The Mods Design

It is said that in optical design anyone can add surfaces, the trick is to take them out while still getting the job done. Minimizing the number of surfaces reduces cost and increases throughput. We have adopted a very simple, traditional design for astronomical grating spectrographs: a

decentered paraboloid collimator and a Schmidt camera. A removable dichroic beam splitter located just behind the slit directs the incoming light to separate red ( X > 500 nm) and a blue ( X < 500 nm) channels. The red and blue optimized collimators produce a 230 mm diameter beam for the red and blue four-position grating turrets. Dispersed light travels from the gratings to red and blue cameras that, while similar in design, use different materials, coatings and prescriptions. MODS includes full multi-slit capability over the arc-min field with a 25 position mask interchange mechanism. Also included are flat field and wavelength calibration systems and a guide and wavefront-sensing camera, which provides the signal to the LBT active optics system.

The opto-mechanical modules are supported on a welded steel space frame. While the frame is designed to have low hysteresis, displacement of the spectra on the detectors due to the variable gravity vector and thermal gradients would be unacceptable. An image motion compensation system, described by Marshall [4], is included to reduce this flexure to an acceptable level. An IR reference beam is launched from the telescope focal plane and passes through all the optics of the spectrograph with the exception that strikes a small "by-pass" grating mounted in a hole in the main gratings. A germanium quad cell in the detector plane generates an error signal which steers the collimators to maintain the spectra in a fixed location on the detectors.

Figure 1. A computer model of the structure and the 10 sub-assemblies of MODs.

A total of 30 stepper motors are used to position the optics. All motors are driven by controllers from Micro-Lynx. Mechanism positions are sensed with proximity switches. Linear motions are accomplished with ball screw slides that include spring-applied/electrically-released breaks. A computer model of the structure and the 10 sub-assemblies is shown in Figure 1.

All mechanisms have been fabricated and are now in testing. The collimator, camera mirrors and small optics are complete. The camera correctors are in fabrication. On site commissioning will begin in the 3rd Q 2006.

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