After you capture a series of long-exposure frames to your computer's hard drive and after you go inside to warm up, it is time to begin the serious task of postprocessing your frames with powerful image processing software. Don't despair. The thing to realize is that the post-processing stage represents half the effort and half the fun! This hobby is different from other forms of astrophotography in that it is highly dependent on powerful image processing techniques, and using those techniques to produce a beautiful final image is a large part of the satisfaction.
The most common methods of post-processing, in the order in which they are generally applied, are:
• Stacking of multiple dark frames to a single dark frame.
• Stacking of multiple flat frames to a single flat frame.
• Selection of best frames.
• Subtracting the dark frame from each raw frame.
• Dividing each frame by the flat field.
• Aligning the frames to one another.
• Stacking the frames.
• Sharpening the stack.
• Level-adjusting the stack.
Many of these steps require software that is specifically designed for processing astrophotos, most of which are available free on the Web. Table 4.6 shows a list of the most popular post-processing programs.
Start by producing a single dark frame that you will use for all of your raw frames. Do this by stacking all the dark frames you took and saving the result. Each program has a slightly different interface, but the basic task is the same.
After dark frame subtraction on each raw frame, divide each result by the flat field frame. At this point you have removed most of the thermal noise and accounted for most of the unevenness in the sensitivity of the CCD. This is what dark frame subtraction and flat field division do. You are left with a series of frames that represent the true theoretical image the camera received, plus a rather large amount of generally random noise. The noise makes the image look fairly undesirable, but stacking will help.
In many cases, a number of your raw frames will be hopelessly degraded. There are a variety of possible degradations, including polar alignment error, periodic drive error, planes and satellites flying through the exposure, blurred images caused by wind gusts blowing the mount, bad seeing and bad focus. Simply jettison any bad frames. In deep-sky imaging this is fairly easy because there aren't many frames to sift through.
You must carefully align all the frames to each other. In most programs you do this by designating one particularly good frame and aligning all the other frames to that frame. There are many ways in which alignment might be required, such as translation, rotation, and stretching. In practice, translational alignment is probably the only method you need to perform if your scope is polar-aligned.
Table 4.6. The most popular post-processing programs used for processing sets of images captured from long-exposure modified webcams.
Registax for Windows
K3CCDTools for Windows iMerge for Windows
Adobe Photoshop for Windows and Mac
Keith's Image Stacker for Mac
At this stage you generate the stack, which is a single operation without much effort on your part. How the image is represented on the screen depends on which program you are using, but you can be sure they are all performing some impressive tricks to squeeze the stack's large dynamic range into the mere 8-bit depth common to most image formats and computer screens. Nevertheless, the actual stack, as stored in the computer's memory, contains the full dynamic range of the combined data.
Stacking performs two feats at once. It increases both the signal-to-noise ratio (S/N) of the final image and its dynamic range (see Figure 4.11). It is the first of these, the S/N increase, that makes the stack look smoother and less grainy than the individual raw frames. The degree to which stacking accomplishes this feat scales with the square root of the number of frames you stack. So when you stack four frames, your S/N goes up by a factor of two. Notice, however, that to get another factor of two (so a factor of four overall by comparison to the raw
frames), you need not eight frames, as you might initially expect (because that is twice the previous stack), but sixteen frames. This explains why, as more and more images are added, the improvement becomes less discernible. For this reason, you should not get too concerned with whether you have 30 or 34 frames, or whether you have 100 or 120 frames. The results will vary by almost unnotice-able amounts in such cases. Once you have a stack you can do a lot of neat things to further refine the final result. Sharpening is generally more useful on planetary stacks, but it can be useful in deep-sky ones as well. There are a few different sharpening techniques, most of which are available in the stacking programs mentioned earlier.
To clean up the noise of the final stack further, there are some advanced methods of noise reduction, most of which use wavelets. It is always preferable to first reduce the noise through stacking, as stacking is the only method of noise reduction that is guaranteed to approximate the true-recorded signal. Other de-noising techniques, such as wavelet-shrinkage and expectation-maximization, make assumptions and approximations in their efforts to reduce noise.
Level adjustment is important. The stack will contain information from the bright cores of nebulae and galaxies as well as dim information from the perimeter of these objects. Proper level adjustment is crucial to bringing up the dim areas without blowing out the bright areas.
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