多謝鄧Sir指導,打完風一定要出去再試!鄧登凳 寫:大家參酌討論一下好了, 指導可不敢啊vegabort 寫: 鄧Sir...關於漂移對軸---鄧氏感嘆號法,小輩有問題想請教..
1)應先試東水平天區還是正天頂區好呢?
2)我試過用CD-1先影東水平天區,星星不只是左右(東南)Drift...而是傾斜Drift 如左上至右下...有時甚至垂直而不是打橫Drift...應甚麼辨好呢?
3)第8點....向近地平的誤差為零或很小時.......如星點向北移, 即鏡頭/望遠鏡的指向比真正的天球赤道偏南了, 極軸指向是偏向西方了(或向北站時的左方), 那要調方位角的螺絲, 使極軸順時針(由上向下望)移。....星點向北移是真正的北方,還是相片上方的意思? 如果向東西drift又怎算?
勞煩鄧sir指導!
1. 我習慣是先做東或西水平區, 因為要順便對焦, 看天頂位對焦痛苦很多。但其實應是先做天頂位會好一點, 因水平是區不可能真正的水平, 很大機會是80%上下誤差加20%左右的誤差。
2. 如果是小許斜向, 基本是南北向的, 可以不理, 因相機沒有小心調角度, 也不一定是正向著東或西。 但如果有很明顯的東西拉線, 那一是在固定赤道儀或固定拍攝設備的連接位有鬆開了的地方, 或是赤道儀的追蹤出了問題(包括小弟試過AstroTrac電壓不夠就會出這個毛病)。東西向拉線問題調整極軸解決不來。
3. 在相機的視像器(monitor)看到的是正像, 所以相片向北的方向就是北方, 由於相機可以透過球狀雲台(ballhead)或鏡頭抱箍等調方向, 上下就很說是什麼。東西拉線問題, 第2點己答了。
Drift alignment using a DSLR camera---漂移對極軸法
今次初訪阴那山, 出現了一些問題, 暫時未知怎樣解決。
星期六的早上兩時多, 有一些天開了, 但仍雲來雲往, 但北極天區是雲封的, 根本看不到極星, 但都設了AstroTrac和300mm鏡頭, 希望天一開便去拍攝。
以往都試過沒有極星, 但用指南針的估計仰角, 都可以在25分鐘內完成三分鐘單張的飄移對軸。但由於雲來雲往, 結果不時見到的不是"感嘆號", 而是好像犘氏密碼般斷開四五節的拉線, 根本不知那裡是頭, 那裡的尾 , 結果只好再拍, 碰運氣。而在接近準確時, 那短的拉線根本沒有光和暗的分別(很可能光區都不是全時間曝光), 根本沒有辨法調, 又要再拍。結果要一個小時多才完成對軸。
請教各位: 有沒有方法在星時光時暗的情況下, 用得到DSLR的飄移呢?
(P.S. 試過用相機對焦點的紅點相作十字目鏡, 但不行, 因著了就不見星)
星期六的早上兩時多, 有一些天開了, 但仍雲來雲往, 但北極天區是雲封的, 根本看不到極星, 但都設了AstroTrac和300mm鏡頭, 希望天一開便去拍攝。
以往都試過沒有極星, 但用指南針的估計仰角, 都可以在25分鐘內完成三分鐘單張的飄移對軸。但由於雲來雲往, 結果不時見到的不是"感嘆號", 而是好像犘氏密碼般斷開四五節的拉線, 根本不知那裡是頭, 那裡的尾 , 結果只好再拍, 碰運氣。而在接近準確時, 那短的拉線根本沒有光和暗的分別(很可能光區都不是全時間曝光), 根本沒有辨法調, 又要再拍。結果要一個小時多才完成對軸。
請教各位: 有沒有方法在星時光時暗的情況下, 用得到DSLR的飄移呢?
(P.S. 試過用相機對焦點的紅點相作十字目鏡, 但不行, 因著了就不見星)
- PeterPeter
- 夸克星
- 文章: 2653
- 註冊時間: 週四 30 11月, 2006 13:25
Peter兄客氣了, 我對極的方式其實很粗糙的, 首先參考北極星盤中北極星的位置, 將極軸鏡中的北極星放於相應位置並記住, 例如10點方向, 然後用200mm(或其它長焦鏡)向天頂附近星星試拍1-2分鐘, 若有拉線則調整北極星向十字線中心靠攏或遠離, 再試拍, 若調整後拉線變長則往相反方向調整, 如是者往反幾次, 一般可達到基本拍攝要求(大約200mm 2-3分鐘), 其實不算得甚麼方法, 只是两軸同時大約調整, 不太嚴謹, 但速度快, 前提是極軸鏡有十字線作參考, 我而家正在研究/練習鄧sir的"鄧氏感歎號法"PeterPeter 寫:鄧sir,我想在這惡劣的情況下,Cori式的對極法或會較可取.
詳細的步驟就要問問Cori兄了.
他的方法不需要理會甚麼原理,
只需不繼嘗試把RA軸上下左右調整,直至拍攝出來的星點不拉線為止.
當調向某一方向會導至更長的拉線時,便需要調向相反的方向.
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- lawrencemok
- 白矮星
- 文章: 741
- 註冊時間: 週一 25 7月, 2011 18:01
I plan to purchase polarie but before that, I'm thinking about how to speed up the ! drift align method, here's the basic idea:
1. Assume you're using the longest focal length lens available to you (e.g. I'm using 250mm).
2. do calculations (quite a lot of steps, i'm not listing here until i'm ready)... to find out the relationship between the length of ! (measure by a real ruler) on your camera's preview screen (assume you zoom to max), to the actual degree of mis-alignment.
