Segments of the Keck Telescope

With its two units of 10 m aperture, the Keck Telescope is historically the first built segmented telescope which surpassed in size the large monolithic telescopes. Its optical design is a Ritchey-Chretien form; the conic constant k = — 1.00379 of the primary mirror slightly deviates from a paraboloid. A set of 36 segments of aperture 1.8 m and thickness 75 mm, are sub-mirrors of the f/1.75 primary mirror. Lubliner and Nelson [42] carried out the analysis for determining the off-axis shape and elastic deformation of the circular meniscus segments.

The stress polishing - preferably called stress figuring , since operated first at a fine grinding stage - was applied by Nelson et al. [50] and Mast and Nelson [43] for the aspherization of the segments. A set of 24 Invar blocks bonded along the perimeter were used for the clamping of radial arms which generated the stressing. Axial forces Fa,k and Fc,k were applied on each arm end via accurate lever systems. These forces are derived from the bending moments Mr and net shearing forces V (cf. Sect. 7.3).

The active optics figuring method provides extremely smooth surfaces as shown by the results of the interferomeric tests (Fig. 7.8).

Because of slight deviations from the theoretical shape, due to figuring residuals and to the cutting of the meniscuses into hexagons, final figuring retouches were

performed by developing the ion beam technique [1, 2, 68]. Another alternative by active supporting of the segments has been investigated [9] but not used. Finally, the manufacturing steps were:

^ Convex Side Polish ^ Stress Figuring ^ Cutting Circular to Hexagonal ^ Boring for Lateral Support ^ Passive Support Mounting ^ Ion Beam Finishing

A detailed summary on the construction of the Keck segments is given by Wilson [69].

7.6 Vase and Meniscus MDMs for Reflective Schmidts 7.6.1 Centered Systems with a Circular Vase-Form Primary

For all-reflective Schmidts, the study of MDMs allows determining the geometry of vase mirrors for obtaining the shape of the primary mirror by active deformation. In the case of a centered system used off-axis, we will demonstrate that the use of radial arms can be avoided by a convenient choice of the rigidity ratio y=D1 /D2 = (t1 /t2)3 between clear aperture and outer ring. In the 3rd-order approximation, using the dimensionless radius p = r/rm with rm as clear aperture radius, the shape of the primary mirror is represented by [see (5.20b)]

where Q = f/d = R/4rm, R is the radius of curvature of the concave secondary mirror and i the incidence angle of the principal ray at the primary mirror (the null powered zone is outside rm at r = \J3/2 rm). With the denotation of Sect. 7.2 in this chapter, let us define Anm mirror coefficients from

This leads to

29 Q3

rm cos I

and A40

0 0

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