The making of 30 in telescope - 16, 4 hrs of polishing

夸克星 註冊時間: 2003-10-09 文章: 2207

Dear ATMs,

In this forum I have been frequently asked about the plywood-paper convex tool used for polishing the 30 inch mirror. I would like to share some of our view-points for this question.

In the first place I would like to express the fact that making a large mirror is a lot different from making a small one. On polishing, say, a 5 inch f/6 mirror, the paraboloid is only 1/2 wave away from a sphere. In other word if you obtained a sphere upon finishing polishing you are very close to obtain a paraboloid. Hence in making small mirror ( up to 12 inch aperture ) we always try our best to secure a spherical surface as far as possible. Thus the pitch lap must be 100% safe-proof. Any imperfection of the pitch lap will not be tolerated, since it will jeopardize the final spherical surface obtained.

However in our case of a 30 inch mirror it has a very fast f/3 ratio. The extremely large aperture coupled with the extremely fast f/ratio implies the fact that the paraboloid will be very very far away from a sphere. Thus in planning the polishing stage we must not aim at making a sphere. Indeed right at the very first moment of polishing we already aimed at making the required paraboloid. We used the same strategy in making the 24 inch, and it had been proved to be a success. The making of a spherical surface is, oddly, meaningless in the case of polising a giant mirror.

Thus we deepen the mirror center while at the same time deliberately produce a turn-down edge at the very first moment of polishing, only to aim at producing the required paraboloid as fast as possible. As told by the experiences of the making of the 24 inch mirror, even with these extreme routines we will discover that the mirror is still too shallow to become a paraboloid after polishing.
Actually the figuring will end up using small laps ( from 4.5 inches to 10 inches diameter ) with local touching on mirror surface.

A plywood-paper convex tool will certainly distort, but will not affect our plan since we no longer need a perfect sphere. As it turns out to be, the lap is quite efficient in doing the polishing job. The extra soft pitch also conforms to the mirror profile only a few minutes after polishing. So we get extra-bonus of no need to press the pitch lap every time when starting the polishing procedure. Soft pitch always produces surface with turned down edge and this is exactly what we need. We only afraid that the degree of turn down edge is not sufficient for the paraboloid.

After 4 hours of polishing the smoothness of the mirror surface has vastly improved – now every part of the mirror surface is able to reflect light at normal incidence. I guess that with a few more hours of working the polishing will be finished.

It』s startle to see how quickly the polishing can be, even with a 30 inch giant mirror.

Best regards
Chan Yuk Lun
9-5-2005