Takahashi FSQ 85 ED 光學討論

檢測望遠鏡光學部份,赤道儀機械組件,目鏡配件和拍攝器材 CCD等...
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MANDII
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文章 MANDII » 週二 15 6月, 2010 14:05

Optic physical structure = number of lens , lens curvature , light path or what ever physically .

Optic paramters = ( source brightness ,focal length/ratio , camera flame size etc .. . what we had dicussed . )

1. Optic strcuture is a fixed factor . It cannot explian the different levels of dark band in a same sope system .
Our previous findings all shows there is a trend/levels of Dark bands . In FSQ 85 it has this case , Even in FSQ 106 it also has this case , and these trend changes with optic paramters .


2. Number of lens is just one of the optic structure factor , Paul raise up the number of lens( 3 or 4 lens easily cause Dark bands ) is the cause . Then , he should catagories all scopes into number of lens( 1&2 or 3&4) , and find out that those 3&4 lens scopes uses can cause Dark bands . But so far , we only see one example of TMB300 scope(1/2 lens) . Although this show the difference , but still not strong enough to say the number of lens is related with Dark bands . Thus , we still cannot denie the possibility of other factors of optic structure .

3. Within a same optic physical strcuture , optic paramters applys samely to determine the levels of dark bands .


The difference is :
In case 1 , we only need to provide 1 or 2 example to prove this .

In case 2 , we need to provide many examples to prove this .

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鄧登凳
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文章 鄧登凳 » 週二 15 6月, 2010 14:16

鄧登凳 寫:Dear Dr Tong,

Are you using Takahashi connectors all the way down to you Canon?

Best Regards,
Tang
推這個問題翻出來...

問題的中心是我們一般接相機用的T2接口, T2本身的螺絲牙是 42mm, 即那是個42mm的口徑通道, 那連接管怎樣都有厚度, 即T2連接管的口徑很可能只是40mm, 而35mm全幅片要43.5mm的大小, 即T2連接系統不足以支持全幅片的 :evil:

我的意思是用全幅片又用T2作為接口的, 就一定有漸暈(vignetting)。[和望遠鏡的光學設計沒有關係 :wink: ]

(正如用APS-C而又用1.25"接口的, 接口的內口徑只得1.125吋(28.6mm), 不足夠對角30mm以上的APS-C感光器)。

而用了減焦鏡, 由於光錐(light cone )角度再收窄, 這個40mmT2孔道的影響會更嚴重。

(所以為全幅片或更大感光器設計的望遠鏡, 很多都會有專用的接環)

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昇仔
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文章 昇仔 » 週二 15 6月, 2010 16:16

鄧登凳 寫:
鄧登凳 寫:Dear Dr Tong,

Are you using Takahashi connectors all the way down to you Canon?

Best Regards,
Tang
推這個問題翻出來...

問題的中心是我們一般接相機用的T2接口, T2本身的螺絲牙是 42mm, 即那是個42mm的口徑通道, 那連接管怎樣都有厚度, 即T2連接管的口徑很可能只是40mm, 而35mm全幅片要43.5mm的大小, 即T2連接系統不足以支持全幅片的 :evil:

我的意思是用全幅片又用T2作為接口的, 就一定有漸暈(vignetting)。[和望遠鏡的光學設計沒有關係 :wink: ]

(正如用APS-C而又用1.25"接口的, 接口的內口徑只得1.125吋(28.6mm), 不足夠對角30mm以上的APS-C感光器)。

而用了減焦鏡, 由於光錐(light cone )角度再收窄, 這個40mmT2孔道的影響會更嚴重。

(所以為全幅片或更大感光器設計的望遠鏡, 很多都會有專用的接環)
I use mostly Taka connectors following Taka's system diagram for FSQ 85. The M42 ring is used to connect the camera.

For the setting without the reducer, an extension tube is used. This is not exactly the same as Taka diagram but it is said that the tube is produced by Taka anyway.

anguslau
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文章 anguslau » 週二 15 6月, 2010 16:29

Wah!! 寫:Light goes straight line, then when all light pass through a single point, both in/out focus light cone should be similar.
Can you agree with this?

Defocusing is just putting the sensor to different parts of the light cone.
Moreover, the sensor plan doesn't change direction, they're parallel to each other when we put the sensor to different location. As a result, the shapes showing on the sensor should be similar, the only difference is 180-degree rotation after passing the focus plan.

(Assume that the elements of optical system don't move when we move the sensor.)
Wah! Sorry, you are correct. I wasn't thinking carefully and got it wrong. For without reducer config of the FSQ, no elements move, the in/out focus image should mirror each other. However, with reducer in place, they can be different. Because the reducer moves (together with the sensor), hence changing the optical system.

