The making of 30 inch telescope - 5, twelve hrs of roughing

[Chanlunlun]

Dear ATMs,

After 12 hours of rough grinding we arrive at f/10.9, or at a focal length of 328 inches. We are still very far away from reaching the target (f/3.3 !). Rough grinding is definitely the toughest part of the whole giant mirror making process. That』s why people always try to escape by using grinding wheels, grinding disc, milling machine ….etc to replacing the process of hand grinding.

The initial design of the 30 inch telescope has just emerged. In order to save material and weight we use as little material as possible. As you can see from the attached diagram the whole telescope has no mounting. A pair of rocker arms support the weight of the whole telescope and at the base a lazy-Suzzie will be used as the turning bearing in alzimuth.

The zenith eyepiece height, being at 103 inches, however, is a little bit too much on the long side. A 3 feet laddeer is required for viewing at high elevation angle.

At the moment this is the simplest design that come up onto my mind. If you have anything better that can make even simpler structure please let me know.

Best regards
Chan Yuk Lun
25-3-2005

[Subaru]

陳sir畫d圖好掂!

[Al]

I grinded a small mirror a few years ago, and have always wanted to make a big one (about 12 inches). However, time and students are what I don't have! It sounds like the use of grinding disc and milling machines can speed up the process. But by how much? What are the drawbacks?

Looking forward to the completion of this exciting project!!!

[Chanlunlun]

I grinded a small mirror a few years ago, and have always wanted to make a big one (about 12 inches). However, time and students are what I don't have! It sounds like the use of grinding disc and milling machines can speed up the process. But by how much? What are the drawbacks?

Looking forward to the completion of this exciting project!!!

Dear Mr. A1,

If you are talking about a 12 inch mirror my suggestion is to grind it by hand. You probably finish the rough grinding within, say, less than 10 labour hours.

Talking about making big mirrors ( 20 inches up ) it seems that a grinding machine is a necessity. Judging from concerning websites and articles it seems that more and more ATMs make their giant mirrors by hand-pushing. The most distinctive one is Bekken who made his 41.2 inch mirror merely by hand ( he has a team, of course, another example of human fresh grinding machine ). Accidentially his telesocpe appears to be biggest amateur telescope in the world at the moment.

While considering alternatives way of rough grinding please bear in mind that no matter which method you are trying to use it is inevitable to build a machine of some sort in the first place. You have to make your own judgement then - the buildiong of such a machine is a time and money consuming process. On the other hand is it better off investing the same time ( or even less ) in hand pushing the mirror, without leaving behind a useless and bulky machine ( unless you want to make another big mirror ) occupying valuable space at home ?

If you arer making a 12 incher, I strongly suggest you to hand push the glass blank. I guess that the roughing stage should last no longer than ten hours. There is no reason to try any alternative way, risking all the dangers.

Best regards
Chan Yuk Lun