Experimenting

Limb Detail

Lately, when the Sun has dramatic features, my skies are cloudy. When clouds clear, the Sun is relatively bland. Such was the case on August 24th when there were no major sunspots. Although we are near the predicted solar activity maximum, there have been few enormous, complicated, photogenic sunspots. Because my observing opportunities are so rare I need to use any one that comes along, even when showpiece surface features are absent. So, on August 24th, I tried to capture details near the solar limb.

This 12-image mosaic, made with a 2X Barlow lens, shows most of the solar disc on August 24th. Two small, barely visible sunspots are rotating around the Sun's western rim on the right. The biggest sunspot, number 1823, is to the right of center. Tiny sunspot 1828, with a dark filament to its lower left, is left of center.

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Next is a solo portrait of the most prominent sunspot, 1823, showing some limb detail to the right of the sunspot.

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Well, that's it for disc detail in this post. I've previously mentioned how difficult it is to capture disc detail and limb detail together in the same image. Limb features are underexposed when disc features are properly captured. Disc features are overexposed when limb features are properly captured. Since there were few disc features to be seen on August 24th, I decided to experiment with camera settings. I tried to adjust exposure time, gain, and gamma factor to capture at least some disc features while simultaneously getting good limb detail. The next image is one experimental result - an 18-image mosaic, made with a 2X Barlow lens, of the entire limb and part of the disc.

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Although disc features are overexposed, you can still see them. The prominences and spicules along the limb show up quite nicely, especially in the larger image obtained by clicking. In this image limb details also include a kind of haze just above the spicules. The haze can be removed manually, but it's quite a tedious time consuming job. Notice the large filament at the bottom to the left of center. You can follow its extent as it crosses from the disc over the edge. It shows how a filament seen against the solar disc becomes a prominence when viewed on the solar limb. Here is a solo portrait of this filament-to-prominence transition:

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A few solar imagers who post their work online somehow manage to capture very high resolution images of regions near the limb. I've seen pictures showing exquisite detail and seamless transition from disc features to limb features. I wish I knew the secret to obtaining these images. The image above is relatively poor compared to some of the best work online.

In the final image below, I blocked out the disc to exhibit limb detail alone. The false yellow color seems to show well on computer screens. I spent hours manually removing the hazy noise in this image. It's the best prominence and spicule image I've produced so far.

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