The making of 30 in telescope - 10, #80 fine grinding over

[Chanlunlun]

Dear ATMs,

If I were asked which part of the mirror making process the most exciting the answer probably is the first fine grinding stage. It is overwhelming to see how a rough and uneven concave surface of a big mirror transforming into an even and perfect sphere.

We had spend 3.8 hrs finishing the #80 fine grinding. Up to now the mirror surface is confirmed to be a perfect sphere.

But one sad news – we measured the focal length again after fine grinding. It reduced to only 91 inches, which means that we have a f/3.1 mirror at the moment. It』s not at all bad since a shorter focal length means smaller zenith eyepiece height, smaller weight and thus more practical of the telescope.

It』s not possible to lengthen the focal length applying TOT method since we do not wish to diminish the mirror thickness. So we just stay with our super fast mirror.

Best regards
Chan Yuk Lun

[Wah!]

似乎磨磨下會撞台角喎!

[sodium]

Dear ATMs,

If I were asked which part of the mirror making process most excited the answer probably is the first grinding stage. It is overwhelming to see how a rough and uneven concave surface of a big mirror transforming into an even and perfect sphere.

We had spend 3.8 hrs finishing the #80 fine grinding. Up to now the mirror surface is confirmed to be a perfect sphere.
Best regards
Chan Yuk Lun

同工具板唔match
係唔係唧係工具板唔係sphere

[Wah!]

所以鏡、板無接觸的地方磨得唔夠有接觸的地方多架!

[Chanlunlun]

似乎磨磨下會撞台角喎!

Dear Mr. Wah!,

No, it won't. Th etable is too small for the mirror surface to touch the corner.

Best regards
Chan Yuk Lun

[Wah!]

個工具板如果最後都拋埋光,就真係可以做個太陽燒烤爐喇!

[Tarepanda]

可以鍍鋁沒?

[elton]

Very interesting news!!!!!! Now the centre thickness is around 9.5 mm thick, still some margin to change the upper/lower position of the workpiece/mirror to increase focal length? Or it is too thin already?
By the way, yesterday I read Mr. Wong's 'blue book' (self make astronomical telescope), it said no. 80 sand means in 1 inches of length can line up 80 granules of sands. Then the no. 40 sand u you used previously must be... SO BIG!!!!!!!!!!!!!!
It is really exciting to see the regular update of the progress of the making of this big big mirror!!!!!!!!!
Add oil! Chan Sir, and little students!

[豆豸苗]

it said no. 80 sand means in 1 inches of length can line up 80 granules of sands. Then the no. 40 sand u you used previously must be... SO BIG!!!!!!!!!!!!!!

40 砂摸落有刮手0既感覺, 之前我用來磨過 12", 好噪的. 不過用在陳 sir 塊大鏡度, 可能磨幾下就碎曬, 自動變曬 80 號砂.
我0係賣砂果度仲見過 20 號砂. 非常大粒.... ^^|||

[Chanlunlun]

同工具板唔match
係唔係唧係工具板唔係sphere

Dear Sodium,

The tool surface is a sphere, but with a plateau at vertex. This small plateau will disappear upon further fine grinding.

Best regards
Chan Yuk Lun