1 hr of roughing grinding of 24 inch mirror

[Chanlunlun]

Dear ATMs,

As cold weather prevails we are not able to carry out grinding of the 24 inch mirror in open area, since cold breeze will certainly hurt.

Consequently we move indoor to carry on with the glass pushing with an objectionable noise of nearly 90 dB due to the #40 carbo . After the first hour of rough grinding, a curvature of about f/24 is generated on the primary mirror. By calculation, we have removed about 14 cubic inch of glass. In order to arrive at the f/3.5 curvature a total of 97 cubic inch of glass has to be removed. Calculation deems to show that we have finished about 14% of work, suggesting an addition of 7 hours of work is required in order to reach the goal. However from my own experience it takes at least 30 hours to get to the required curvature by a full size tool. If we are using a sub-diameter tool with mirror on bottom, we still required longer working time. It doesn』t seem that I』ve made a valid calculation.

The focal length of the mirror is obtained by measuring the central depth, or the sagitta, of the mirror. The measurement of sagitta is conveniently carried out by a piece of straight aluminium angle bar and a vernier caliper. The measurement is accurately since the vernier caliper is a precise tool.

Tomorrow the mirror will receive a continuous of at least 4 hours of rough grinding, by that time we will have sufficient data to tell how much more time we still need to finish the rough grinding stage.

One last thing - It has been said that it's very very tough to push a 24 inch mirror on top a 24 inch tool. I've tried it, it's not difficult at all, it's indeed quite confortable.

Best regards

Chan Yuk Lun
6-2-2004