Sky & Telescope - January 2021 - 62

Galactic Advice

faintest outer extents of galaxies are
often exciting features that we would
like to highlight in our images and that
require significant noise reduction.
Some examples include arc-like shells
or stellar streams that hint at ancient
acts of galactic cannibalism. The difficulty is creating the selections or
masks that apply a smooth transition
from the high-signal areas where little
or no noise reduction is required.
One of the easiest and most powerful techniques I employ is to simply
magnify and process images at 200%
or greater while working with noise-reduction tools. At this
scale, I can see if I've made a reasonable choice with any given
process, or if I've gone too far. A galaxy with a silky smooth
outer halo and sharp, small-scale features in the brighter portions (including H II regions and foreground stars) can make
a jarring transition for the eye to follow and forces the viewer
to concentrate on the imager's processing choices, rather
than the galaxy itself.
An edge-on spiral galaxy that illustrates this point well is
NGC 5866 in Draco. Without applying strong non-linear compression and selective noise reduction, it would be impossible
to see its narrow dust lane, yellowish core, and bluish arms

COMPOSITIONAL BALANCE Usually stars play a minor
role in a galaxy image and don't require special treatment.
The exception is when some bright stars, like the ones
north of NGC 5792, play a supporting role in the composition. In such cases, it's often beneficial to retain their
color and not have them appear as over-exposed dots.

62

JANUARY 2 021 * SK Y & TELESCOPE

W JARRING TRANSITIONS Another minor
detail to be aware of is the transition between
areas that require noise reduction and the
regions surrounding stars. In this close-up,
the pixelated noise surrounding each star will
draw viewers' attention, forcing them to focus
on this minor oversight in the final image.

simultaneously with its complex outer
halo. With these considerations the
otherwise featureless Spindle Galaxy
is transformed into an edge-on galaxy
with a visible history of galactic interaction with large tidal tails and shells.

A Good Plan
One final approach I use when processing galaxy imagery is
to take inventory of all the interesting attributes and details
in my subject early on. In most cases, significant details and
features within a galaxy should be visible in the calibrated
but unprocessed data in its linear form. If, later in processing, details and colors emerge that were not initially evident
in the raw data, then there's a good chance these details
are simply artifacts introduced by the processing. Additionally, by noting real features early in your plan, you can make
processing choices better suited to reaching a final result that
renders all of them well.



Sky & Telescope - January 2021

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Sky & Telescope - January 2021 - 1
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