DIY 7 inch Cassegrain Telescope - 1, Designing

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Chanlunlun
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文章: 3844
註冊時間: 週四 09 10月, 2003 21:06

DIY 7 inch Cassegrain Telescope - 1, Designing

文章 Chanlunlun » 週四 01 3月, 2012 15:05

Dear ATMs,

If atming is one of your favorite hobbies you sure have a well collection of telescopic items as time goes on. For me the collection includes eyepieces, worm gears of different sizes, small lenses, optical glass and in particular, a 7 inch primary with a 2 inch hole perforated at center. This mirror was intended to be a Cassegrain primary long time ago. For one reason or the other this Cassegrain never came into reality. I belief it’s time to continue my dream which once started more than 30 years ago.

Here’s the spec :-

Aperture :- 7 inches
Secondary diameter ( with holder ) :- 2.3 inches
Final focal length :- 126 inches, at f/18.

The secondary of Cassegrain is a hyperbolic convex mirror which is particular difficult to make. I don’t intend to make one. Instead I try to use the back side of a 50mm binocular objective to be the secondary.

A special spherometer was made to measure the radius of curvature of the backside of the 50mm binocular objective – it is at 441.2 mm, a very appropriate value for a Cassegrain secondary. In this I am able to avoid to grind and polish such a small mirror.

After determining the radius of curvature of the secondary, the whole thing was designed by ray tracing software ( OSLO light ). The final result rolled out not bad – effective focal length at 3200 mm, if the secondary is left spherical it will be about 1/3 wave departed away from the best hyperbola. This small deviation should present little problem for final figuring of the secondary.

A very accurate hand drawing was then made in order to determine the length and diameter of the baffle tubes. They are important as Cassegrain are notoriously known for its light leakage. An imperfect baffling system will surely destroy the contrast of the final image.

That is it for all the designing work.

This project is just for fun. While SCTs are so cheap and so dominant in the market today, a homemade Cassegrain can hardly gain anything practically beneficial except to enjoy the process of DIYing.

Best regards
Chan Yuk Lun
1 March 2012
附加檔案
1.jpg
Hand drawing to determine the baffling tube diameter and length.
1.jpg (34.98 KiB) 已瀏覽 5479 次
2.jpg
The binocular objective to be used as the Cassegrain secondary. It is in fact the objective lens of a Meade 8 X 50 finder.
2.jpg (61.65 KiB) 已瀏覽 5479 次
3.jpg
Measuring the radius of curvature of the backside of the objective lens by a specially made spherometer.
3.jpg (61.39 KiB) 已瀏覽 5479 次
最後由 Chanlunlun 於 週一 26 3月, 2012 15:15 編輯,總共編輯了 1 次。

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bingsze
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文章 bingsze » 週五 02 3月, 2012 09:29

繼續努力,當七吋成功了,就會有十二吋、二十吋………啦!

到事就看到自製的好處了!祝成功! :D 奸爸爹!

PTS
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文章: 7678
註冊時間: 週日 28 10月, 2007 08:50
來自: HK

文章 PTS » 週五 02 3月, 2012 09:38

bingsze 寫:繼續努力,當七吋成功了,就會有十二吋、二十吋………啦!

到事就看到自製的好處了!祝成功! :D 奸爸爹!
I have a 60-cm f/3 paraboloid mirror, with a 150-mm central hole. Is Mirror of God interested in converting this into a Cass system?

pts

anyone
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文章: 14194
註冊時間: 週六 07 2月, 2009 12:13
來自: hong kong

文章 anyone » 週五 02 3月, 2012 09:57

PTS 寫:
bingsze 寫:繼續努力,當七吋成功了,就會有十二吋、二十吋………啦!

到事就看到自製的好處了!祝成功! :D 奸爸爹!
I have a 60-cm f/3 paraboloid mirror, with a 150-mm central hole. Is Mirror of God interested in converting this into a Cass system?

pts
a 60-cm f/3 paraboloid mirror ? :shock:
a big monster !

Chanlunlun
夸克星
文章: 3844
註冊時間: 週四 09 10月, 2003 21:06

文章 Chanlunlun » 週五 02 3月, 2012 10:08

PTS 寫:
bingsze 寫:繼續努力,當七吋成功了,就會有十二吋、二十吋………啦!

到事就看到自製的好處了!祝成功! :D 奸爸爹!
I have a 60-cm f/3 paraboloid mirror, with a 150-mm central hole. Is Mirror of God interested in converting this into a Cass system?

pts
Dear PTS,

What a giant piece of mirror. We are of course interested.

What is the thickness of your mirror ? We have to calculate whether a 24 inch Cassegrain will overload our Paramount or not .

Best regards
Chan Yuk Lun

PTS
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文章: 7678
註冊時間: 週日 28 10月, 2007 08:50
來自: HK

文章 PTS » 週五 02 3月, 2012 10:12

Chanlunlun 寫:
PTS 寫:
bingsze 寫:繼續努力,當七吋成功了,就會有十二吋、二十吋………啦!

到事就看到自製的好處了!祝成功! :D 奸爸爹!
I have a 60-cm f/3 paraboloid mirror, with a 150-mm central hole. Is Mirror of God interested in converting this into a Cass system?

pts
Dear PTS,

What a giant piece of mirror. We are of course interested.

What is the thickness of your mirror ? We have to calculate whether a 24 inch Cassegrain will overload our Paramount or not .

Best regards
Chan Yuk Lun
It is a light weight one, The back is sort of honey-comb structure. I can pick it up in the wooden crate with one hand.
I am not too sure about its quality. May be just good for spectroscopy work.


pts

Chanlunlun
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文章: 3844
註冊時間: 週四 09 10月, 2003 21:06

文章 Chanlunlun » 週五 02 3月, 2012 13:36

PTS 寫:
Chanlunlun 寫:
PTS 寫:
bingsze 寫:繼續努力,當七吋成功了,就會有十二吋、二十吋………啦!

到事就看到自製的好處了!祝成功! :D 奸爸爹!
I have a 60-cm f/3 paraboloid mirror, with a 150-mm central hole. Is Mirror of God interested in converting this into a Cass system?

pts
Dear PTS,

What a giant piece of mirror. We are of course interested.

What is the thickness of your mirror ? We have to calculate whether a 24 inch Cassegrain will overload our Paramount or not .

Best regards
Chan Yuk Lun
It is a light weight one, The back is sort of honey-comb structure. I can pick it up in the wooden crate with one hand.
I am not too sure about its quality. May be just good for spectroscopy work.


pts
Dear PTS,

It looks promising. Hopefully I can have time to come and take a look on that giant mirror.

Best regards
Chan Yuk Lun

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Steve
中子星
文章: 1257
註冊時間: 週二 15 4月, 2008 22:16
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文章 Steve » 週五 02 3月, 2012 13:57

好快又有巨鏡睇了...
祝一切成功

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Eddy So
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文章: 12903
註冊時間: 週四 04 12月, 2008 22:35
來自: 中男海

文章 Eddy So » 週五 02 3月, 2012 19:57

Steve 寫:好快又有巨鏡睇了...
祝一切成功
x2

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