CFHT Information Bulletin Number 37, Semester 97II

Telescope Upper-End Seating and Measurement Project

Rohendra Atapattu, CFHT (

Our telescope uses an ingeneous mechanism to change upper-ends. It makes centering a new upper-end on the telescope, locking it on, and releasing it from the crane a one-step operation. Over a year ago increasing difficulty during upper-end changes to smoothly release and seat each upper-end began plaguing us. A project was undertaken to evaluate and maintain the locking devices, and also accurately measure each upper-end to determine the as-built dimensions and the effects of early modifications.

Although dismantling the locks on the telescope sounds simple, the 4 locks called "birdsheads" (due to their resemblance of a bird beak), each weigh close to 400 kg and were installed during telescope construction with no provision for overhaul. It required special tooling which was designed and fabricated in-house to remove each birdshead. The summit crew and the mechanics group worked four separate engineering shutdowns to remove, service, replace and adjust each birdshead segment. During the course of the maintenance we discovered a few reasons for the problems we were encountering with the birdshead mechanism. Some years ago different sized guide bolts had been fitted in an attempt to get the system to work properly. This resulted in binding and ultimately in deformation of the guide plates. Interference fits were also discovered between sliding components. The guide plates were replaced and machined for proper clearance and uniformly sized bolts were fitted.

In parallel, the effort to accurately measure the telescope and upper-ends proceeded without interuption to regular observing. To measure the telescope upper-end mount ring a three legged trammel was developed and built by Wiley Knight. During one of the maintenance shutdowns, he suspended himself in trapeze-like fashion and rotated with the dome inside the telescope top ring to measure the defining ring. For measuring the mating surfaces of the upper-ends, Dan Sabin developed and built a fixture that enabled accurately profiling each flange. As a results of these measurements, overlaying all the mating surfaces allowed us to complete the project by setting the upper-end handling ring drive segments such that they mate smoothly with the telescope lock segments and these in turn were adjusted equally to provide uniform clearance for all upper-ends (Figure 15).

CFHT Information Bulletin Number 37, Semester 97II
Copyright © 1997, Canada-France-Hawaii Telescope