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Image Processing

Recording the images (on video)
Image Stacking

Recording the images .
Since the main topic of this is video imaging here follow some tips on the optimum recording of images , part of the information found on the VideoAstro mailing list , part of it is from my own experience .

Since I do imaging from a balcony there is some additional turbulence . When using long focal lengths (high magnification) the focussing is very tricky . The best thing will be to place the telescope outside well in advance of the session , something not so practical in an unstable climate as the one in the Netherlands . It is not uncommon to see Jupiter or Saturn "dance" and "morph" on the video screen . This makes the advantage of video imaging even more important since the very short instants of a good image is difficult to catch .

When the imaging starts , be prepared to spend some time setting the monitor to an appropriate level . I have found that initially I set the brightness level too high resulting in underexposed images . Take a look at the Solar gallery page and compare the two full disk solar images in the beginning . Notice that the limb darkening gives rise to ring shaped darkening in the first (underexposed) image whereas it is smooth in the second image .

Be prepared to record a lot of video and discard at least 95% of the material .

Image stacking
Until the beginning of 2000 I did all stacking of images manually using the Photoshop image processing software . This is a very time consuming activity , and the appearance of Robert Stekelenburg's AstroStack software has been quite a revolution for image stacking video astronomers . Thanks to Rob for making the software available as freeware .

Why do image stacking ? - The answer is simple : stacking of almost identical images reduces noise (seen as grain in the image) , everty time you double the number of images stacked the noise is reduced by half (3dB) Since noisy images will look even noisier with image processing this is very important for video astronomy . The same technique , of course , can be successfully applied to integrating CCD images as well as Photographic images , too .

How many images should be stacked ? - There is no single answer to that - if very low noise and high quality images have been recorded it is sometimes not even necessary to do any stacking . This is typically the case with solar and lunar images , though not always .
With planetary images it is recommended to stack as many good images as you can possibly find in a piece of the recording , again depending on the noise in the single images . For images made with small (60-125mm aperture scopes) I would recommend using up to 100-200 stacked images .This is now within reach using the AstroStack program . Agood wxample can be found at Ole Nielsen's image gallery page : An image of Saturn made afocally with a camcorder "looking" into a 125mm SCT , 72 images were stacked in this case , and the image is excellent for a video image through a telescope
For the lunar eclipse images I used 64 images for stacking in order to reduce the noise to acceptable levels .
For deep sky and constellation imaging etc. ("Camera Direct") I will recommend using as many images as you can find time to stack , up to several hundreds . The method I would use here would be the following : I stack batches of about 25 images , make some processing on these images in order to improve brightness , contrast and possibly some sharpness . The resulting images are then stacked and processed further . Until now I have only stacked up to about 100 images in this way since my recordings have been made without any motorized tracking . With tracking the images will be somewhat better aligned for longer , allowing for more images to be easily stacked . Of course for deep sky etc. I use the "Nightshot" or "Low Lux" setting of the camcorder in order to record anything at all . This gives 8 identical video frames at a time , meaning that only 1 out of every 8 video frames should be used for stacking . This is a feature not yet implemented in the AstroStack program , but I believe Robert is working on it .

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Last update 21 January 2006 by Jan Andersen.
Background image : Video image of the Pleiades
Top caption : Lunar Eclipse 21-01-2000