The 84 year old Hooker telescope at the Mount Wilson Observatory, originally used by Hubble to measure the expansion of the universe, is ready for the 2001 observing season. Over the last week, the telescope's 100-inch primary mirror got its yearly "face lift", where its dirty old aluminum coating was removed and replaced by the evaporation of a new coat of aluminum onto its surface. The mirror was originally cast at the St. Gobain Glass Works in France and shipped to California in 1905 where it was figured and brought up the mountain in 1917. This was the first large astronomical telescope mirror to be aluminized (1935) and prior to this time had to be silvered every few months to maintain its reflectivity. On being rolled out of the coating lab the mirror was hoisted about 50 feet straight up to be replaced in the telescope. The opticians for the 100-inch are, from left to right, Sean Hoss, Observatory Superintendent and on the right, Scott Teare, New Mexico Tech. (Photo courtesy of Larry Evans.)

1.0 Introduction

The large telescopes at the Mount Wilson Observatory are recoated every year in order to optimize their performance. It is necessary to keep the coatings in good shape as these telescopes are about the same size as the secondary mirrors on the world's largest telescopes. There are two main considerations for this: 1) it has been observed that the coatings on these mirrors degrade in a little over 1 year; 2) there are 3 mirrors used to get light to the focal surface so the light loss due to the telescope alone is given by (1-pR_i), where p indicates the product of each of the three surface reflectivities, R_i. As a result, the telescope losses alone for a 90% reflective coating is about 27% (conversely it transmmits about 73%). In the simplest observations, the growth of pinholes over in the tertiary mirror alone a year of use of the telescope reduces the reflective surface to about 60% of the available area. In this case if there are no other degradations (and unfortunately there are) the losses become (1 - 0.9*0.9*0.54) = 54% or 46% throughput. While this is only about 0.7 magnitude loss of light, this represents a considerable loss of light for adaptive optics systems and particlarly laser guide star systems where 6 reflections of the telescope are used so this becomes a about 20% throughput.

It is suspected that this degradation is due to the a combination of the poor air quality above Los Angeles, chemical damage from pollen (the telescopes are in a well treed national forest) combined with the coating operations being less than ideal. We have limited control over the first to and are actively working to improve the coating operations. We have recently introduced an additional cold trap (completed in 1999), upgraded the thermal coating power systems (completed in 2000) and are introducing more detailed procedures. In 2001 we are working to minimizing dust contamination and controlling the air quality during the cleaning and surface preparation process through to the application of the coating.

The aluminization process and equipment are not complex, however there are a number of features to coordinate and many standards that need to be met in order to obtain high reflectivity surfaces. The purpose of this document is to streamline the operations and optical work in aluminizing large telescope mirrors.

The aluminization process has been broken down into its basic components as listed in Table 1: