Corona During Total Solar Eclipse
August
21, 2017, Farewell Bend State Recreation Area, Oregon
by
Stephen D. Blazier
I wanted to see how
much of the sun’s corona I captured on that beautiful day, so I combined my
deepest exposures during totality. My longest exposures are 1/13
second at ISO 1600 and f/13, using my Nikon D700 with an AF Nikkor ED 180mm
f/2.8D IF lens coupled to a Kenko Teleplus PRO 300 DGX 2x AF Teleconverter, for
an equivalent focal length of 360mm.
I
took 37 of these
RAW images. I chose ViewNX 2 to color balance and convert to 16-bit TIFF for
further processing. To get the most out
of these I needed to take advantage of both the 14-bit pixel depth and the
signal to noise improvement from combining multiple frames. I converted the TIFF images to floating
point, aligned them using a Fast Fourier Transform algorithm, and then normalized
them by offset and scale. I used a sigma
rejection to get rid of outlier pixels, and averaged the rest together. I could have summed them, but since I was
working in floating point, the result is equivalent.
Those of us used to teasing faint objects out of the noise
have experience mapping from a higher dynamic range to the limited display
devices available, but in this case, I wanted both bright and faint detail in
the result. I chose to learn about High
Dynamic Range (HDR) processing.
It was
not easy. I find most HDR documentation
is either recipe based (e.g., specifying to click certain buttons in a program
without explaining what they do, then experiment with a set of parameters), or
analyze technical research papers.
I
chose a Debevec creation model, a triangular weighting function, and gamma
response curve to produce the HDR image.
Other creation models produced somewhat similar results, but like most
things with HDR, deserve further exploration.
To visualize this HDR image on standard devices, I applied Rafal
Mantiuk’s operator, as described in “A Perceptual Framework for Contrast
Processing of High Dynamic Range Images”, ACM Transactions on Applied
Perception 3, 3 (2006), pp. 286-308.
The
result is interesting in several respects, but does not match our actual visual
experience of the eclipse. We saw an
inner solar corona of pure white light.
I don’t believe any output device today can match the brilliance of that
inner corona. In my processed image, the
corona spans across the field of view, but it faded into the twilight-like sky
before that from our vantage in Oregon.
Moreover, we saw no hint of the Maria on the moon when we were watching.
Thanks for sharing your experiences viewing and recording the moments of totality with us. You went into another realm with complex post processing it seems. Well done. So far I haven't seen any real life amateur imaging that includes the Earth shine, except in a magazine.
I think Stephen explained in a separate email that the bright spot to the lower left was Regulus.
Keith
