A standout image of the Sun
How to construct a stunning solar image by concentrating on activity at the Sun’s edge
hen will this all be over? We’ve all been asking the question during COVID-19, but it has also been on the minds of solar astrophotographers for an entirely different reason. Our Sun entered the depths of solar minimum back in 2018 and we are wondering when it will wake up. What can a solar photographer do when the Sun constantly shows a blank face?
Our answer is to look at the Sun’s edge, while remembering to stay safe – never look at the Sun directly and always use solar filters on any equipment that is pointed towards it. Even on days when the disc is spotless, the limb often sports handsome prominences. These arcs of hydrogen plasma are photo-worthy subjects for any astronomer equipped with a hydrogen-alpha (Ha) filter. Using a 90mm Ha Coronado etalon filter mounted on a 92mm f/4.8 refractor gives us a versatile setup that can cover wide-field and closeup imaging. With a small black and white streaming camera we can shoot wide to capture the full solar disc or, by adding a Barlow lens, zoom in on specific activity.
On the day of the ‘A Little Fireworks’ capture, in July 2018, the prominences were spotted with the telescope and eyepiece, before switching to the camera to capture the frames for processing. We captured two sets of data, one for the prominences and one for the surface detail. As atmospheric
Wturbulence was low, we were able to add a Barlow lens to increase the magnification, improving the view of the prominences. Critical sharp focus was attained with ease by using live view on a laptop screen.
We tuned the filter for brightness and detail, set gamma to low and turned the gain off. The capture duration was set for one minute, which yielded a stream of 1,800 individual frames. At this magnification, an active prominence can show movement over a short time, so it’s important to plan the duration of capture so that movement does not result in any blurred detail after stacking.
In the frame
Software such as RegiStax or AutoStakkert! can be used to analyse the stream of frames, sorting them from sharpest to blurriest. We selected the best 25 per cent, which were aligned and stacked ready for processing. Be sure to use multiple points (see image, opposite, top) when aligning the frames to maximise sharpness across the entire field of view. The resulting stack will provide a smooth image, allowing further processing to tease out the finest details.
Open the stacked image in Adobe Photoshop or your favourite image-processing software. Most dedicated astronomy image-processing programs will provide a sharpening tool so select this. Great attention should be paid to selecting settings that render additional detail but do not add artefacts.
is an artist and astrophotographer who is based in Buffalo, New York. He won the ‘Our Sun’ category at the IIAPY in 2019 with ‘A Little Fireworks’
Three variations of image presentation: (left) the prominences with a black occulting disc and a blue circle to show Earth’s size to scale; (middle) with added chromospheric detail; (right) the final result with colour