Looking at the test images by Sing Chai (for without reducer config), I am not sure we are seeing different in/out shapes. The defocused image for in focus is not as large (or blurred) as the out focus one. So it will appear more circular. The two need to be similarly defocused before we can see clearly.

On the point of whether Petzval design more prone to this problem. I think they may be. It is because there are two optical elements near the end of the optical train. But they are still quite far inside of the focal plane. Given such a scope is already a fast design to start with, the last two element needs to be extremely large in order to minimise vignetting. On the other hand, non-Petzval designs do not have such elements for potential vignetting. Minimising vignetting for a reducer is relatively easier because the reducer is much closer to the focal plane compared to the 3rd/4th Petzval elements.

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Subaru
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文章 Subaru » 週二 15 6月, 2010 16:44

anguslau 寫:On the point of whether Petzval design more prone to this problem. I think they may be. It is because there are two optical elements near the end of the optical train. But they are still quite far inside of the focal plane. Given such a scope is already a fast design to start with, the last two element needs to be extremely large in order to minimise vignetting. On the other hand, non-Petzval designs do not have such elements for potential vignetting. Minimising vignetting for a reducer is relatively easier because the reducer is much closer to the focal plane compared to the 3rd/4th Petzval elements.
This phenomena does not only apply to Petzval, but also on telephoto design.

I was using a low cost 400mm f/5.6 telephoto lens some 2x years ago for DSO photography with film, and noticed this effect, thus it is something rather common, and I believe it is not treated as a problem in the field of astrophotography.
最後由 Subaru 於 週二 15 6月, 2010 16:45 編輯,總共編輯了 1 次。

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willis
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文章 willis » 週二 15 6月, 2010 16:44

昇仔 寫:
I use mostly Taka connectors following Taka's system diagram for FSQ 85. The M42 ring is used to connect the camera.

For the setting without the reducer, an extension tube is used. This is not exactly the same as Taka diagram but it is said that the tube is produced by Taka anyway.
昇仔 Confirmed that what u have tested about the vignetting is correct. From Japan Tak official website that the image circle for FSQ-85 is:
44mm (luminosity is 60%) (With reducer or extender)
http://www.takahashijapan.com/fsq85ed.html

This means what u estimated, 50-60%, is agree with Tak's info. [Good Job] [Good Job] [Good Job]


We can also find the FSQ-106 info. The image circle of FSQ-106 is:
44mm (luminosity is 60%) (With reducer or extender)
http://www.takahashijapan.com/ct-produc ... 106ED.html

Which means Tak defines the tolerance of the image circle is 60% luminosity.

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MANDII
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文章 MANDII » 週二 15 6月, 2010 16:49

Anybody can explain the reason of using different F-ratio can cause different dark band , under the same optic design scopes ?

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sn-10
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文章 sn-10 » 週二 15 6月, 2010 16:52

昇仔 寫:
鄧登凳 寫:
鄧登凳 寫:Dear Dr Tong,

Are you using Takahashi connectors all the way down to you Canon?

Best Regards,
Tang
推這個問題翻出來...

問題的中心是我們一般接相機用的T2接口, T2本身的螺絲牙是 42mm, 即那是個42mm的口徑通道, 那連接管怎樣都有厚度, 即T2連接管的口徑很可能只是40mm, 而35mm全幅片要43.5mm的大小, 即T2連接系統不足以支持全幅片的 :evil:

我的意思是用全幅片又用T2作為接口的, 就一定有漸暈(vignetting)。[和望遠鏡的光學設計沒有關係 :wink: ]

(正如用APS-C而又用1.25"接口的, 接口的內口徑只得1.125吋(28.6mm), 不足夠對角30mm以上的APS-C感光器)。

而用了減焦鏡, 由於光錐(light cone )角度再收窄, 這個40mmT2孔道的影響會更嚴重。

(所以為全幅片或更大感光器設計的望遠鏡, 很多都會有專用的接環)
I use mostly Taka connectors following Taka's system diagram for FSQ 85. The M42 ring is used to connect the camera.

For the setting without the reducer, an extension tube is used. This is not exactly the same as Taka diagram but it is said that the tube is produced by Taka anyway.
There's a wide t ring for connecting the camera, this wide t ring has a diameter of 50.8mm.

anguslau
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文章 anguslau » 週二 15 6月, 2010 17:00

PaulNg 寫:I agree that the proof is pretty convincing... but I've been looking at some images, like this... taken by TMB130 with flattener, 6X7 E200. It shows no dark banding like that...
I agree, no such problem can be observed in this image. But since the reducer is specifically designed to cover the large format and its final speed is relatively slow at f/6, it is quite possible that the design caters for a nearly fully-illuminated image circle.

