Composite Solar Eclipse Imaging Techniques |
The spectacle of a total solar eclipse
elicits powerful reactions tempered by cultural
beliefs. Traditionally, the Chinese have interpreted
solar eclipses as a dragon swallowing the sun, Hindus as
Rahu devouring the sun, and South Americans as a puma
feasting upon the sun. More sensually, Polynesians
have construed solar eclipses to represent the passionate
embrace and lovemaking between the sun and the moon.
Regardless of interpretation, most total solar eclipse
observers are deeply moved by the experience.
Indeed, Charlemagne’s son, Emperor Louis, died shortly
after viewing a total solar eclipse in 840. Whether
this contributed to Louis’ death is not certain, but
during a recent eclipse, a colleague was so awestruck that
he felt a jolt in his chest caused by a cardiac
arrhythmia.
In an effort to capture the grandeur of a
total solar eclipse and to facilitate its study, early
observers sketched its appearance. With the advent
of photography during the 1800’s, details could be
recorded more precisely, even though a single exposure
could only capture features of the corona within a limited
dynamic range. To overcome this drawback, the idea
of producing a composite solar eclipse image based upon a
series of individual photographs of different exposures
was conceived. Thus, prior to the total solar
eclipse on July 29, 1878, the U. S. Naval Observatory
encouraged photographers to fit cameras with the largest
and longest focused portrait lens available and to expose
a photographic plate each 30 seconds during totality,
alternating between exposure times of 3 or 5 and 6 or 10
seconds (depending upon the speed of the lens). The
Naval Observatory collected these negatives and from them
created a composite sketch to reproduce all the details of
the corona that were revealed on the individual images
(Fig. 1).
Figure 1.
Eclipse Composite, 1878.
Not until more than 100 years later were
eclipse images combined into a photographic image.
Wendy Carlos used “a complex compositing method” in the
traditional darkroom to assemble details from three to
eight eclipse images. In the November 1994 issue of
Sky & Telescope, Steven Albers published a
composite created using thousands of lines of Fortran
computer code to process and digitally combine five
eclipse images. In the same issue, Kazuo Shiota
published a composite of six eclipse images that were
processed using Adobe Photoshop. The “technobabble”
in the article indicated that each image was processed
using Photoshop’s radial blur filter in spin mode to
create an unsharp mask, but details were sparse.
Gerald Pellett described more fully the
techniques he used to produce composite eclipse images in
an article entitled “Eclipse Photography in the Digital
Age,” published in the January 1998 issue of Sky &
Telescope. Like Shiota, Pellett used Adobe
Photoshop to apply the radial blur filter in spin mode to
each individual image. After experimenting with
different amounts of spin, Pellett concluded that 10
degrees seemed optimal for composites of seven to fifteen
images. He then subtracted each blurred image from
the corresponding original image, offsetting these images
by adding 128 to all pixel values and applying a scale
factor of 1 to produce a series of “difference”
images. These difference images were added
sequentially to create an “edge-detection” filter.
As each image was added, the result was scaled by 1 and
offset by subtracting 128 from all pixel values.
Pellett then multiplied the pixel values in the
edge-detection filter by those in the longest eclipse
exposure image (1/8 second in his sequence) to create a
composite image showing enhanced coronal detail. In
a modification of this technique intended to create a more
visually realistic (though less-detailed) composite image,
Pellett multiplied each difference image by the pixel
values in its original image, then added these modified
difference images using scale factors greater than 1 (such
as 1.4 for his seven-image composite), followed by
negative offsets. Others, including Benjamin
Gomes-Casseres, have suggested additional methods to
combine modified difference images, such as sequentially
adding the images by twos or using the DOS program ADDPIX
to add and average the pixels in RAW images.
Although these methods enhance coronal detail, they can be
quite tedious and time-consuming. An alternative
approach, described by Adobe’s Russell Brown, expands the
effective dynamic range of a series of individual images
by combining them in Photoshop using layer masks.
To create the composite eclipse image in
this article, I scanned seven film images taken during
mid-totality with exposure duration ranging from 1/60 to
1/2 second at f/8; three of these images are shown in Fig.
2.
Figure 2. Bracketed eclipse exposures, Zambia 2001.
By combining these images using Adobe
Photoshop, I obtained a composite photo that expanded the
dynamic range of the individual images to reveal detail in
both the inner and outer corona, thereby approximating the
actual appearance of the eclipse (Fig. 3).
Figure 3. Composite eclipse image, Zambia 2001. This approximates the actual appearance of the eclipse.
I then extracted additional features
of the coronal structure—even details that were not
visible when observing the eclipse—by applying a radial
blur filter analogous to that described by Pellett.
By changing the amount of unsharp masking and subsequent
blending, I adjusted the final composite image to optimize
the detail in the coronal structure. The specific
steps I utilized in Adobe Photoshop to create and sharpen
this composite image derived from scans of transparency
film are described in the article I wrote for the May 2006
issue of Sky & Telescope magazine, which is
available for purchase
as a PDF file from the magazine
(despite the listing, this is not an article on "elipse"
photography). The techniques remain valid, but with the
software currently available, I would incorporate Adobe
Camera Raw and HDR software into the process.
Applying these relatively
simple steps, I obtained composite images that showed an
amazing amount of coronal detail. Nonetheless, no
attempt to capture the grandeur of a total solar eclipse
would be complete without a dramatic image that requires
no compositing at all: the diamond ring (Fig. 4).
Figure 4. Diamond ring and solar prominences, Zambia 2001.
This
fleeting spectacle may be the most dramatic sight during
a total solar eclipse.
Copyright 2006, Maurice Hamilton. All rights reserved.