Homemade Hyperbolic Astrograph - 1, Optical Layout

自製天文望遠鏡 牛頓望遠鏡 杜蘇式 卡式 設計和磨鏡方法<br>
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Chanlunlun
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Homemade Hyperbolic Astrograph - 1, Optical Layout

文章 Chanlunlun » 週四 17 6月, 2010 12:55

Dear Star Shooters,

Hyperbolic astrograph has been around for some decades but it’s still not very popular. There exists commercial available ones ( the most famous one may be the Takahashi Epsilon ) but the price is unusually high. No wonder the difficulty in manufacturing of the deep hyperboloid primary mirror contributes greatly on the price tag of such system.

Things become interesting recently. The emergence of the MPCC in a very reasonable price has triggered vast discussion about coma-free Newtonian in various forum. However a deeper investigation leads to the tragic conclusion that to eliminate coma, using a MPCC, the price to pay for is to allow under-corrected spherical aberration coming into the system. Most importantly this is unavoidable if we insist on using parabolic primary mirror.

Fortunately this is not entirely the case. If the speed is limited to slower than f/5 the MPCC is quite a piece of marvel. At faster speed spherical aberration becomes so obvious that star images start to get very fat and resolution decreases appreciably. At faster speed, we need to change to shape of the primary into hyperbolic, in order to correct for the S.A. of the MPCC. Thus this leads again to the hyperbolic astrograph design, which is exactly the same as the “Epsilon”.

The experience in Komeko tells that in star shooting speed is a very key factor upon success. The speed of f/2.8 is very desirable to capture both “deep” enough and to finish fast enough before clouds come. So I wonder whether such a system could be built, at home, in an amateur sense.

Attached please find the optical layout of the proposed system. The first thing that may catch attention is the unusually big secondary diagonal mirror. The system is built specially for QHY8 or APS-C DSLR. To allow for such a big field a 50% diagonal is a must. ( Actually it is already a bit too small but design constraint cannot allow a bigger diagonal ).

The primary mirror is a hyperbolic mirror which must be custom made to well match the behavior of the MPCC. How do I know how deep a hyperbola should be made ? I don’t . Being an amateur I plan to work in an amateur way – to figure the primary mirror upon a star until a null figure appears. In this way at least we eliminates spherical aberration. How about coma ? I don’t know, I just hope for the best.

Here is the specification of my homemade “Epsilon” :-

Aperture : 102 mm
Focal length : 350 mm f/3.5
Optical layout :- Hyperbolic Astrograph.
Image circle : 26 mm at about 70% vignetting.
Target camera type : QHY8 or any APS-C CCD or DSLR.

Frankly speaking there is no optical design involve in anyway. I just make a gamble. Being an ATMer why not build an astrograph for fun ?

Best regards
Chan Yuk Lun
17 June 2010.
附加檔案
Optical Layout_small.jpg
My Epsilon
Optical Layout_small.jpg (35.3 KiB) 已瀏覽 10004 次
最後由 Chanlunlun 於 週四 17 6月, 2010 22:16 編輯,總共編輯了 3 次。

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MANDII
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文章 MANDII » 週四 17 6月, 2010 12:57

哇吓! 好似系新镜制作来播 :D
顶顶了,在HK衣家都仲有人会落手做呢样野,难得。

[Good Job]

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bingsze
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文章 bingsze » 週四 17 6月, 2010 13:51

幾時有野睇呀?

以你的紀錄,一星期應該冇問題?!

但有沒有足夠測試工具? :D

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funoooo
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文章 funoooo » 週四 17 6月, 2010 13:58

Support!!!

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cwlam
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文章 cwlam » 週四 17 6月, 2010 14:25

funoooo 寫:Support!!!
x2 :D :D :D

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PeterPeter
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文章 PeterPeter » 週四 17 6月, 2010 14:31

造多幾支賣給同好的話就更好 :D

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Subaru
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文章 Subaru » 週四 17 6月, 2010 14:41

It's great to hear this project, and I am sure it is just a piece of cake and only a beginning as a proof of concept for Chan sir.

