2017 Total Solar Eclipse, High Dynamic Range Corona

The subtle and ethereal beauty of the corona, the Sun's outer atmosphere, is seen in a composite image of the total solar eclipse of August 21, 2017. Here the corona spans more than 5 solar radii and actually extends out of the field of view. The bright star Regulus (Alpha Leonis) is at top left, and magnitude 5.26 Nu Leonis is at right. Red prominences can be seen on the west limb of the Sun as well as Earthshine on the Moon.

The Solar Corona

Intricate detail in the corona is caused by the Sun's magnetic fields which get twisted up and when they release a tremendous amount of stored magnetic energy may contribute to the heating of the corona. The corona is very thin, but has a temperature of more than a million degrees Kelvin and actually gets hotter farther away from the Sun.

The solar corona can only be seen during a total eclipse of the Sun. It is one of the rarest and most beautiful sights in nature and seeing it may be a once-in-lifetime experience. The corona extends millions of miles into space and is made up of super-heated plasma at about 2 million degrees K. The light from the visible corona comes from three sources:

The K (Kontinuierlich) Corona is closest to the limb of the Sun and is made up of sunlight scattered by free electrons. The Fraunhofer lines of the photospheric spectrum are smeared out so that the spectrum of the K corona is almost a pure continuum. The K-corona is the brightest part of the visible corona out to 1.5 solar radii.

The F (Fraunhofer) Corona is comprised of sunlight scatter by, or reflected off, dust particles and actually merges into the Zodiacal light. The F corona is the brightest part of the corona beyond 1.5 solar radii from the Sun's limb.

The E (Emission) Corona is made up of narrow spectral emission lines produced by ions in the hot plasma. The E-corona is much fainter than the K and F coronas.

Scientists list several other types of coronas which are generally only visible in the infrared:

The T (Thermal) Corona is caused by thermal emission in the infrared region by interplanetary dust.

The S (Sublimation) Corona is made up of the emission of low ionized atoms produced by sublimation of dust particles in relatively cold parts of the corona.

The Fl (fluorescence) Corona is caused by resonance fluorescence of molecules and free radicals produced by interactions of the K- and F-corona, solar wind and sunlight.

The IRF (Infrared fluorescence) Corona is caused by infrared fluorescence of silicon nanoparticles produced by interaction of the K- and F-corona and sunlight.

Photographing the Corona

The Sun's corona has a tremendous brightness range spanning more than 14 photographic stops. Normal cameras can not record this much of a difference in brightness in a single exposure. The correct exposure for the sky background and outer corona is 2 seconds at f/6.3 at ISO 200. The correct exposure for the bright red prominences along the Sun's limb is 1/4000th second at f/6.3 at ISO 200. The long 2 second exposures will completely overexpose the bright portions of the scene, and short 1/4000th second exposures will not record any of the faint outer corona.

To deal with this huge dynamic range, a series of different exposures are made during totality, each correctly recording just one part of the corona. They are then composited together with special high-dynamic range techniques which more closely approximate what the human eye can see. The human eye, a remarkable visual instrument, can encompass almost the entire brightness range of the total eclipse. Only the Earthshine, which was recorded in the longest exposures, was very difficult to see visually.

Image Acquisition

Image Acquisition was completely automated with Solar Eclipse Maestro.

Processing the Corona

Each individual image of the corona at various shutter speeds were calibrated with 64 flats, 64 bias in Images Plus. Exposures longer than 1/15th second were dark subtracted with 64 matching dark exposures. Exposure sets were then averaged together in Images Plus. A background color image was created from an HDR Pro stack in Photoshop CC 2017. Each individual exposure set was then blurred with a radially graded filter and subtracted from the original manually to produce a "difference" image. Difference images were then added together manually to create a luminance HDR image that spanned the full brightness range of the corona. The luminance detail image was then combined with the color HDR background image. All HDR processing in Photoshop CC 2017.

Total Time Invested

General life preparation: 23,243 days 10 hours 20 min 33.8 seconds

Eclipse specific research and preparation: 378 hours

Fly, drive and scout time round trip: 28 hours

Totality: 1 minute 55.8 seconds

Total exposure time during totality: 15.99951171875 seconds

Processing time: 18 hours

Exposure Data

Scope: Astro-Tech AT65Q

Astro-Tech AT65Q Working Focal Length: 420mm

420mm Focal Ratio: f/6.5

f/6.5 Camera: Unmodified Canon 7D Mark II

Unmodified Canon 7D Mark II Filters: None

None Exposure: High-Dynamic-Range Composite of 36 frames at 14 different exposures: 1/4000th second x 6 frames 1/2000th second x 6 frames 1/1000th second x 2 frames 1/500th second x 2 frames 1/250th second x 2 frames 1/125th second x 2 frames 1/60th second x 2 frames 1/30th second x 2 frames 1/15th second x 2 frames 1/8th second x 2 frames 1/4th second x 2 frames 1/2th second x 2 frames 1 second x 2 frames 2 seconds x 2 frames

High-Dynamic-Range Composite of 36 frames at 14 different exposures: ISO: 200

200 White Balance: Daylight

Daylight Mount: Losmandy GM8

Losmandy GM8 Guidescope: None

None Autoguider: None

None Location: Bandit Springs, Oregon

Bandit Springs, Oregon Date: August 21, 2017

August 21, 2017 Processing: Photoshop CC 2017

Special Thanks

Special thanks to friends and family who helped me with this eclipse - especially to Drew Maser and Joe Stieber, who lent me equipment without which my pictures would not have been possible. Also thanks to Steve Mattan for lending me a wide-angle lens; Jim Sweeney for help driving and scouting; Arnie Rosemoff for driving a lot of the equipment back home; Dave Neunheuser for machining some counterweight locks; Brandon and Michael - my two sons, for computer support; and my wife, Kathy, for moral and emotional support and for putting up with me working 18 hours a day preparing in the three weeks leading up to the eclipse, for being there with me to experience this special event, and for putting up with me working 18 hours a day in processing the images in the two weeks after the eclipse!