Capture the Sun with Your PST
High-quality hydrogen-alpha photography of the Sun can be done on a modest budget.
In this magazine's February 2005 issue, page 96, I reviewed Coronado's Personal Solar Telescope (PST) a groundbreaking instrument that brings hydrogen-alpha observing to amateur astronomers at an affordable price. For the review, I took a few images through the instrument in an attempt to approximate the view through the eyepiece. While my images were successful, they highlighted the PST's relatively small "sweet spot" that shows Hα features at maximum contrast. Thus the entire solar disk isn't evenly illuminated, and prominences are visible on one side of the Sun while not on the other until the PST's tuning ring is turned to display that area properly. Due to this shortcoming, I originally deemed the PST to have limited use as an imaging platform. Nevertheless, taking those images lit a fire of interest, and I decided to try pushing this little scope to the limits of its potential.
What I discovered is that excellent photographs of the Sun through the PST are possible once a few obstacles are overcome. In addition to the uneven illumination, the PST has limited back focus. While not a problem for visual use, the focal plane is very close to the top of the 1¼-inch eyepiece holder, and there is not enough back focus for an SLR camera. I had to shoot my photos using either eyepiece projection or a focal imaging (aiming a camera's lens into an eyepiece).
My first afocal images were taken with a borrowed Nikon Coolpix 990 and a Tele Vue 19-millimeter Plössl eyepiece. Soon, however, I purchased a Canon PowerShot A85 point-and-shoot camera. While I can't offer opinions on other camera models for use with the PST, the Canon turned out to be extremely well suited for solar imaging. I also purchased a commercial adapter to connect this camera to a Tele Vue 20-mm Plössl eyepiece.
By experimenting, I found that my best exposures were 1/160 second, with focus set at infinity and the camera's built-in optical zoom set to maximum (3x). Focusing was done with the higher-magnification 11x digital zoom, and critical adjustments were made with the PST's focus knob. Sunspots are the easiest to focus on, but filaments and prominences are useful when no significant spots are visible. I found that a small video monitor attached to the camera made focusing easier, because the larger image made it easier to see when the best focus point was established. When a monitor isn't available, I use the small screen on the back of the camera, placing a dark cloth over my head to shield the screen from bright sunlight and help me see low-contrast features.
To overcome the PST's uneven illumination, I capture several exposures, turning the scope's tuning ring slightly between each one. I then combine the pictures to average the detail captured in each exposure, thus creating an evenly illuminated photograph of the entire solar disk. I use from 3 to 20 frames, but I tend to capture at least three images at each setting of the tuning ring in an effort to get at least one during a moment of good seeing. Similar to what planetary and deep-sky astrophotographers experience, the more frames I stack, the smoother the resulting image and the more it can withstand subsequent image processing.
I use my Canon PowerShot A85 in black-and-white mode. Other cameras without this feature can still capture good Hα images in color mode, as James M. Weightman explained in his article in the July 2004 issue, page 137.
Once I've captured my exposures, I download them to my computer. The easiest way to align and stack a series of images is to bring them into the freeware program RegiStax. However, because my frames differ from one another due to the PST's uneven illumination, the same solar features don't always show on every frame, so RegiStax may not have a consistent alignment feature. In such cases, I manually align each image in Photoshop. I start by opening all the frames, discarding the blurriest ones, and pasting the best ones over the first image in the series. Using keyboard shortcuts, I can make quick work of selecting the image (Control+A), copying (Control+C), pasting (Control+V), and closing the now-copied image (Control+W). When my image stack is complete, I save the file as a Photoshop document (PSD).
To align each image (each "layer" in the jargon of Photoshop) to the base image (called "Background" in the Layers palette), I first hide every layer except the Background and Layer 1 by clicking the small eye icon to the left of each layer I want to hide. I then change the blending mode of Layer 1 from Normal to Difference. This causes the two layers to cancel out when detail is aligned, and to highlight non-overlapping detail. I use the Move tool to roughly slide Layer 1 around until it cancels out most of the Background, and then switch to the arrow keys on the keyboard to make the final, small adjustments. Once these layers are aligned, I change the blending mode of Layer 1 back to Normal. I repeat these steps until all layers have been aligned to the Background, then save the file.