FAQ - 有關 Autoguider 導星鏡注意事項

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funoooo
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FAQ - 有關 Autoguider 導星鏡注意事項

文章 funoooo » 週五 13 8月, 2010 10:24

有關導星鏡及主鏡比率,各有各不同見解!最誇張是有人用 1:8 去追星成功。為此想再查詢大家意見,再作整理後成為大家參考。

以我所知如下:
1. 導星鏡及主鏡焦距比率,最少是 1:3。 最好能是 1:2。以C8為例:它的焦距是 2000mm,理想導星鏡焦距是1000mm。 如果以 1:3 為例,副鏡都要焦距 666mm.
2. 但有人用 1:8.2 去導星。
http://hkastroforum.net/viewtopic.php?p=194994#194994
不過是極少數例子。
3. 導星鏡一般直徑是 50mm ~ 60mm。當然越大越好,可以不作微調下找到導星。 否則需要調較副鏡找尋導星跟蹤。
4. 在調焦方面可以不用太準,會使追星時容易點。
5. 用 Off-Axis Guider + Sensitive CCD 可做到 1:1 比率,並且減少因導星鏡經緯儀剛性不足而導致失誤。 (ref from Raymond Comment) 但相對面對視場內沒有光星作 Guiding 風險。


如果大家有任何意見及錯漏,請賜教! 賜教! [跪拜禮][
最後由 funoooo 於 週五 13 8月, 2010 16:00 編輯,總共編輯了 3 次。

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Wah!!
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文章 Wah!! » 週五 13 8月, 2010 11:01

肉眼導星和自動導星是有不同的.
不同的自動導星系統是有不同的.

HK.Raymond.Tse
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文章 HK.Raymond.Tse » 週五 13 8月, 2010 15:38

The best guider focal length should be 1:1.

Use Off-Axis guider (to prevent flexture of 2 scopes) with a high S/N & sensitive guider (to find guide star in the very narrow fov of the OAG).

Raymond Tse

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funoooo
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文章 funoooo » 週六 14 8月, 2010 09:47

隔離台 (coffee 台) 社長有一篇意見,非常有見地。有時間大家參考一下!

http://www.astrocafe.hk/viewtopic.php?f=7&t=422

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noodle
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文章 noodle » 週六 14 8月, 2010 10:10

funoooo 寫:隔離台 (coffee 台) 社長有一篇意見,非常有見地。有時間大家參考一下!

http://www.astrocafe.hk/viewtopic.php?f=7&t=422

你其實可以參考早前買的"The New CCD Astronomy"
Ch 5 :p192-225 ,

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文章 sn-10 » 週六 14 8月, 2010 10:27

Relative focal length is not the only consideration, you also need to consider the pixel scales (aka image scales) between your guidescope and your imaging scope.

Also, try googling and reading some articles on subpixel guiding, it's very helpful for you to understand what is actually going on in autoguiding, it's more complicated than you think. If you understand the mechanism, you would know why we don't need 1:1 matching in terms of pixel scale (not focal length).

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funoooo
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文章 funoooo » 週六 14 8月, 2010 10:39

noodle 寫:
funoooo 寫:隔離台 (coffee 台) 社長有一篇意見,非常有見地。有時間大家參考一下!

http://www.astrocafe.hk/viewtopic.php?f=7&t=422

你其實可以參考早前買的"The New CCD Astronomy"
Ch 5 :p192-225 ,
Good. Check 下先。 我現在只看到 Chapter 3. :oops:

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funoooo
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文章 funoooo » 週六 14 8月, 2010 10:40

sn-10 寫:Relative focal length is not the only consideration, you also need to consider the pixel scales (aka image scales) between your guidescope and your imaging scope.

Also, try googling and reading some articles on subpixel guiding, it's very helpful for you to understand what is actually going on in autoguiding, it's more complicated than you think. If you understand the mechanism, you would know why we don't need 1:1 matching in terms of pixel scale (not focal length).
深了少少,要找時間研究研究!

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鄧登凳
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文章 鄧登凳 » 週六 14 8月, 2010 13:07

sn-10 寫:Relative focal length is not the only consideration, you also need to consider the pixel scales (aka image scales) between your guidescope and your imaging scope.

Also, try googling and reading some articles on subpixel guiding, it's very helpful for you to understand what is actually going on in autoguiding, it's more complicated than you think. If you understand the mechanism, you would know why we don't need 1:1 matching in terms of pixel scale (not focal length).
I agree with you.

Pixel scale is the key. Most autoguiding software can detect variation of intensity of light on single pixel so autoguiding can start to react when the brightness varies by about 10%.

However a pixel size ratio of 1:10 is not possible because
(1) The guide camera takes images not intanteously but over a period of time. Say over 0.5 second (for very sensitive ones) or over 4 seconds (for those with poorer sensitivities). So if very sensitive one can really (not exactly correct) works with 1 to 10, the poor sensitivity ones can work only with 1: 1.25!!
(2) We never have perfect seeing. Air current is alway changing the intensity of light slightly, so an autoguiding system should only react to change of light intensity caused by position error but not chasing the seeing. So the reaction level (called agressiveness in most software) is set and that is usually no more than one third intensity level change.

It is where the so called one-third rule comes.

Note: for those using computerless autoguiding, there is no control of agressiveness, 1:2 pixel ratio is usually recommended.
Many good astrophotography guides suggest careful adjustment of agressiveness according to seeing of the night.

p.s. I am not aware of an astrophotographic CCD or DSLR with 24 mircon pixel (other than using 3X3 binning all the time) on which someone's example is based on. In reality pixel size of the image taking camera and the guide camera is similar and all in the range of 5 to 10 mircons.

BTW, inadequate guiding (say 1:8.2) is better than no guiding (such as the guide camera fails to find a star).

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sn-10
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文章 sn-10 » 週六 14 8月, 2010 19:04

In fact, one should do longer exposures to avoid seeing fluctuation, no matter how sensitive the chip is (either defocus the star or add a filter).

Also, image scale =/= pixel size scale, we are not comparing pixel sizes.

As discussed earlier, longer exposure will do the job because seeing variation is averaged out. Another trick, which has been mentioned many times in this forum, is to use a red filter.

FYI, also mentioned many times, there's a 100mm focal length "telescope" called efinder used as a standard guidescope in STV.

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文章 鄧登凳 » 週二 30 4月, 2013 17:39

A good reading about sub-pixel guiding by Craig Stark, author of PHD guiding software: http://www.stark-labs.com/help/blog/fil ... curacy.php

How accurate a modern autoguide program can be depends on the Signal to Noise Ratio. If the camera is sensitive enough and having low noise, accuracy can be 1/250 of a pixel. A 1/10 pixel accuracy should not be difficult to achieve with a reasonable bright star and a reasonable guide camera.

A similar discussion in Chinese, which also considers the issue of atmospheric seeing, can be found here:
http://www.hkastroforum.net/viewtopic.php?t=24910

In general, with a guide camera of about 5 micron pixel size, for atmospheric seeing of >1.5 arc sec (very good seeing on ground level), a subpixel accuracy of 0.15 pixel (quite noisy situation), a guide scope of 100mm focal length is adequate.

總的來說, 以5微米像數大小的導星相機, 大於1.5角秒的視寧度(地面來說很好的情況), 亞像素導星的準確度為0.15像素(噪訊相當高), 那一支100mm焦距的導星鏡己足夠應付任何焦距的主鏡。

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