Large Mirror from Smaller Segments

Making a Large Mirror from Smaller Segments 

The University of California and the California Institute of Technology have adopted this approach for their Keck Observatory 10-m telescope (see Fig. 4), completed in 1992. Russia also has announced plans for a 25-m SMT utilizing a spherical primary to avoid the problems of making aspheric segments. 

Optical combination at a single, final focus of light received at separately mounted telescopes. To maintain coherency between separate light beams, one must equalize the optical path distance (OPD) between the source and the final focus for all telescopes, a difficult condition to meet if telescopes are widely separated. 

Placing the array of telescopes on a common mounting with a means for optically combining the separate light beams. All OPDs can be equal (theoretically), thus requiring only modest error correction to obtain coherency between telescopes. This approach is known as the multiple mirror telescope (MMT). 

Electronic combination after the light has been received by detectors at separate telescopes. Image properties will be those due to the separate telescopes, and coherent combining is not presently possible. Strictly speaking. 

---