USING WIDE GRATING IN THE SPECTROGRAPH ONLY HAVING ONE CAMERA TO PROVIDE LARGE DISPERSION WITH HIGH LIGHT EFFICIENCY

In recent high efficiency Cassegrain Spectrographs it has been proposed that the light should be fed into two independent channels, the separate red and blue channels are obtained by using a diehroic mirror (OKE, J. B. et al. 1982). Both channels will comprise a collimator, several interehangeable gratings, a camera and a light detector.Because of the limited space at the Cassegrain focus, each channel can have only one camera with similar focal length. The problem of providing a wide range of dispersions, as is required for astronomical foci, is usually solved by utilising several inter-changeable gratings with different line spacings and different angles of incidence α. To minimize light loss caused at the grating, the angle of the diffraction has to be made smaller than the angle of incidence and the physical constraints required at the instrument as a fixed angle between the camera and the collimator axis. Hence, for a high dispersion system having a large angle of incidence a very wide grating and a very large camera aperture are required and the conditions are very difficult to meet. C. G. Wynne and S. P. Worswick (1983) suggest the addition of an anamorphotic lens system before the slit. These lenses reduce the collimated beam size before the grating in the dispersion direction only, which is a very good solution to the problem. There is however another Solution to the same problem using a super wide grating as suggested by Jiang Shi-Yang in 1979. For example, it.is possible to use a grating with height of 15.4 cm/N and width of 20.6 cm and use a collimator of diameter 15 cm but of focal length only 1/N of that usually used. Then, for high dispersion with large angle of incidence it is not necessary to add any extra lenses but for the low dispersion case with a small angle of incidence one can add similar anamorphotie lenses either behind or in front of the slit to widen the collimated beam size in the direction of dispersions only. By this means it is. possible to ensure that the beam will fill the complete width of the grating. The advantages of this solution are (ⅰ) unlike Wynne's method, the anamorphotie lense is only needed for low dispersion and low resolutions so a small amount of aberration and light loss is not unacceptable. (ⅱ) The focal length of the collimator is fairly short and it does not require the us of an inverse Cassegrain or the addition of a folding mirror. (ⅲ) This type of grating is easier to manufacture and should have better quality and lower price. (ⅳ) The size of the collimated beam in the direction parallel with the slit is decreased to 1/N of its normal value. Hence it may be possible to move the focal plane totally out of the entrance light beam by using an off axis Schmidt camera. For example, if N=3 and the grating has size 154×206 mm squared, the size of the collimated beam in the direction parallel to the slit is only 50 mm. For a camera with 450 mm focal length, and with the in edge of the entrance beam being 22 mm outside the axis of the camera, the smallest monochromatic focal ratio is still larger than f/2. This means that we can use any electronic detector with about 40 mm diameter without any obscuration.