Telescope Tubes and Instrument Considerations

FIGURE 9 Serrurier truss used to maintain primary-to-secondary alignment as the tube rotates. Equal deflections and parallelogram action at both ends keep the optics parallel and equidistant from the original optical axis. Similar flexure is designed Into most large telescope tube structures. 

One can reach a constant-gravity focus (instrument location) on an alt-az telescope by simply diverting the beam along the altitude cross-axis to the mounting structure that supports the tube. This is called the Nasmyth focus after its Scottish inventor. The instrument rides the mounting as it rotates in azimuth but does not experience a change in gravity direction. 

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