In astronomy, the term cophasing or phasing describes the process of controlling the individual segments in a

segmented mirror A segmented mirror is an array of smaller mirrors designed to act as segments of a single large curved mirror. The segments can be either spherical or asymmetric (if they are part of a larger parabolic reflector). They are used as objectives ...

or a telescope so that the segments form a larger composite mirroring surface. Cophasing implies precise, active control of three

degrees of freedom In many scientific fields, the degrees of freedom of a system is the number of parameters of the system that may vary independently. For example, a point in the plane has two degrees of freedom for translation: its two coordinates; a non-infinite ...

of each individual segment mirror: translation along the

optical axis An optical axis is an imaginary line that passes through the geometrical center of an optical system such as a camera lens, microscope or telescopic sight. Lens elements often have rotational symmetry about the axis. The optical axis defines ...

(piston) and rotation about two axes perpendicular to the optical axis (tip-tilt). Each segment of the segmented

telescope A telescope is a device used to observe distant objects by their emission, Absorption (electromagnetic radiation), absorption, or Reflection (physics), reflection of electromagnetic radiation. Originally, it was an optical instrument using len ...

is a solid body having 6 degrees of freedom exposed to the gravitation force, wind blowing, and other mechanical forces. If the position of each segment is not controlled the resolution of the whole telescope will be the same as if telescope had the diameter equal to the size of one segment. To achieve a resolution commensurable with that of a monolithic telescope of the same diameter the segmented surface must be controlled with a precision better than

\lambda/40

surface rms. Projects for future

extremely large telescope The Extremely Large Telescope (ELT) is an astronomical observatory under construction. When completed, it will be the world's largest optical and near-infrared extremely large telescope. Part of the European Southern Observatory (ESO) agency, ...

s (ELTs) generally depend on the use of a segmented primary mirror. While the basic technologies required for segmented telescopes have been demonstrated for the 10m

Keck telescope The W. M. Keck Observatory is an astronomical observatory with two telescopes at an elevation of 4,145 meters (13,600 ft) near the summit of Mauna Kea in the U.S. state of Hawaii. Both telescopes have aperture primary mirrors, and, when c ...

or GTC telescope, ELTs of diameters form 50 to 100 m represent a qualitative change with respect to wave front control related to segmentation in comparison with the current 10 meters technology.

References

{{reflist, refs= {{cite journal , title=Fizeau interferometric cophasing of segmented mirrors , last1=Cheetham , first1=Anthony C. , last2=Tuthill , first2=Peter G. , last3=Sivaramakrishnan , first3=Anand , last4=Lloyd , first4=James P. , display-authors=1 , journal=Optics Express , volume=20 , issue=28 , date=December 2012 , pages=29457–29471 , doi=10.1364/OE.20.029457 , pmid=23388772 , bibcode=2012OExpr..2029457C , doi-access=free Telescopes