Stacking

I have to confess that after I have sent a final draft to a book publisher, I will discover something later that I wish I had included. When I produced the 2nd edition of How to Photograph the Moon and Planets with your digital camera, all of the photographs and techniques used just the best image of several. I have since discovered that the quality of photographs can be improved greatly by stacking. This is the act of combining several images of the same object together to form one image that contains all of the detail of the individual images. At least, that is the intention and it is not always achieved but I have found, with experience, that there is a very noticeable improvement.

The great news is that this can be applied to images of the Moon as well as deep sky images. If you are taking short exposure photographs of objects like the Sun (with suitable filters), the Moon and brighter planets, then about 30 or as many as can be practically taken. If you are taking planetary photographs or close-ups of the Sun or Moon, you will be using higher magnification. Unless you have a mount with a motor drive, it will take a lot of patience to take as many as 30 shots and even more if you want to take over 100 and need to track manually. This is also true if you are taking a number of widefield shots with a DSLR or compact digital camera to capture constellations.

There are no real hard and fast rules on this but I recommend that you use about the best 60% of individual images and exclude the rest from the stack. Although Microsoft ICE was designed to automatically stitch a number of adjoining photographs to make a mosaic (which it does rather well) it can also be used to stack multiple photographs of the same object. Registax (Versions 5 and 6) can also be used to stack still images, as well as the movie files it was designed to stack but it does not handle large images well and will often freeze the computer or complain that there are not enough computing resources (usually memory). All programs that are capable of stacking and are free can be found here: Phil's Software Downloads.
Please also be aware that stacking can take from several minutes up to an hour.

As an example, I took ten individual shots of Cassiopeia on September 7th. Although they were 30 second exposures using a DSLR, the same procedure can be used with a compact digital camera.


Fig 1 Individual frame of Cassiopeia

Figure 1 shows an original shot of Cassiopeia. I took nine others but I also subtracted a dark frame from each. This involves taking a photograph with something dark covering the lens (in this case a fleece!) and using the Subtract function available in some editing packages to remove some inaccuracies introduced by the camera. I then stacked the frames and used a technique described in a previous article to produce the final image. This even shows some sign of the Milky Way background.


Figure 2 Final Image

Fig 3 shows a lunar image taken on July 29th, where I stacked the best 9 of 20 images.

Figure 3 Moon

Being a solar fan, I could not neglect our own star. The original photo was taken in September 2011 but I had taken 7 images of a solar region. I stacked the best five to obtain a nice close-up of some sunspots in Figure 4.

Figure 4 Sunspots

To summarise, I would recommend stacking as a useful method of obtaining better results with modest equipment. It doesn’t turn bad photographs into good ones but can make some sub-standard ones acceptable and average ones good. Further improvements can be made by using the (well-documented) Deep Sky Stacker for constellations and deep sky objects. Webcams and their modern equivalents can be used to record video files, which can then be converted into still images using Registax or similar tools, such as Avistack. Again, this is a well-documented method, stretching back over more than a decade.
Philip Pugh is the co-author of the 2nd edition of How to Photograph the Moon and Planets with your digital camera by Springer-Verlag and lead author of three other books.

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