505 mm for Xo = 735 nm at the detector. This X, Y locus of the Xo spectral elements can be straightened by rearranging the output of the fiber array - spectrograph slit input - in a lateral circle arc in the Y-direction with an opposite lateral sag of ratio fCol/fCam = 4/1.5.

Table 4.5 Optical parameters of LAMOST spectrographs R = 2,000. Blue channel [370-590 nm] - Red channel [570-900 nm]. Unit [mm]

Element

Radius of curvature

Axial separation

Figure

Aperture stop

0 0

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