Sky & Telescope - March 2020 - 32

Eclipse Imaging

Revealing Coronal Loops

p Applying the blurred image to its corresponding image stack using the Subtract blending mode produces a detail map that is then
stretched to reveal low-contrast details within the solar corona as well
as some lunar features.

p Applying the subtracted
image to the raw stacked
image using the Multiply blending mode produces a detailed, though
noisy, picture. Reducing the opacity of the target between 50% and
75% can achieve a more pleasing result.

To bring out additional ﬁne detail in the corona, open the ﬁrst
stacked image (group 1.tif) and blur it using the Gaussian
Blur ﬁlter (Filter > Blur > Gaussian Blur). In the ﬁlter window,
change the Radius to 3 or 4 pixels (the larger value will add
more contrast to your completed photo, though at the cost of
suppressing the smallest-scale features), depending on the focal
length of the optics you shot with. Save this image with a name
that reﬂects the process used, such as "group 1 blur.tif".
Keep this blurred image ﬁle open and reopen the original
stacked image (group 1.tif). Next, apply the blurred image
to the original by selecting Image > Apply Image from the
dropdown menu. In the Apply Image command window, ﬁrst
change the Source from group 1.tif to group 1 blur.tif. In the
Target section, change the Blending mode to Subtract, and
increase the offset to 128. Click OK, and a very low-contrast
map of small-scale differences between your original image
and the blurred version remains, though it will appear gray
with a dark ring where the limb of the Moon is located. We
need to signiﬁcantly compress the dynamic range of this
image by opening the Levels tool (Image > Adjustments > Levels). In this window, change the low value slider at left to 126,
and the high value slider at the right to 130, and click OK.
This mapped image should now display the edges of coronal loops and streamers, and some background stars, albeit
with a lot of noise. Save this image as "group 1 subtract.tif".
Next, you need to apply this detail map to the original
stack. Open group 1.tif and again select Image > Apply Image
from the dropdown menu. Here you have several options
depending on how aggressive you want the processing to
appear. Change the Source to group 1 subtract.tif and the
blending mode to Multiply. Immediately, you'll see lots of highcontrast loops within the corona, though it will be extremely
noisy and rather "overcooked." When applying this process, I
prefer to lower the Opacity to about 75% before clicking OK.
This result displays a lot of coronal detail but is very noisy.
You can reduce the noise by opening the Camera Raw Filter
again and dropping the Texture slider to -50.

p Left: To correct the sharpening artifact around the lunar limb, return to the original stacked image and crop it with the Moon centered. Middle: Create a blurred image using the Radial Blur filter set to Spin at 3 pixels, then apply it to the cropped Moon picture and stretch the result using the Levels
tool. Right: Applying the radial-blurred subtraction map to the cropped Moon image with the same settings used for the full eclipse stack results in an
image showing coronal detail right to the limb of the Moon.

MARCH 2 02 0 * SK Y & TELESCOPE

Sky & Telescope - March 2020

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