…continuedCapture the Sun with Your PST
The next step is to average these layers to combine the features recorded in them, as well as to increase the signal-to-noise ratio. (By stacking images, random noise is reduced, while detail that repeats is reinforced.) In Photoshop, the best way to average exposures is to change the opacity of each layer sequentially from the bottom to the top of the stack. The rule for achieving a proper average is to have each layer’s opacity set according to its location in the stack divided into 100. For example, the Background layer will be 100%, Layer 1 (the second image in the stack) will be 50%, Layer 2 will be 33%, and so on. I find a practical limit for this technique to be about 20 layers, with anything more offering no visible gain. At this point I flatten the image stack and save it as a new file, preferably in a format such as TIFF that doesn’t compress the data.
Once I have a stacked image with roughly even illumination across the entire solar disk, it’s time to enhance the contrast and sharpen the detail. I start by opening the Curves palette and adjusting the image to display the surface features in high contrast. This usually subdues prominences, so I save the file with a new name, such as "disk curve.tif." I then open the original file again and apply another curve that enhances the contrast of the prominences, this time at the expense of disk detail. I save this image as "prominence curve.tif." Next, I'll bring these processed files into RegiStax separately and use the program's excellent wavelet filter to sharpen them. I prefer this technique to Photoshop's Unsharp Mask because of the extended control RegiStax provides. Once they’re sharpened, I save the images as bitmap (BMP) files and bring them back into Photoshop to combine.
I start with the disk image and open Color Range from the Select menu. Using the Eyedropper tool and holding the Shift key, I click on different parts of the area outside the Sun's disk until the Selection Preview displays a view with the background completely white and the solar disk blacked out. I then click OK. I now have a selection line around the solar disk, but slightly farther away from the disk than I want. I then use the drop-down menu Select > Modify > Expand, and increase the area of my selection by about 4 pixels, which reduces the radius of the circle around the solar disk. I also use Select > Feather and soften my selection by 2 pixels. I copy this and paste it onto my prominence image. Usually I need to align the disk and prominence images, using the same procedure as before.
I now have a decent image of the Sun, but there are still a few steps remaining. I like my solar images to be in color, but images through any solar Hα telescope are only red. I prefer a color palette of yellow, orange, and red, which highlights contrasting features better than a single red palette. To colorize the picture, I convert the image from Grayscale to RGB color and adjust the color channels using the Curves function. I first select the prominence layer. Using the drop-down menu Image > Adjust > Curves, I reduce the blue channel to zero, lower the green curve’s midpoint, and boost the red channel so that my prominences have a fiery red appearance. I then select the disk layer and adjust the curves slightly differently, reducing the blue channel to zero, raising the red midpoint, and adjusting the green curve until my disk is an orange ball with reddish filaments.
Once I'm satisfied with the color adjustments, my final step is to reduce the limb darkening on the solar disk. I make a duplicate layer of the disk image by selecting the drop-down menu Layer > Duplicate Layer. I carefully use the Clone- Stamp tool to replace prominent features with similarly shaded areas around them until I have an image of the disk devoid of large-scale features. I then apply a Gaussian Blur filter set to a radius of 25, which eradicates small features. I'm left with a soft layer that shows mainly the disk illumination I wish to correct. With this layer selected, I now use the drop-down menu Image > Adjust > Invert to make a negative image of the disk illumination. By changing the layer-blending mode to Overlay and lowering the opacity to 50%, I get a view with edge darkening reduced to an acceptable level. I can now flatten the image layers and save the final result.
Some of the techniques I use in Photoshop can be accomplished with other software, and I often jump between several programs. But Photoshop tends to be my "do everything" program when all else fails, and all the steps I describe here usually take me less than an hour from image capture to final product.
By following these steps, you too can produce excellent images of the Hα Sun on a small budget. And unlike nighttime astrophotography, you won't lose any sleep over it!
On clear days, assistant editor Sean Walker spends lunchtime outside the Sky & Telescope offices with a dark cloth over his head.