24 inch mirror - 9 hrs of parabolizing

自製天文望遠鏡 牛頓望遠鏡 杜蘇式 卡式 設計和磨鏡方法<br>
佛科試鏡法 佛科刀片儀 光柵試鏡法 光柵片
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註冊時間: 週四 09 10月, 2003 21:06

24 inch mirror - 9 hrs of parabolizing

文章 Chanlunlun » 週三 24 3月, 2004 09:01

Dear ATMs,

After 9 hours of parabolization we finally got half way done. The mirror now has reached 50% of full correction and we are looking forward to finish rough parabolization in the coming 5 or 6 working hours.

At the moment all parabolization work was done with the 14 inch lap, ( this same lap was also used for polishing ) as the 9.5 inch parabolization lap had failed. Our strategy of parabolization is that we deliberately turn down the edge and deepen the center of mirror so that the combined effect is to force the mirror into a paraboloid shape. In this way minimum amount of glass is needed to be removed, which means that maximum labor is saved.

The Offner tester was used for the first time tonight. My first impression was that it vastly distorted the shape of the slit as viewing into the mirror. A terrible oblate spheroid with very exagerated turned up edge was observed. This is normal as when the mirror is corrected back into a flat profile ( null figure ) the figure will automatically be a paraboloid, so to speak.

The style of the Offner tester is terrible, it doesn』t look nice. As from computer simulation for maximum sensitivity the front lens of the tester has to be moved backward 322 mm from the knife edge, which introduce a very strange outlook on the equipment itself. Nevertheless the tester, in practice, was proved to be quite easy and comfortable to be handled with.

We are now quite close to the end of optical work of the 24 inch telescope. Mechanical construction work will be started in due course.

Best regards
Chan Yuk Lun
The Offner tester under use. The best and most convenient way to test such a big mirror is to put everything onto the floor.
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The completed Offner null tester. The silver cylinder on left is the lens barrel which holds the compensation and field lens. The surface of the front lens has to be separated from the knife edge by 322 mm, hence the shape.
Offner_tester2.jpg (57.25 KiB) 已瀏覽 5791 次
The computer ray tracing result of the Offner tester together with the paraboloid under examination. This is an output from the ray tracing program OSLO. It s not possible to accurately test the mirror without simulation from computer.
oslo.gif (15.29 KiB) 已瀏覽 5791 次


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