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注意观测日食大气现象----影带

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发表于 2008-7-29 09:21 | 显示全部楼层 |阅读模式
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全食前后几分钟在地面上能看到明暗相间的条纹涌动,在白地或白墙上会比较容易看出来。一直忘了这个碴儿,这会儿你们都出发了我才想起来。好像一直没见大家讨论过这个事儿,希望你们能看到这个帖子并互相转告。如果没有合适的墙面或地面,可以在地下铺大片的白纸。
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/ k" G+ n* q2 F' p97年我在漠河的经验是食既前3-5分钟就开始看到斑马纹状的影带飘动,当时面前是大片的未经踩踏的白雪,所以条件算是极佳。
2 a4 C; V/ |) W/ p+ a2 B3 i* V随着食既的临近,影带飘动加快,也逐渐不太规则,扰动比较厉害,食既前若干秒感觉已经像是沸腾状。
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楼下附上搜索到的中文和英文介绍网页各一。英文网页很详细,有照片,有video,有参考文献。据说世界范围内影带(shadow bands)的成功照片和video都很少,爱好者不妨一试。
 楼主| 发表于 2008-7-29 09:22 | 显示全部楼层
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1 f0 s7 n% I0 w' |: P/ ~ http://news.sina.com.tw/article/20080728/625913.html4 c/ c$ p" o! A
日食問答:什麼是影帶
3 M  q5 H4 o) R" {/ S5 r北京新浪網 (2008-07-28 21:00) " ^$ h, Q1 W* K* E; r

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* q- N: @- w5 E2 a  觀測者在日全食發生的前幾秒看到的大部分短暫現象中,就包括影帶。它們看起來像很多模糊的緞帶,只要將一張幾英尺見方的白紙放在地上,就能看到它們。它們看起來就像池底陽光留下的漣漪。而且每個日食的影帶可見度都各不相同。19世紀的觀測者將它們解釋成是一些類型的衍射現象產生的干擾帶。然而,太陽很難是一個『點源』,而且影帶的模式比你能期望衍射現象產生的模式y騔H意。# b! |7 M* m7 _6 q; K
  不過有關這一現象的一種最簡單的解釋是,它們在大氣的騷動干擾過程中產生。在光線穿過大氣中的漩渦時,出現了折射現象。非常遙遠的地方的光源看起來就像一些『閃光』,但是在大型天體附近,射來的光能分裂成干擾束,到達地面再進行重新組合,形成斑駁的光斑和暗色帶狀物,或者是帶狀物的一部分。在全食附近,太陽的影像只是一個僅有幾弧度秒寬的細細的新月形,它的大小大約就跟在地面上看到的大氣漩渦的大小一樣。之所以會產生帶狀物,是因為太陽的影像的一邊比另一邊長。影帶的移動速度並不像我們期望的那樣,並不是跟日食同步,它的速度是由大氣漩渦的運動速度決定。(孝文)
 楼主| 发表于 2008-7-29 09:24 | 显示全部楼层

英文网页,没时间翻译了

http://www.strickling.net/shadowbands.htm3 z/ g. d# F$ Y5 Q" Y  g
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Shadow bands during a total solar eclipse
by Dr. Wolfgang Strickling
On 21.06.2001 i experienced a total solar eclipse under optimal conditions for my first time. As our video recording of the shadow bands succeeded unexpectedly well, i looked for more information about this interesting phenomenon afterwards .
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1 o# I. X, y3 h2 b9 i$ X. i Contrast enhanced and processed screenshot from our Hi8-Video, 18 seconds before the second contact. 7 t1 `, H& N( r  X& C9 p
Videotaped by Dr. Andreas Dahm. Time is UT +2 h 5 z4 u% A2 u5 X! O
For a highly resolved picture click on the right picture!

: p: B( g$ K* H4 s# bOn the  left you see an animated GIF of the shadowbands
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  f, w6 v$ A& [  KDuring the central 10 minutes of the eclipse a video camera filmed a white cloth 1.4 x 2.4 m . More details, short videos and images can be obtained  on my 2006 observation report page and my 2006 observations page (more detailed).

