My Current H-A Solar Imaging Procedure

By Ed Abel

From the Coronado Web page:

“About solar filters...
The most effective method of viewing the sun is by isolating the Hydrogen-Alpha wavelength at 656.3nm and rejecting all other types of light. At this wavelength you are able to view all the features of the chromosphere, which originates just above the photosphere. These features include flares, prominences, filaments, spiculae, faculae, and sunspots. As the sun is constantly changing, the view through a Coronado filter is always new and exciting. Whether you watch a sunspot group march across the surface or a prominence explode thousands of miles into space there will always be something unique to view through our filters.”

“My” current H-Alpha solar imaging equipment includes Ron Lee’s Daystar 0.5 Angstrom filter and his SBIG STV camera. Bill McHale has loaned his 5” Vixen refractor, the aperture of which has been masked to 4”, producing an approximate f/8 system. When coupled with a 4 X Powermate, the f/32 images are quite stunning. Also being evaluated is my Astrovid 2000 Video Camera.

The most dramatic surface details surround sunspots. The Umbra and Penumbra are visible as dark and somewhat contrasting lighter areas. However, the spot’s surrounding surface features reveal the darker filaments, which can swirl, split, and be delicate or massive. The Solar limb reveals ever-changing prominences consisting of complex clouds or streamers of gas above or in the chromosphere. They can be seen with loops, jets, sprays, etc.

After the scope is correctly filtered and pointing at the Sun (NO small task), the live images possible with the STV camera make the VERY difficult manual focusing task more manageable. An electric focuser then allows very fine-tuning of the image without causing vibrations in the scope, which would severely impair final focusing. Even then, the air turbulence caused by solar heating causes significant image fluctuations and many, many blurred images. Capturing the camera output to video at 30 frames per second allows post-capture selection of the best frames in a 20 or 30 second “movie”.

The second most difficult operation is exposure and contrast control. Fortunately, the STV provides a wide control range. The ease and swiftness of camera response allow capturing either a surface detail (at, for example .004 second) or a Limb feature (at, for example .07 second) all within a matter of seconds. Capturing solar surface details with the Astrovid camera produces images, which do not have the grayscale range of the STV, and are thus less accurate depictions of the surface detail changes. The Limb details however do not depend on grayscale and are quite dramatic with the Astrovid.

Post-capture processing involves Histogram shaping and subsequent sharpening. I personally see details more clearly when they are displayed in color and have found that Yellow accentuates the surface better for me. I therefore colorize the images. Since I cannot simultaneously capture both the surface features (requiring less exposure) and the limb prominences (requiring more exposure), I composite two individually captured images when I want to display both surface and limb details in one picture.