THE UNITED KINGDOM INFRARED TELESCOPE
ANNUAL REPORT
1995 AND 1996

6. UKIRT Upgrades

6.2 Secondary Mirror

A new secondary mirror was delivered with the rest of the top-end systems and is producing good performance with the tip/tilt system. The secondary does have some problems, however. It was manufactured from a zerodur substrate by an innovative technique involving figuring followed by substantial lightweighting by ultrasonic abrasion. This technique had been successfully tested on a flat prototype before the new secondary was commenced. However the new mirror, unlike the flat, does show significant print-through from the lightweighting process. The reason why this occurs with a curved optic but not a flat one is not currently understood. The amplitude of the print-through is not enough to affect the Strehl ratio to a significant degree, but would seriously impede any use of higher-order adaptive optics on the telescope. The secondary also has a turned down edge, of lesser width and amplitude than that of its predecessor but still enough to degrade the ultimate performance of telescope to a very significant degree: the limiting Strehl ratio at 2 microns is probably reduced to 0.7 as a result of this aberration.

A final problem arises from a thermal mismatch between the zerodur mirror and the invar used in its attachment points, which has a slight, but significant, difference in its expansion coefficient. Since the attachment points must be glued on at room temperature, while the telescope is normally used at around 0 C, significant distortion is introduced, manifesting itself as spherical aberration and higher-order (R) trefoil. The spherical aberration can be reduced using the primary mirror control system at the price of most of its dynamic range, but the trefoil cannot. The residual effect again induces a reduction in the limiting Strehl ratio, this time by a factor of 0.8. The attachment system has been redesigned to eliminate this effect. (The apparently simple solution of gluing the attachments at 0 C will not work, as adhesives which set at 0 C are not available.)

In summary, although the telescope is functioning well, the secondary problems do prevent its full performance from being achieved and a new, defect-free, secondary is highly desirable. In fact it is most desirable from an operational standpoint to have a spare secondary as this permits developments and modifications, e.g. application of new high-tech coatings, without requiring substantial down-time. Avenues to obtain a new mirror without the defects discussed above will be actively pursued.