Several hundred might be close to being sharp and just a bit distorted. Twenty of those frames might be very sharp and undistorted. Where do you draw the line? From a typical webcam run on, say, Saturn, you might collect 3,000 frames in a 300-second period. However, you really do not want to stack those images that are blurred or badly distorted. Of course, if you stack thousands of images the final result will look incredibly smooth. Even stacking hundreds of crude 8-bit dynamic range webcam frames increases the bit depth of the image noticeably. But, with planetary webcam frames, the pattern is no longer fixed if the images are stacked in a random fashion (as random as the planet's movements). With long exposure CCD images, a dark frame or bias frame is often taken to eliminate this fixed pattern noise. A major factor enabling noise reduction is that there is often a subtle fixed pattern to noise for each CCD chip. The fact that the planet wanders around a bit at the whim of the telescope drive is not a disadvantage here (as long as it does not wander too much). Once you have a master reference frame you will then want to align and stack as many good frames as possible, with respect to that master frame, to reduce the noise.
(In fact, when you become more experienced, a master reference image, produced using Registax' Optimizing page "Create" function can be assembled from a short sequence of 50 frames, rather than a single, noisy, frame.) Needless to say, the master reference frame must be chosen with care. Frames such as these are the master reference images that Registax needs to use to align and stack other good images onto and this is the key to how Registax delivers the goods. In those frames the planet's shape may well be within an arc-second of the true shape all around the limb or rings if seeing conditions are good. Out of several thousand frames there will always be a few single, noisy frames, where the seeing was close to perfection for that single 0.1 second period. Stacking also increases the dynamic range of an image such that subtle shades and contrast differences in one noisy 8-bit frame are transformed into a smooth final result where the most subtle luminance variations are revealed. Of course, a few, rare, individual frames may well be nearer to geometric perfection than the final result, but they will be far noisier. In addition, on images of reasonable quality (i.e., not highly distorted) it produces an average image, not one where one side of a planet, or its rings, is deformed. Stacking reduces the pixel-to-pixel noise by a factor roughly equal to the square-root of the number of frames stacked.
It would be incredible if the image of a planet were to remain steady and undistorted for several minutes at a time: indeed, it never happens! What does this imply for a frame-stacking program? It is important to remind ourselves why we stack frames at all. Remember, the light is traveling through 30 kilometers of the Earth's seething atmosphere.
In the webcam era it is now possible to video a planet for many minutes, at 0.1 second intervals, and to see just exactly what happens from split-second to split-second. One thing that needs to be appreciated from the outset is that even on the best nights of atmospheric stability a planet never just sits there, without a single quiver. I would certainly have valued a Registax "Idiot's Guide" when I started using it!
#Astronomy avi stacking software software#
As a Registax user myself, I hope that an explanation of the software from a user's (rather than the designer's) viewpoint will be of use.
#Astronomy avi stacking software full#
However, a full understanding of Registax' strong and weak points can be of great use. However, few amateurs who use Registax seem to have the spare time or the inclination to examine exactly how it works and to assess which options are the best in different situations. We have already examined the remarkable Registax software developed by Cor Berrevoets, a software package that has made the stacking of thousands of AVI frames a routine and reliable operation.
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