3. Find out how many turns you need to turn your gear head handles to correct the mis-alignment......and ultimately... simplify the equation so that x mm on screen = y degree of turn in gear head.
I had done some very rough calculation for my case, a 1 degree mis-alignment, will appears as a ~1mm length of ! for an explosure of 10 minutes, and I turn my gear head ~45 degree to correct it.
With this steps I hope to make my alignment within 1 degree accuracy (assuming I can clearly measure the length of 1mm on my screen)... by spending just 20 minutes. (10 minutes per axis)
1. Assume you're using the longest focal length lens available to you (e.g. I'm using 250mm).
2. do calculations (quite a lot of steps, i'm not listing here until i'm ready)... to find out the relationship between the length of ! (measure by a real ruler) on your camera's preview screen (assume you zoom to max), to the actual degree of mis-alignment.
3. Find out how many turns you need to turn your gear head handles to correct the mis-alignment......and ultimately... simplify the equation so that x mm on screen = y degree of turn in gear head.
I had done some very rough calculation for my case, a 1 degree mis-alignment, will appears as a ~1mm length of ! for an explosure of 10 minutes, and I turn my gear head ~45 degree to correct it.
With this steps I hope to make my alignment within 1 degree accuracy (assuming I can clearly measure the length of 1mm on my screen)... by spending just 20 minutes. (10 minutes per axis)
Good thinking.lawrencemok 寫:I plan to purchase polarie but before that, I'm thinking about how to speed up the ! drift align method, here's the basic idea:
1. Assume you're using the longest focal length lens available to you (e.g. I'm using 250mm).
2. do calculations (quite a lot of steps, i'm not listing here until i'm ready)... to find out the relationship between the length of ! (measure by a real ruler) on your camera's preview screen (assume you zoom to max), to the actual degree of mis-alignment.
3. Find out how many turns you need to turn your gear head handles to correct the mis-alignment......and ultimately... simplify the equation so that x mm on screen = y degree of turn in gear head.
I had done some very rough calculation for my case, a 1 degree mis-alignment, will appears as a ~1mm length of ! for an explosure of 10 minutes, and I turn my gear head ~45 degree to correct it.
With this steps I hope to make my alignment within 1 degree accuracy (assuming I can clearly measure the length of 1mm on my screen)... by spending just 20 minutes. (10 minutes per axis)
The length of the "!" may vary a bit depending on the position of your pointing.
My experience was that even without a ruler, you can guess the amount of adjustment required with two to three trail-and-error adjustments.
Moreover, currently with the AstroTrac TT320X-AG using focal length of 300mm or shorter, polar alignment using polar scope is accurate enough. With the iOptron SkyTracker and a 180mm lens, again polar scope alignment is good enough. Given Vixen polar scope is known to be quite accurate, I speculate that drift alignment is not necessary for a 250mm-or-less lens.
地球的極軸會變化的, 一種變化是一個短循環的小變化, 叫錢德勒擺動(Chandler wobble), 見http://www.earth.sinica.edu.tw/~bfchao/ ... 08-090.pdf, 這個在不太長焦的望遠鏡下, 根本察覺不到。vr2xsc 寫:鄧Sir, 地球極軸的角度是否不變的? 我看過其他同好他們在天檯set 定在極軸位置, 之後又好似之後又唔見佢地再對,是否唔須要再set. 唯一我們四處走又睇唔到北極星, 所以每次都要對餐飽(一兩個鐘都天光啦), 北極星的仰角大約在22.5 度(香港).我可否用一個精度高的(+/- 0.01度)仰角尺度準一次之後, 以就用返呢個數係唔係百份百準呢? 之後祗估左右北位就OK呢? 因為如果加埋上下左右真係好難估!
另一個是較長期(26000年)的打圈, 叫做歲差(axial precession), 請參考http://case.ntu.edu.tw/hs/wordpress/?p=3966, 請個是個大圈, 一萬二千年前和一萬四千年後, 織女星會是極星。由於歲差, 每年極軸的移動差不多一角分(arc minute), 兩三年下來, 就會出現對極不準的問題。不過, 如果用了雙軸自動導星, 十年內的差異, 導星會解決了。
一般仰角器和指南針的準確度是0.2度角(12角分), 不足以用來準確對軸。
如果有星, 用DSLR做漂移, 三分鐘一幅計, 應20至45分鐘內可完成。用CCD做, 更應10-20分鐘內完成。
- lawrencemok
- 白矮星
- 文章: 741
- 註冊時間: 週一 25 7月, 2011 18:01
Thanks for your advise 鄧sir~ I'd take your suggestion ...... I guess just trial and error with the help of writing down logs of ! length + gear knob rotation should be good enough
Also thinking that for drift alignment, even if east / west horizon is not low enough doesn't matter (like in PTC) ...
I can first correct AZ error using the stars near the intersection of the meridian and equator,
then I aim camera to lowest possible position in either East / West.... since AZ error was already corrected, any ! should be solely due to ALT error....
Also thinking that for drift alignment, even if east / west horizon is not low enough doesn't matter (like in PTC) ...
I can first correct AZ error using the stars near the intersection of the meridian and equator,
then I aim camera to lowest possible position in either East / West.... since AZ error was already corrected, any ! should be solely due to ALT error....
- lawrencemok
- 白矮星
- 文章: 741
- 註冊時間: 週一 25 7月, 2011 18:01
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