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6 V4 q7 T/ Q2 V  ?# T. x( M Unfortunately there exist only few photos of the shadow bands world-wide. So there ist often still published a rather bad lithography of the shadow bands of the eclipse 1870 in Sicily and South Italy (see picture left). 7 m+ ?. h& z: |$ v" q# i# b7 f
The best theory for the emergence of the shadow bands is published by Codona 1986 [3]. His theory meanwhile accepted by the most scientists. Codonas scintillation theory is able to explain very well also subtle photoelectric observations .
& [/ |# F* r  ], l3 V6 G  nAfter Codona the shadow bands at ground level result from interference of light rays, taking a somewhat different way in the atmosphere when crossing its turbulences and density variations . 4 t7 m2 }, ]( w
The best observation conditions for such interferences can be expected from point ligth sources. On the other hand, the more extended the source of light is, the more less will such interferences be perceptible. Nevertheless you may observe the so-called " heat waves " on very hot days on homogeneous structured surfaces. In general, they are nothing different than the shadow bands.   ^$ f  |9 N6 p
During a solar eclipse however the solar crescent becomes more and more the shape of a slot. While a point light source would produce a spotted interference pattern, the pattern produced by this slit-shaped solar crescent is smeared to bands.
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Above: Photos after image processing, 45 s, 25 s and 10 s before second contact of the 2006-03-29 eclipse
The wavelength (band distance) of the shadow bands is expected to decrease to 2nd resp. to increase after 3rd contact, see images above, taken on 2006-03-29. My observations of 2006 show the relation of shadow band distance to contact time very clearly (see right graph). / C# ?3 \- d% q+ ^/ j; \3 B
Left: The shadow bands orientate parallel to the projected picture of the solar crescent. Their direction of motion is percepted always in a right angle to their orientation and is resulted from the wind direction in the creating air layers. . y7 H  u2 [1 I- O- }# H
(according to B. W. Jones)

% G' E1 o+ a2 @/ L- g: J) uRight: function of the shadow bands wavelength resp to time.
8 B* I* c" w, t  ?% P( `The orientation of the resulting interference bands is therefore parallel to a projected image of the solar crescent on the projection surface. So the shadow bands orientate directly before or after the totality parallel to the edge of the moon's shadow. In larger distance from the totality are they right-angled to the center line.) z, J& E( X% l' i9 v
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[ 本帖最后由 秋叶夏花 于 2008-7-29 09:32 编辑 ]
 楼主| 发表于 2008-7-29 09:25 | 显示全部楼层

英文网页 (续)

We should expect an orientation following the equation8 g! h" e$ }% B
   Ab = As -90° + ArcTan (Tan (Pa) / Sin (e)); l, ]& ~9 ]3 A7 i( t) Y9 A
with
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  • Ab the Azimut of shadow bands
  • As the Azimut of the sun
  • Pa the Position angle of mid of  the crescent (appr. 2nd resp. 3rd contact) to Zenit and
  • e the elevation of the sun
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The height, in which turbulence cells creating perceptible shadow bands may lie, depends on the angular dimension of the source of light. The above mentioned " heat waves " can be produced only by convection cells a few meters above the ground. Higher cells will average their effects away, since the sun is not a point light source. The more narrow the source of light is, the higher may the causing cells lie. So the convection cells, which are mainly responsible for the shadow bands, have heights between some hundred meters at the beginning of the visibility and up to a few kilometers directly before the second or immediately after the third contact. 2 U0 P3 I1 m8 E' ~9 a
The movement of the shadow bands is caused by winds in the different atmospheric levels. The direction of motion appears always perpendicular to the orientation of the shadow bands, since one cannot recognize parallel shifts of the bands with the human eye. The velocity of the shadow bands depends therefore on the wind velocity! With zero wind speed they will hardly move and therefore will not be noticeable. On the other hand, if the wind is blowing very fast, the movement is so rapid that the eye can not follow the low-contrast structures any longer and therefore an observer will  not see the shadow bands, although they are well provable with fast photometers [4]. For good observations wind velocity should lie in the range of one to few meters per second. $ U' |% E5 a! j# ?2 s+ Y9 h
Codonas scintillation theory explains also some of our observations: 5 Q4 B' N$ \" C: G
The orientation of the shadow bands changed, because our camp did not exactly lie on the center line, but some kilometres south of it (coordinates 31° 01.45 ' east, 16° 24.90 ' south, 487 m above sea level). The solar crescent was situated at the second contact approximately parallel to the horizon, after the third contact it was inclined to the horizon of about 43°. The shadow bands lied therefore before the totality perpendicularly to the sun's direction, afterwards they were twisted about 50° against it. Shadow bands are parallel before and after the totality only if the observer is placed exactly on the center line! $ A0 f/ P! X0 P2 Q
The observed increase in contrast near totality is predicted by Codona's theory as  well as the decreasing of the band distance. It was at the beginning of visibility appr. 30 cm and before the totality approx. 10 cm. ; C( E, p+ p; c
The theory explains also the conditions for optimal visibility of the shadow bands: ) M8 k; N! ~1 i! W
  • Long solar eclipses are more favorable for the observation of the shadow bands than short ones, because during  a short eclipse the solar crescent is approximately semicircular and thus too extended (see fig. below). In longer lasting eclipses it resembles an ideal slot far more. Annular eclipses do not produce shadow bands, or, if they do, then they produce very low in contrast and turbulent shadow bands.
    4 b2 U7 j7 P3 @  [/ ` Simulation of eclipses in the last five minutes before the totality
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    a) short 1-minute eclipse:
    ( l" `7 c$ l6 D0 v/ t$ Vthe crescent is very curved
    b) long 7-minute eclipse: : l6 t( \- F1 e; B
    the crescent is nearly a slot.
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  • Good seeing is unfavorable, since no turbulence cells develop. ("bad seeing is good for shadow bands "). 9 G6 g4 i3 k  M! [& m
    Therefore observation places at sea level are more favorable than higher locations. 5 P. h7 Z* z) ^% e& g6 g  |
  • Moderate wind velocities in middle  atmospheric heights let the shadow bands become well visible. Very strong wind results in fast movements so that the eye cannot follow any longer. With zero wind speed the shadows are nearly without motion and therefore hardly remarkable. + X3 }6 V7 v& S& z; G
  • A small elevation of the sun over the horizon will produce stronger contrasts than eclipses near then zenith.
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  • The shadows bands are visible in the two minutes before and after the totality. However, most observers do not see the shadow bands for such a long time. They are best to be seen about 20 seconds distance to totality.
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Some observers report shadow bands very similar to eclipse shadow bands occuring at sunrise or sunset behind mountain ridges or linear clouds. They share a similar geomtry like solar eclipses. Good transparency and the presence of appropriate air turbulencences seem to be conditions for the occurrence of such kinds of shadow bands.The distance to the ridge or the elevation above the horizon seem to be less important.
 楼主| 发表于 2008-7-29 09:27 | 显示全部楼层

