SpectroCam's optical path is shown in Figure 1.1. The basic design is that of a Littrow spectrograph, in which the collimated beam incident on the diffraction grating is nearly parallel to the reflected beam. The instrument is mounted at the f/70 Cassegrain focus of the 200-inch telescope. Two flat mirrors, one outside and one inside the dewar, redirect the incoming beam to two spherical mirrors, the first of which images the telescope's secondary mirror on to the second. The second mirror then converts the f/70 beam to the desired f-ratio of 12.3, and acts as a Lyot stop to prevent light from outside the secondary from proceeding farther down the optical path. The f/12.3 beam is focused onto a slit wheel, then passes to an off axis paraboloidal mirror where it is collimated. The same parabloid focusses light returning from the grating wheel onto the detector. Just before reaching the detector the beam passes through a 10-position filter wheel.
A grating carousel carries two diffraction gratings for the low and high resolution spectrograph modes, and an optical flat for camera mode. The carousel is driven by a special Grating Scan Mechanism which can switch between the three optical elements in <= 5 seconds and position them to an accuracy of a few arcseconds.
SpectroCam-10 was designed to be a long-slit spectrograph first and a camera second, and the desire to include multiple modes as well as keep the optical design fairly simple and the dewar size to a minimum led to a number of compromises. The most important is that the unvignetted angular field of view is limited to about 16'', or 64x64 detector pixels. A larger field would have required a larger dewar window, and larger and more complicated optical elements to correct off-axis aberrations. The slit lengths are also restricted to 16'', but all 128 pixels in the spectral direction are illuminated in both spectrograph modes.