I cannot find the detailed description of the TMB reducer, I saw a description of a gaint field flattener for the scope here which says it is 78mm in diameter: http://www.astrotech.it/it/downloadarea ... %20A&M.pdf. For a flattener this size, it should be able to fully illuminate a major portion of a 6x7 sensor (56mmx67mm; diag 87mm).

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MANDII
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文章 MANDII » 週二 15 6月, 2010 17:14

So far I can read that the design cause the dark bands .
That is very technical things regarding optic design .
However , just simply saying , how to explain the different levels/promonience of dark bands in same system ?

Anybody can answer my this simple question ?

anguslau
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文章 anguslau » 週二 15 6月, 2010 17:14

Subaru 寫:This phenomena does not only apply to Petzval, but also on telephoto design.

I was using a low cost 400mm f/5.6 telephoto lens some 2x years ago for DSO photography with film, and noticed this effect, thus it is something rather common, and I believe it is not treated as a problem in the field of astrophotography.
Now it looks obvious that this phenomenon is not that uncommon. Perhaps we were just expecting too much and over-reacting a little. :lol: But I guess people just tend to expect text book stars when paying for premium gears such as a Tak astrograph... :wink:

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鄧登凳
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文章 鄧登凳 » 週二 15 6月, 2010 17:20

昇仔 寫:
I use mostly Taka connectors following Taka's system diagram for FSQ 85. The M42 ring is used to connect the camera.

For the setting without the reducer, an extension tube is used. This is not exactly the same as Taka diagram but it is said that the tube is produced by Taka anyway.
Strange! I do not know why Takahashi use a T2 ring (42mm screw) in its light path for something intended for full frame.

I have two diagrams showing the light path.

Referring to the diagrams

tan(alpha) = [(85-43.5)/2] / 450 = 0.0461

alpha = 2.64°

tan(beta) = [(85+43.5)/2] /450 = 0.1424

beta = 8.11°

Theoretical (without 40mm restriction) light cone at corner = 2.64 + 8.11 = 10.75°
[with restriction, the angle of the light cone perpendicular to the paper is still this value]

tan(gamma) = 1.75 / 75 = 0.0233

gamma = 1.34°

restricted light cone (on the plain of the paper) = 8.11 – 1.34 = 6.77°

That will create clearly an olive shape.

Estimated vignetting = 6.77 / 10.75 = 63%

My conclusion: the 42mm (T2) connector is the culprit.
附加檔案
angle calculations.JPG
angle calculations.JPG (16.55 KiB) 已瀏覽 6672 次
Light path.JPG
Light path.JPG (16.29 KiB) 已瀏覽 6672 次

anguslau
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文章 anguslau » 週二 15 6月, 2010 17:22

MANDII 寫:So far I can read that the design cause the dark bands .
That is very technical things regarding optic design .
However , just simply saying , how to explain the different levels/promonience of dark bands in same system ?

Anybody can answer my this simple question ?
ok, I'll try and see if I can rephrase my understanding to make it clearer to you.

When there is vignetting on an optical system, part of the aperture has been obstructed. The partically obstructed aperture will typically be an olive shape. Such shape of aperture will produce diffraction patterns which shows the dark bands we are discussing here. As vignetting becomes more severe, the aperture will be obstructed more and the olive shape is more prominent (ie a thinner olive shape). The more elongated shape will produce darker (and hence more prominent) bands.

In an other words, more vignetting, more dark bands.

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昇仔
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文章 昇仔 » 週二 15 6月, 2010 17:25

anguslau 寫:
Subaru 寫:This phenomena does not only apply to Petzval, but also on telephoto design.

I was using a low cost 400mm f/5.6 telephoto lens some 2x years ago for DSO photography with film, and noticed this effect, thus it is something rather common, and I believe it is not treated as a problem in the field of astrophotography.
Now it looks obvious that this phenomenon is not that uncommon. Perhaps we were just expecting too much and over-reacting a little. :lol: But I guess people just tend to expect text book stars when paying for premium gears such as a Tak astrograph... :wink:
We talk like we are over-reacted, at least for people looking at these discussions. For me I am just curious. And we are already making textbook stars or something close to that quality. :wink:

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Wah!!
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文章 Wah!! » 週二 15 6月, 2010 17:34

I think "Light path.JPG" maybe over-simplified:
1. Light cone projecting to the edge is not full aperture of the primary lens.
2. Light cone projecting angle may not be the same as the drawing.

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