英文网页 (续)

An explanation more easy to understand is possible by using a ray-optic explanation instead of Codonas wave-optic. Like a refractor, whose image may be described by light rays as well als by light interference.
2 |  \8 i; N$ ~% r' d8 F" c' hAs a result of atmospheric turbulence and density fluctuations, the solar light rays are refracted. So parts of the atmospheric turbulence cells may work like positive optic lenses producing a real image on the ground. Normally, the images of the noneclipsed sun are too large so that they average themselves and remain invisible. Only if the solar crescent is small enough and the images become as small as the atmospheric turbulence cells are, we can see the shadow bands. They result as a superimposition of multiple crescent images and orientate along the tangents of them. The diameter of the atmospheric cells is about 10 to 20 cm. So, if the crescent becomes narrower, the distance of the shadow bands will decrease and their contrast will increase.
% p: j$ ]$ P  J' U2 U+ I, [" lFrom the size of the shadow bands it is possible to calculate the focal length of our atmospheric lenses. We get a range of some hundred meters to 2 km.
2 Y6 z" P, y" z8 Y2 D0 tOther theories tried to explain the shadow bands by Fresnel diffraction at the lunar limb. Although such diffraction should be expected, it does not seem to play a role in the production of shadow bands. As the diffraction pattern will move with the lunar shadow (ca. 1 km/s!), ist is too fast to recognize. It should also be expected only a very short time around the 2nd and 3rd contact (ca. 1 to 2 sec). I suppose, the contrast will be very low, much less than the measured 2 .. 4 % of our shadow bands. Nevertheless the distance of the diffraction rings (1 cm to meters, depending on the width of the crescent) matches our observations, although it will increase in approach to totality... 2 F# N0 K& t0 Y) {/ C
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Observation of shadow bands
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The shadow bands can easily observed and recorded with amateur means. As those observations have scientific interest, eclipse travelers should take appropriate equipment with them [4]. In order to record also high frequency variations of the shadow bands, exposure times of max. 1/100 second should be used. Because of the reduced brightness around the totality photographies are difficult to be made. The brightness lies in the range of 10 to 100 Lux  (that is 1/1000 to 1/10000 of noon brightness!), so high speed films and fast lenses are required.
0 C5 ~. `5 z1 Z. E5 x# iFast CCD-video or digital cameras will  probably provide better results. Due to rapid change in brightness near the totality, you should switch on automatic exposure and switch off autofocus! Take a projection surface of 1 x 1 m size minimal, better a larger one and note its orientation, size and the geographic coordinates of your location. In case of videotaping, film a carefully adjusted clock or a GPS clock before and afterwards to have good time information later. You may start your camera several minutes before totality and then observe the eclipse visually. Some minutes after totality, stop your camera with your observations finished.
' O- w- d: W0 H7 b( u% ?Meanwhile also good photoelectric observations are possible with amateur means. You should use registration frequencies of about 1 kHz in order to register high speed changes. For more details see the publications of B.W. Jones [2].  
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) s; K! {0 U9 f: v. r: ^To study contrast an the development of the shadow bands in detail, it is possible to take an intensity profile, for instance with  LIMOVIE. With  SPECTROGRAM you can make a fourier power spectrum of the intensity plot after converting the intensity graph to a WAV-file (done with csv2wav). The upper graphs show the spectrum before second contact (marked as C2), the graph right the development after third contact (C3) of the eclipse 2006-29-03. It can be seen, that the shadow bands do not develop continuously, but that there may occur short periods of less activity. Longer measurements reveal that the shadow bands activity begins several minutes before naked eye can watch them on the video.
* V1 B0 [8 }, a6 X* AGenerally: Prefer such devices, which you can start some minutes before the totality! So you will not not forget your measurements and can enjoy the eclipse visually.
+ H' K2 U# P; N5 S. I. y4 P4 |The weather and wind conditions, especially wind speed, wind direction and cloud movement should to be noted as well as geographic coordinates and visual impressions. An interesting location for shadow bands observation is the zone of grazing eclipse at the border of the band of totality. There the shadow band swill be seen over a long time and they will rotate, as the solar crescent changes its orientation in the sky!
9 y' w2 o, @) N7 G1 BDownload my MPEG-2 videofiles for the $ r, I) f2 J/ `. _/ [, `

