Sky and Telescope - January 2018 - 38

Image Processing
t WORKING IN
LAYERS Every
adjustment to an
image should be
done on a new
layer, in order
to preserve the
original image;
changes can be
undone if necessary. A copy of the
original photo is
created, then this
layer can be used
to adjust particular
aspects of the
original, such as
color or contrast.
t COLOR
BALANCE Most
night-sky images
coming out of your
camera will have
a reddish-brown
cast. This is often
easiest to correct
on the Color layer
using the Auto
Color command,
or using the author's free plug-in
filter WhiteCal
(shown at left).

as the conditions change. I set my camera color proﬁle to
daylight, so all my RAW images often feature a warm, brown
color cast that I then correct on the computer. There are
two quick ways to perform an easy but often effective color
balance. Note that while I describe these steps while using
Photoshop CC, users can use similar approaches with other
image-processing software.
Since I'm working with color information, I begin by opening the layers window in Photoshop and creating a duplicate
layer from my original image. I then change the layer blending mode of this new layer to Color, and retitle the layer from
the default title "layer 1" to "color." I then select this new
layer and apply the Auto Color function from Photoshop's
pull-down menu (Image > Auto Color). This sometimes
produces a good result. If the colors appear a bit washed out,
I can bring them back by increasing the color saturation
(Image > Adjust > Hue/Saturation).
If Auto Color doesn't produce the desired outcome, a second approach is to try the plug-in ﬁlter WhiteCal that I wrote
several years ago (https://is.gd/QcKYT0). The plug-in works
by adding it to the ﬁlters folder in Photoshop, then selecting a
spot in the image that you consider to be a good white reference using the Lasso or Magic Wand tool. Select something
that should be a neutral color, then run the ﬁlter using Filter
> DeepSkyColors > WhiteCal. The plug-in will then adjust the
colors on the entire image so that the average color of your
selection is neutral.
But either approach can sometimes fail. When using
WhiteCal, I tend to experiment with different white reference
selections until I'm pleased with the result. If there's nothing
in the image that I can use as a white reference, I'll try selecting a small area of the sky background, preferably near the
top where the sky color tends to be more neutral.
Which of these tools give the best results for any given
image is something you'll need to experiment with every time
they're used. More often than not, the color of your image
should improve with just a couple of clicks. Once you're
satisﬁed with the color balance, bring up the Hue/Saturation
tool to intensify the color. Moving the saturation slider to
somewhere between 10 and 30 usually works well.

Selective Processing

p LAYER MASKING A critical step in nightscape processing is to address the sky and the landscape separately through layer masks. Two
copies of the background layer are created (called Land and Sky) below the color layer, and a layer mask is then added to the latter. Paste
another copy of the original image into the layer mask, which can then
be adjusted using the Levels tool to make a high-contrast mask.

JA N UA RY 2 018 * SK Y & TELESCOPE

At this point you should have two layers: the original image
(called background in the Layer window) and a color layer. It's
time to create two new layers, both also duplicating the original layer by clicking on the background layers and selecting
Layer > Duplicate Layer from the pull-down menu. Rename
these new layers "Land" and "Sky." The layer list should then
appear as follows, from bottom to top: Background, Land,
Sky, and Color.
These layers are necessary because nightscapes recorded
when the Moon isn't up tend to have a nice, bright sky but
dark land features, appearing almost like a silhouette. You
might not have this problem if there was some light illuminating the foreground - street or car lights, or your own

https://www.is.gd/QcKYT0

Sky and Telescope - January 2018

Table of Contents for the Digital Edition of Sky and Telescope - January 2018