best astrophotography software
If I may ask a rudimentary question: what is the purpose/goal of stacking astrophotography photos? I understand merging exposures for hdr, but what is the deal with stars, the moon, whichever? I would like to find out as my girlfriend loves everything astronomy.
There are a number of different kinds of astrophotography that benefit from stacking. I will just answer your question in the area I'm most knowledgeable - lunar/planetary imaging. For that type of imaging, one of the best programs is Registax, but others use Avistack. Registax is free, and I believe Avistack is also. Unfortunately both of these programs are Windows only. So you'll have to use something like Bootcamp or Parallels to run a virtual Windows session.
So why stack planetary images? Bear with me. First off planets have an incredibly small angular size (measured in arc seconds). The largest planet, Jupiter is around 45 arc seconds - 0.013 degree! So to get a decent number of pixels in the image you must use very long focal lengths - 10,0000 mm is not too long. With that kind of magnification, the effects of distortions caused by air of differing temperatures becomes a problem. Astronomers call this effect "seeing". On a night of poor seeing (most apparent by the stars doing a lot of twinkling), if you were to look at say a magnified view of the Moon, it would be like looking through the surface of disturbed water. The image would be swimming around, blurring and distorting. Even on a night of better seeing, there will be some distortions present. For a planet, being much smaller in angular size than the Moon, the effect is worse. So how do you get around the effects of atmospheric seeing.
One method is to capture a large number of short exposure images, either video (AVI) or multiple still images (JPEGs). If you have a sufficient number of images there will be some in which the instantaneous seeing is better - that one image is sharper the most of the other frames. What is so powerful about programs like Registax is that they evaluate every frame and give it a quality assessment. It then, in a sense, reorders the frames in sequence from best quality (sharpest image) to worst quality (the most blurred by seeing). The photographer then chooses a quality limit, beyond which any lower quality images are excluded from further use. In effect you just keep the best images with the best resolution and discard the blurry ones.
The program then registers the images using alignment points and then stacks the images. The result is that the noise of each image, being random, tends to average out to be zero, while image details (which are always in the same location - not random) tend to be reinforced. Even details that are not apparent on individual frames will become visible.
This process also works well on the Moon - even better since it has a much larger angular size, 1/2 degree. Here's a couple of examples of a stacked image of the Moon.
Here's an old Stacked image of Saturn:
Take a look at the Astrophotography Talk Forum on DPReview for more information on astrophotography.
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