Sky and Telescope - March 2018 - 36

Imaging Aesthetics

astronomical deconvolution ﬁlters, nothing can completely
make up for a poorly focused image.
Fortunately, we can mitigate the effects of seeing in our
images in several ways. Monitoring the weather and seeing
conditions is easy using websites such as the Clear Sky Chart
(cleardarksky.com) or smartphone apps including Scope
Nights that give detailed predictions to let you know when the
air is steady or not.
You can still take sharp astro-images in poor seeing if you
switch your pixel scale to better match the conditions you're
shooting in. For instance, if the seeing produces bloated stars
measuring about 3 or 4 arcseconds across, you can switch to
a short-focal-length instrument with your camera that produces an image scale of around 4 arcseconds per pixel. Also,
if you're using an astronomical CCD or CMOS camera, you
can bin the pixels, which groups four adjacent pixels together
to function as a single pixel, reducing the resolution of your
camera and masking the ill effects of poor seeing. If the seeing is really bad, you can simply switch to low-resolution,
wide-ﬁeld imaging, targeting entire constellations, or even
photographing nightscapes. The trick is to match your equipment to the conditions you're shooting under.

p NOT BLACK Establishing good contrast in a deep-sky photo doesn't
simply mean making the sky black and the stars white. Setting the
background sky to black would have lost all of the beautiful dust in this
exquisite image of NGC 7497.

Tracking

M A RCH 2 018 * SK Y & TELESCOPE

calculates how long an exposure can be with a particular
lens: 500 / ﬂ = T, where ﬂ is the focal length of your lens, and
T is how long you can expose before stars noticeably trail.
This works especially well with today's low-noise DSLR and
mirrorless cameras operating at high ISO settings, permitting
you to capture subjects like the Milky Way over a picturesque
landscape, meteor showers, and auroras. In fact, some trailing in a nightscape image doesn't detract from the overall
scene, so you can shoot up to 30 seconds with ﬁsheye and
other extremely wide-angle lenses.

Color, Contrast, and Saturation
Unless you're shooting the Moon or concentrating on a speciﬁc monochromatic wavelength of light such as hydrogenalpha, pleasing color is an extremely important aspect in the
best astrophotos. Note that I say pleasing rather than accurate
- color perception is somewhat subjective, and most people
experience color slightly differently. False-color narrowband
imaging has become quite popular as well.
That said, a natural-color image of the night sky taken
from a dark site should generally have a neutral sky background. The reddish-brown cast recorded in nightscape
photos should be corrected (S&T: Jan. 2018, p. 36), with the
exception of reddish or greenish natural airglow. Likewise,
a high-resolution image of a spiral galaxy should still allow
your eye to determine where the galaxy ends and the sky
background begins. Determining this boundary in a sprawling nebulous ﬁeld can be tricky but still manageable.
Even tri-color narrowband images of deep-sky objects,
which rely on various false-color palettes or combinations,
still require a pleasing color balance in order to present the

A DA M BLOCK / M OUN T LEM M ON SK YCEN TER / UNIV ERSIT Y OF A RIZON A

One aspect unique to astrophotography, compared to other
types of photography, is tracking. Because our planet rotates,
objects in the sky outside of the atmosphere are perpetually moving with respect to your camera and telescope.
This requires a way to cancel out that movement using a
motor-driven mount aligned to Earth's rotational axis. Good
tracking is most important in telescopic close-ups of galaxies, nebulae, and star-clusters - faint targets that require
long exposures to reveal them properly. Perfect tracking also
ensures you're able resolve the smallest details possible for
your equipment. Otherwise, small-scale features become
smeared, mimicking poor focus.
Few mounts track perfectly - most have small, repetitive errors known as periodic error that require corrections
during an exposure. You can deal with this by attaching an
additional camera (called an autoguider) to a guide scope or
off-axis guider that monitors a single star during the main
exposure, and automatically makes small corrections to
ensure a perfectly guided image.
Fortunately, perfect tracking isn't necessary for every
astrophoto. Because the Sun, Moon, and planets are relatively
nearby and bright, they can be adequately recorded in exposures of a fraction of a second and thus only require enough
guiding to keep the target on your camera's detector.
One type of astrophotography that doesn't require tracking at all in most cases is nightscape photography. Shooting
the night sky over a picturesque landscape with a simple
camera-on-tripod setup and wide-angle lens produces such
low resolution that you can get away with exposures of, say,
10 seconds without any noticeable trailing.
There's a simple formula known as the "500 rule" that

http://www.cleardarksky.com

Sky and Telescope - March 2018

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