在 Wikipedia 挖寶就對了.
越看越多了, 只可惜字很多, 沒有圖示.
底下說了一大堆, 其實重點就是:
speckle imaging 就是一連串的短時間曝光影像, 其實就像 video imaging,
利用 speckle imaging (or image stacking) 可以用來對付大氣不穩定所造成的成像模糊問題, 提高影像解析度.
Speckle imaging
這名詞 10 多年前就在中大天文所看見過, 只是完全不知道是啥東西,
原來 image stacking 也是 speckle imaging 的 一種,
以下節錄自 Wikipedia:
Speckle imaging (also known as video astronomy) describes a range of high-resolution astronomical imaging techniques based either on the shift-and-add ("image stacking") method or on speckle interferometry methods. These techniques can dramatically increase the resolution of ground-based telescopes.
The principle of all the techniques is to take very short exposure images of astronomical targets, and then process the images so as to remove the effects of astronomical seeing.
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This breaks down due to the practical limits imposed by the atmosphere, whose random nature disrupts the single spot of the Airy disk into a pattern of similarly-sized spots covering a much larger area (see image of binary on right).
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For many years telescope performance was limited by this effect, until the introduction of speckle interferometry and adaptive optics provided paths to remove this limitation.
Speckle imaging recreates the original image through image processing techniques. The key to the technique, found by the American astronomer David L. Fried in 1966, was to take very fast images in which the atmosphere is effectively "frozen" in place. For infrared images, exposure times are on the order of 100 ms, but for the visible region they drop to as little as 10 ms. In images at this time scale, or smaller, the movement of the atmosphere is too sluggish to have an effect; the speckles recorded in the image are a snapshot of the atmospheric seeing at that instant.
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There are a number of different speckle imaging methods. In one technique called shift-and-add (also called image stacking), the short exposure images are lined up by the brightest speckle and averaged together to give a single output image. In the Lucky Imaging approach, only the best few short exposures are selected.
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Of course there is a downside: taking images at this short an exposure is difficult, and if the object is too dim, not enough light will be captured to make the analysis possible. Early uses of the technique in the early 1970s were made on a limited scale using photographic techniques, but since photographic film captures only about 7% of the incoming light, only the brightest of objects could be processed in this way. The introduction of the CCD into astronomy, which captures more than 70% of the light, lowered the bar on practical applications enormously, and today the technique is widely used on bright astronomical objects (e.g. stars and star systems).
Another limitation of the technique is that it requires extensive computer processing of the image, which was also hard to come by when it was first applied. Although the almost-universal Data General Nova served well in this role, it was slow enough to limit the application to only "important" targets. Again, this limitation has largely disappeared over the years, and nowadays desktop computers have more than enough power to make such processing a trivial task.
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