PROCESSING SOLAR
IMAGES
Processing H-alpha
Solar Images
1. Use a monochrome camera that produces an AVI video file. Monochrome cameras made by The Imaging Source, Point Grey Research, and Lumenera, work well for solar imaging. You can shoot h-alpha images of the Sun with a color camera too, but you will find that you have to do extra processing to pull out the detail from the red, green, and blue channels. The red channel will often be overexposed, the detail you want to capture will be in the green channel, and the blue channel will be very dark. Therefore you end up processing the green channel only, and throwing away the red and blue channels. Use a monochrome camera to save yourself the extra work.
2. The solar disk detail in h-alpha is usually much brighter than the prominences on the Sun’s limb. Therefore it is best to shoot two separate AVI files, one exposed properly for the disk, and another exposed properly for the prominences (with the disk overexposed). The two images will be combined later in Photoshop to produce a single image that shows both well. I usually try to shoot at least 600 frames per AVI file. You can shoot more, but if you shoot too many frames, your computer may have problems processing the AVI file, depending on how much memory your computer has.
3. Load the AVI file(s) into a program that can sort through video frames, sort out the best quality ones, and then stack them to create a high quality, low noise, image. Programs that are popularly used for this include Registax, and AutoStakkert!2. These programs have other useful features that can be used to enhance the image such as wavelet sharpening.
4. In Registax, load the AVI file you wish to process. Step through the individual frames to find and select a sharp frame when the air was steady, to be used as a reference frame for stacking. I usually use 80% for the number of frames to keep. Once stacking is complete, the screen for wavelet sharpening will appear. For full disk images that have very small features, I only adjust the lowest slider, and not very much, to keep the image from looking overprocessed. With higher magnification images, such as of individual sunspots, I start out by adjusting the lowest sliders first, and then the succeeding higher numbered ones, until the image is sharpened enough, without looking over processed, or noisy. Save the processed monochrome image as a 16-bit PNG file.
5. Now load the PNG file(s) into your favorite image editor, such as Photoshop, or the GIMP. Use the clone stamp tool to edit out imperfections in the image, such as dust donuts, crop out unwanted parts of the image, and adjust the brightness (using curves and levels) and the brightness and contrast.
6. To make a mosaic from several images that you would like to combine into one image, use the Photomerge function in Photoshop, and then crop off the uneven edges.
7. Some people like to invert (make a negative of) the image at this point because they believe it adds a 3D appearance to the detail on the disk. If you want to do this, make sure you do it before adding any color. I prefer positive images however, because they look more natural.
8. To add false color, use the curves tool. For the disk detail, I like a yellow orange cast. Select the red channel, and pull the line up a bit in the center to give the image a reddish cast. Select the blue channel, grab the upper right end of the line and pull it down to the bottom. Select the green channel, and pull the line down a bit in the center. You should now have a yellow orange image. Continue to adjust the curves until you are happy with the final color.
9. Save the processed file as a 16-bit PNG file. For e-mailing and posting on my website, I use Photoshop to convert the image to 8-bit, downsize it to no more than 1200 pixels wide, and save it as a JPG file.
Processing CaK Solar Images
1. I use the same technique to process CaK images as I do h-alpha images, until I add false color (step 8). Some people like their CaK images to appear deep purple. I find it hard to see the details on the disk however, using that color. I prefer a blue color instead. Select the red channel, and pull the line down a bit in the center to give the image a purple cast. Select the blue channel, and pull the line up a bit in the center add blue. Select the green channel, and pull the line up a bit in the center. Continue to adjust the curves until you are happy with the final color.
2. Prominences are visible in CaK, but they are much dimmer with respect to the brightness of the disk, than they appear in h-alpha images.
Processing White
Light Solar Images
1. I use the same technique to process white light images as I do h-alpha images, except that only one AVI file is required, because no prominences are visible in white light. When adding false color (step 8), I prefer a yellow color instead. Select the red channel, and pull the line up slightly in the center to give the image a reddish cast. Select the blue channel, and pull the line down a bit in the center to add yellow. Leave the green channel alone. Continue to adjust the curves until you are happy with the final color.
2. Unlike h-alpha or CaK imaging, a color camera will work nearly as well as a monochrome camera to capture sunspots, unless you use a green (continuum) filter to enhance the visibility of granularity. Then you will have to eliminate the green color cast.