I believe some larger astrograph like 10" f/3.3 will come after Chan sir finishes the test with this little gem.

How about Chan sir to install the MPCC into your ε-130 for the null test before starting the mirror?

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Wah!!
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文章 Wah!! » 週四 17 6月, 2010 14:54

Is the MPCC mentioned above from Baader?
I've checked it's spec:
http://www.baader-planetarium.com/pdf/mpcc_e.pdf
I only could find "parabolic mirror" from the file, how do people know it works better for hyperbolic mirror?

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Subaru
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文章 Subaru » 週四 17 6月, 2010 15:04

Wah!! 寫:Is the MPCC mentioned above from Baader?
I've checked it's spec:
http://www.baader-planetarium.com/pdf/mpcc_e.pdf
I only could find "parabolic mirror" from the file, how do people know it works better for hyperbolic mirror?
Yes, MPCC is now from Baader and was having a brand name of Celestron / Baader.

I think Chan sir would like to do an experiment to prove this suggestion.

anyone
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文章 anyone » 週四 17 6月, 2010 15:05

期待快些現真身! [完美計劃]

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MANDII
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文章 MANDII » 週四 17 6月, 2010 15:16

已,讲讲下衣家好似5只系要整支镜,仲要系某些理论上有一特破播!
一句讲洒! 香港地5同大陆,造得呢样野的就系难上加难的事,佩服!


未见其成,先见其志。

Chanlunlun
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文章 Chanlunlun » 週四 17 6月, 2010 16:28

bingsze 寫:幾時有野睇呀?

以你的紀錄,一星期應該冇問題?!

但有沒有足夠測試工具? :D
Dear BingSze,

BingSze my friend, you are, may I say, too optimistic.

The problem is not only optical but also mechanical. The whole OTA must be very precisely made and rigid enough not to flex on an equatorail mount.

I will speed up the progress, if special tools can be available soon. For the first time I will work on metal but not plywood.

This project will be rather long term, I'm afraid.

Best regards
Chan Yuk Lun
最後由 Chanlunlun 於 週四 17 6月, 2010 22:18 編輯,總共編輯了 1 次。

anguslau
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文章 anguslau » 週四 17 6月, 2010 17:11

Great to hear such a project! And I am very interested to see the progress.

Just for discussion purposes:

There already existed some excellent commercially made correctors/reducers for parabolic systems (such as Philip Keller), why you choose to build a hyperbolic mirror instead? Sure the MPCC is cheaper, but I am not sure about its quality for serious imaging. I've tried it on my SN-10 (f/4) and very disappointed at the results. Perhaps as you mentioned, it only works well for slower systems.

I find the Tak E180 producing excellent results. The only culprit is to make accurate alignment, which is a hit-and-miss exercise given the Tak tools. I guess you can measure the hyperbolic mirror of the E180 and make equivalent smaller aperture (for portability) or larger systems, utilisation the existing corrector? Will this work? I think such smaller or larger f/2.8 systems will be very interesting. :roll:

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鄧登凳
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文章 鄧登凳 » 週四 17 6月, 2010 17:33

Dear Mr Chan,

I have no experience in making or designing Newtonian astrograph. I just want to learn more from you on the design.


Your focal plain has a radial distance of 110+55=165 from the principal axis. The mirror has a radius of 102/2 = 55. So is there a reason why the MPCC is not moved much closer to the principal axis? Moving the MPCC closer to the principal axis also allows the use of a small secondary mirror and reduce central obstruction.

There are many cheap 4 to 5 inch Newtonians around, why don't you use their cell/body and just focus your effort on the optical part?

Best regards,
Tang

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梅西爾
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文章 梅西爾 » 週四 17 6月, 2010 18:34

鏡神又出新鏡?

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