The making of 30 in telescope - 27, mirror nearly finished.

夸克星 註冊時間: 2003-10-09 文章: 2207

Dear ATMs,

After many many hours ( we haven』t counted yet, but many ) of figuring the mirror finally approaches to a very nearly perfect parabola. What is left, after the smoothing action, is a 2 inch width of edge zones suffering from both a raise and turned down edge, which will present no real problem.

In the Offner tester we can feel the flat surface covering a total diameter of 26 inch, with a zone 2 inches width surrounding. We have been fighting through a very long way.

The figuring lasts much longer than that of the 24 inch mirror mainly because it has a much larger surface area ( 56% more ) and more importantly, it has a f/ratio of 3 instead of 3.5. The later factor makes the mirror more difficult to figure into a parabola, especially we started up with an Oblate Spheroid.

Now the journey is very near to its end point. We have to think about the preparation of silvering process, another problem associated with such a big mirror.

Best regards
Chan Yuk Lun
19-9-2005

夸克星 註冊時間: 2003-07-02 文章: 5234

快啦快啦, 就快有0.76米鏡睇.

紅巨星 註冊時間: 2003-06-28 文章: 464

全港最大望遠鏡誕生！！

星際雲 註冊時間: 2004-05-09 文章: 47

congratulation!!!
The thinnest big mirror.
The world record is going to be broken down.

A small question: How long will a silver cover last?

夸克星 註冊時間: 2003-10-09 文章: 2207

Subaru 寫到:

快啦快啦, 就快有0.76米鏡睇.

Dear Mr. Subaru,

We have walked throught the 99th steps of the 100 steps journey. However the last step may take a longer time, as things always happen like this.

Even though we have finished the optical work there are still much left to be done :-

1. Silvering of mirror - in this case we have to deposite a lot of silver over a large area. Huge amount of silver nitrate.

2. Mechanical - a rough calcuolation shows that in order to make use of this big telesciope in advantage a power of 200 X is the best. At this magnification a simple Dobsonian design is inadequate. We need to make somne form of tracking for this telescope.

3. Diagonal mirror - sooner or later we have to grind the flat mirror to replace the small 4 inch diagonal. Our goal is a diagonal with 6.5 inch minor axis.

4. Cassegrain secondary mirror - if f/3 is too fast to create excessive coma ( which I don't think so ) a Cassegrain secondary convex mirror will be grinded to change the whole layout into a Cassegrain design. There are two advantages - first teh coma will be vasstly reduced and zenith eyepiece height can also be lowered into a convenient position, getting rid of carrying a ladder to site.

So the journey is far from finished. Indeed the advanture has just barely begun.

Best regards
Chan Yuk Lun

白矮星 註冊時間: 2005-09-11 文章: 686

連埋OTA﹑座﹐看天頂星時候﹐真係要擔梯。應該把這個香港盛事告知天下。

超新星 註冊時間: 2005-09-10 文章: 6

Can someone tell me what is the second large telescope in HK?
Is it also hand made by Chan Yuk Lun? Thank

白矮星 註冊時間: 2005-09-11 文章: 686

I think even on a global level 30" hand grind primary mirror is a remarkable achievement.... what is the biggest globally ?

夸克星 註冊時間: 2003-06-28 文章: 9061

全人手磨既大鏡應該唔多, 可能最大都係41"吧??? Rolling Eyes

夸克星 註冊時間: 2003-10-09 文章: 2207

plotter 寫到:

Can someone tell me what is the second large telescope in HK?
Is it also hand made by Chan Yuk Lun? Thank

Dear Mr. Plotter,

The second largest telescope in Hong kong is a 25 inch Newtonian. However its primary mirror was bought from USA instead being a home made one.

The second largest telescope with a home made mirror is a 24 inch Newtonian, also made by me and my students.

Best regards
Chan Yuk Lun

夸克星 註冊時間: 2003-10-09 文章: 2207

小山寫到:

congratulation!!!
The thinnest big mirror.
The world record is going to be broken down.

A small question: How long will a silver cover last?

Dear Mr. Tiny Hill,

The life time of silver depends very much on how you use your telescope and the structure of the OTA.

In order to fully evaluate the life of silver film I had coated 4 mirrors and set them under different environments.

Mirror 1 :- 10 inch f/8.4 parabola. This mirror was coated 18 months ago and it has never been put into use. It rest on the store room on my home, dry air, face down on a wooden plate. after 18 months the silver tarnished a little, but still in quite good condition.

Mirror 2 :- 4 inch f/6.8 parabola, ( LMCC telescope ). The mirror is coated 14 months ago and is situated at the bottom of a conventional aluminium closed Newtonian telescoep tube. This telesocpe has been kept busy, both in my home and in Water Mouth, for visual observation and for prolonged lunar mosaic photography, for a whole year. When not in use the tube was carefully closed by the supplied plastic tube cover and put inside a specially made silk velvet pocket. The mirror is still under excellent condition even nowaday, no trace of silver tarnishing occur so far.

Mirror 3 :- 4.4 inch f/6.2 parabola (the Ugly telescope ), silver coated 14 months ago. . The whole telescope, with the equatorial mount, is just set up in my study room at home. Again the mirror is situated in a closed aluminium conventional Newtonian telescope tube. When not in use it is closed with a plastic tube cover and nothing else is done. The reflectivity of the silver seems to remian unchnage, but turned a little yellowish. the condition is not as good as the LMCC telescope.

Mirror 4 :- 24 inch f/3.4 parabola, coated 15 months ago. This telescope has been under very busy usage in Water Mouth for the past year. The mirror is situated at the bottom of an open truss telescope tube. The mirror, together with its silver coating, had been attacked by rain water twice. When not in use the whole mirror, together with its mirror cell , is covered by a wooden box ( not air tight ). The silver coating is severely tarnished and needs recoating.

So we may draw the conclusion that in order to fully protect the silver coating a closed telescope tube is a must. Furthermore, from my experiment, it seems that a silk velvet pocket which forms an outer skin for the the mirror closure provides a very good extra protection. I belief that silk has a very good filtration effect to cut off the Sulphur contaminant in air.

I will take these consideration in the designing of the 30 inch telescope tube.

Best regards
Chan Yuk Lun