終於可以理番塊 gregorian 主鏡.

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豆豸苗
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文章 豆豸苗 » 週二 08 7月, 2003 12:28

鬼 寫:我都好想睇下你塊gregorian 主鏡磨成點呀?
係唔係上次好靚果塊呀?
拋光曬, 但係之前受過污染有少少花痕. 個面有少少低邊咁0羅. :?

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sodium
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文章 sodium » 週三 09 7月, 2003 09:54

gregorian:正image

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文章 訪客 » 週一 21 7月, 2003 00:16

Hi all,

One of the unfavourable characteristic that people argue is that the tube is longer.

Anyway, I think it is not a critical point. I still think that it is very strange not many people (amateur and factory) build gregorian. Concave mirror should be easy to make (we can independently test the secondary but for Cassigrain we have to put the two mirrors together for a test). This is an advantage, and this advantage dwarfs the above-mentioned disadvantage.

By the way, I still remember that Gregorian has another important characteristics: focus forms in between the two mirrors, therefore a mask can be placed at this point. Coronagraphy 日冕儀 make use of this design.

Regards,
啟明 David
:wink:

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文章 訪客 » 週一 21 7月, 2003 00:35

From the book: Telescope Optics

Cassegrain: Typical primary mirror (paraboloid), interchangeable between a Newtonian and Cassegrain. Favourable for most amateur.

Dall-Kirkham: spherical secondary easier to make. Ellipsoid primary is not difficult to make. In general, simple to construct. Far stronger coma than classical Cassegrain thus narrow field.

Ritchey-Chretien: Both primary and secondary are hyperboloid: almost no coma but with astigmatism and field curvature. For wide-field use. Mainly used on professional telescope.

Pressmann-Camichel: not a well-known variant of Cassegrain. Spherical primary with oblate ellipsoid. Very narrow field because of severe coma. (The book shows a very severe coma). My opinion, however, on this design is that it may be used by some very large telescopes. Why? I have heard that someone want to build a very large but very cheap telescope, by manufacturing hundreds of small (up to 2m diameter) mirrors. To make the mirror at low cost, the simpliest method is to make identical spherical mirror, to form the primary.

Regards,
David 啟明 :wink:

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豆豸苗
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文章 豆豸苗 » 週一 21 7月, 2003 00:37

Anonymous 寫:By the way, I still remember that Gregorian has another important characteristics: focus forms in between the two mirrors, therefore a mask can be placed at this point. Coronagraphy 日冕儀 make use of this design.

Regards,
啟明 David
:wink:
Hi hi~~ I know there is a focus forms in between the two mirrors. However, I am not quite understand on usage of the "mask" you have mentioned here tim~ :? :? :?
Would you mind to explain it in more details? :oops: :oops: :oops:
Thanks Thanks~~

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Wah!
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文章 Wah! » 週一 21 7月, 2003 00:48

係gregorian主鏡的第一個成像,放一個圓形,可以剛好把太陽的光擋去,只留下太陽外邊的氣體光線繼續射到副鏡,所以可以專門觀測太陽的日冕.

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豆豸苗
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文章 豆豸苗 » 週一 21 7月, 2003 00:51

Wah! 寫:係gregorian主鏡的第一個成像,放一個圓形,可以剛好把太陽的光擋去,只留下太陽外邊的氣體光線繼續射到副鏡,所以可以專門觀測太陽的日冕.
係??? 咁神奇? 咁咪即係人做日食?? :roll: :roll: :roll:

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Wah!
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文章 Wah! » 週一 21 7月, 2003 00:54

係呀,不過一定要有一個追得非常準確的赤道儀追蹤(仲要係"太陽時"的方式追!),否則太陽一走位,粒CCD就會馬上報消!(呢種望遠鏡絕對唔可以用眼睇架!)
阻住太陽光球/色球的圓形,散熱都好講學問架!,同埋佢d光的反射方向都好考究.

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豆豸苗
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文章 豆豸苗 » 週一 21 7月, 2003 00:57

Wah! 寫:係gregorian主鏡的第一個成像,放一個圓形,可以剛好把太陽的光擋去,只留下太陽外邊的氣體光線繼續射到副鏡,所以可以專門觀測太陽的日冕.
我想問, 咁一般望遠鏡做唔做倒呢樣野?

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Wah!
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文章 Wah! » 週一 21 7月, 2003 00:58

應該可以,但係首要係解決散熱/光同埋追蹤的問題.

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文章 訪客 » 週一 21 7月, 2003 22:14

Wah! You are correct. That is why the mask is called the "energy ejection filter".

It is a metalic cone (so that light is reflected at the other direction) with a cooling system. I do not know why others type are not use. I need to browse more books to confirm this. But to put such a complex structure (the cone and the coolant) at the other end of the tube is not justisfied. I believe modern telescope will not use this design.

Guiding, on the other hand, should not be a serious problem, given that you are not too aggressive :P to look at the innermost corona.

Regards,
David 啟明

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Wah!
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文章 Wah! » 週一 21 7月, 2003 22:25

am..個mount一停電就......

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文章 訪客 » 週一 21 7月, 2003 22:26

One more thing that I missed. It is not possible for us, the Hong Kong amateur to build such a telescope to observe the corona. It is because we are living at sea-level. The atmospheric condition does not permit us to do so. Go to a high mountain then you will have the chance.

I remembered many many years ago, on the cover of "Tenmon Guide", there was a picture of the prominence. The photographer used a remote building as the "natural" mask to cover all of the Sun, except the prominence. This is a very dangerous way to take photo, however.

Regards,
啟明

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Wah!
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文章 Wah! » 週一 21 7月, 2003 22:34

我有想過做呢種鏡架...不過都只係想想而已......
我最後決定做一個spectrohelioscope(中文唔知點叫),而家磨緊其中兩片凹鏡.

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Wongsir
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文章 Wongsir » 週一 21 7月, 2003 22:34

Anonymous 寫:One more thing that I missed. It is not possible for us, the Hong Kong amateur to build such a telescope to observe the corona. It is because we are living at sea-level. The atmospheric condition does not permit us to do so. Go to a high mountain then you will have the chance.

I remembered many many years ago, on the cover of "Tenmon Guide", there was a picture of the prominence. The photographer used a remote building as the "natural" mask to cover all of the Sun, except the prominence. This is a very dangerous way to take photo, however.

Regards,
啟明
啟明,

Thanks telling us a lot about Gregorian and other telescopes.
Thanks again for sharing your knowledge with us.

Wongsir
21-7-2003

:) :)

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