Sky & Telescope - January 2021 - 61

Retaining Color
One of the biggest challenges when processing galaxy imagery
is retaining color in the central regions. Increasing the brightness and color in a galaxy while maintaining a smooth look in
the fainter regions of the field often produces a near-colorless
nucleus. The key to maintaining color in the core of a galaxy
is to preserve it early in the processing workflow. Color can
appear diluted when the brightness of an image exceeds 80%
of the maximum value. This is due to how our eyes and brain
perceive brightness and color. While there often is a colorless
core in many galaxies, this should generally be the size of a
typical star in the image. Not long ago, the digital development
process (DDP) was the go-to tool used to manage this difficult area in most galaxies. Today, there are far more facile and
powerful tools, such as HDRMultiTransform in PixInsight, that
compress the dynamic range in an image while allowing for
small-scale enhancements in the bright regions.

Star Treatment
Without exception, galaxies appear to float behind a foreground of stars residing in our own Milky Way. Quite often
the stars are uniformly distributed across the field and play a
lesser role in the final composition. The galaxy (or galaxies) is
the focal point of the image, and, to the degree that processing choices affect the stars, I may permit them to be modified
by the same processes, or in some cases, I may create masks
to shield them from these effects targeting the main subjects
of my picture.
Sometimes it's easy to become overly concerned with
retaining star colors in the final composition even when
those decisions negatively affect the appearance of the
galaxy. Examples in which this can happen include galaxies seen through prominent arms of the Milky Way, such as
NGC 6946, NGC 6674, and IC 342. In such cases, it's often
a better approach to de-emphasize the stars in these fields so
that the galaxy isn't competing with them for the viewer's
attention. The exception is when a star (or group of stars) is
very bright, you can use it as a balancing weight within the
framed composition. This is usually done by placing your
galaxy at one side of the image frame with the star (or stars)
at the opposite end. Using these stars as a compositional element, it then becomes important to retain as many attractive qualities of the stars as possible.

Noise Reduction
Due to their faintness, galaxies present challenges in managing the amount of noise reduction applied to an image. Many
nebulae have smooth brightness and color transitions across
large areas. The faintest portions of these nebulous regions
may look like the background sky itself and, often in the
interest of contrast, are made to appear very dark without
taking away from the overall impact of the image. By comparison, all but the nearest galaxies are smooth objects with
small highlights of color and detail. Even modest noise-reduction techniques can adversely impact these elements. But the

S OVER CONTRAST Top: Increasing the contrast of dust lanes in a
galaxy may be appealing but can lead to loss of color and other details in
the image. Bottom: This picture of NGC 4698 avoids this tempting step,
allowing other interesting details in the galaxy like the golden tone of reddened starlight in its dust lanes to shine through.
sk yandtelescope.org * JANUARY 2 021

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Sky & Telescope - January 2021

Table of Contents for the Digital Edition of Sky & Telescope - January 2021

Contents
Sky & Telescope - January 2021 - Cover1
Sky & Telescope - January 2021 - Cover2
Sky & Telescope - January 2021 - 1
Sky & Telescope - January 2021 - Contents
Sky & Telescope - January 2021 - 3
Sky & Telescope - January 2021 - 4
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