Keck Primary Mirror Figure Changes from Mirror Support Repairs

Repairs to the W.M. Keck Observatory primary mirror segments are needed to stop the cracking at the bonded supports. The repairs include changes to the design of the axial and radial inserts and the way these are bonded to the mirror backsurface. In this paper, we present finite element analyses of a primary mirror segment including whiffletree and radial mirror support system to determine the effect of the modified supports on the mirror figure. Displacements of the front surface are calculated for a number of operational and assembly fit-up cases, and differential displacements between the old and new support system are found. Zernike coefficients are calculated for front surface displacements and differential displacements. Stresses on the glass surface at the radial pad bond locations are also determined. The results show that the residual deformations of the mirror front surface with the new supports are similar to those with the old supports, differing only in the immediate vicinity of the axial inserts and radial pads, and the impact on the image quality of the Keck telescopes is negligible.

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