$ f( }* {; m& L' m' D3 u4 ?(If you get problems in playing MPEG-2 files, download VLC media player or  Microsoft's newest newest media player. The standard one often does not play MPEG-2.)
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[ 本帖最后由 秋叶夏花 于 2008-7-29 09:31 编辑 ]
 楼主| 发表于 2008-7-29 09:28 | 显示全部楼层

英文网页 (续完)

Hints to digital image and video  and signal processing
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- g" A: o, e7 F1 W7 LFor good and smooth projection screens it is easy to analyze the video or photos. If the projection screen is not smooth, but eg. a folded cloth, it is often necessary, to eliminate structures of the cloth digitally. : a1 n6 l# ?5 w# `7 G9 R
For still pictures i extract the 25 frames of one second video and averge them by adding them (the German GIOTTO freeware does this very well). The result will be inverted to get a negative image and superimposed with 50% transparency to one of the original frames.
9 u9 j2 J. g1 yFor processing a video clip, i get an averaged video sequence, by making a multiple superimposition of the same videoclip to itself, each clip separated by one frame in time (1/25 sec at 25 fps). To get an one-second averaged clip (in PAL resolution at 25 fps (, NTSC at 24 fps IMHO)), i superimpose this clip 25 (NTSC 24) times. In my video software (Ulead Media Studio) the clip on track "V1" gets 0% Transparency, V2 gets 50%, v3 67%, and any other track Vi gets a transparency of 100% * (i-1)/i with i = 1 to 25. So track 25 gets 96% transparency. 3 b. D2 a: }$ k) m
From the superimosition i create a video file, invert it to get a negative and superimose it again with its original clip at 50% transparency to average nonchanging structures. All these steps are possible with the same video software and no other software is required. (But, on my pentium 800 i need 45 mins for a 1 minute clip to calculate the average film...). Another, faster method is to superimpose the original video and an inverted and by 1 to 3 frames shiftet copy of it with 50% transparency. But i expect some artefacts and uncontrolable errors, making an exact analysis impossible, so i prfer the first, more precise method. + j' U3 U6 D+ E% a9 v
The result is a smooth grey film, showing only differences between the actual frame and the average of the 12 frames before and 12 frames after. It is optimal for coontrast enhancing amd further video processing.   Q6 D4 g% \* O) j, J

# r$ R$ ^2 t* q9 x1 c% tLiterature and internet links:
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© Dr. Wolfgang Strickling, Germany

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[ 本帖最后由 秋叶夏花 于 2008-7-29 09:29 编辑 ]
发表于 2008-7-29 11:26 | 显示全部楼层
日食的项目很多,如何兼顾和统筹将是个很紧要的问题,做得好,收获才大。这个影带观测项目适合团队型做较佳,单枪匹马型很难兼顾。
发表于 2008-7-29 13:12 | 显示全部楼层
证实!; ?* {6 P4 W0 {4 {/ _
97年我在漠河就看见过!
发表于 2008-7-29 21:06 | 显示全部楼层
好神奇啊,明年准备
发表于 2008-7-29 23:22 | 显示全部楼层
真的,值得关注,那才叫“万道金光”,我也终于体会到课本上为什么形容阳光是“一缕一缕”的了。
发表于 2008-8-12 20:10 | 显示全部楼层
我们在金昌观测日全食时观察到了影带现象,非常明显!
发表于 2008-8-13 11:31 | 显示全部楼层
在伊吾我也注意看了这种影带,不过现场有大片大片的云,当云遮住太阳的时候,地面阴影变化的感觉就很明显,真正食甚开始的时候,地面亮一片暗一片的,反而没有前面看到的云遮阴影明显,所以没有注意到,更没有拍下来了。明年再试。

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