The Cameras

Historically, cameras for astronomical spectrographs have needed faster f-ratios as telescope size has increased. This simply reflects that the desired slit width in arc-sec and physical pixel size remained relatively constant. LBT and MODS represent a significant departure from this trend in two ways. LBT (and many of the current generation of telescopes) will produce seeing-limited images that are significantly smaller than previous images. The basic seeing limited-slit width for MODS is 0.6 arc-sec. The MODS cameras have a large focal plane, 120^43 mm, and will be outfitted with detectors with high pixel count, 4096x4096 initially and 8192x2880 when our custom detectors are ready. This allows us to bin pixels for low resolution work in average conditions and still operate at higher resolutions, both spatial and spectral, when conditions warrant. In addition, since LBT is an adaptive optics telescope, MODS has been designed to take advantage of modes such as ground-layer correction by providing scales as fine as 0.12 arc-sec/pixel. The combination allows us to use a relatively slow f/3 camera. The decentered Schmidt design has the advantages of no vignetting and ghosts from the detector are not returned to the grating. The computer model of the MODS blue cameras is shown in Figure 2. Figure 3 shows a picture of the one of the cameras, without optics, on its handling cart. The support structure of four spectrograph cameras [5] consists of a center bulkhead, which mounts to the spectrograph structure, and cells for the primary mirror and corrector which are attached to the center bulkhead with a thermally compensated truss system. The center bulkhead holds the filter wheel and the shutter.

Figure 2. The computer model of the MODS blue cameras.
Figure 3. A picture of the one of the cameras, without optics, on its handling cart.

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