http://www.skyandtelescope.com.au 61Recognising what goes into an
outstanding image can help to
improve your own work.
W
e’ve all marvelled at the amazing celestial images
produced by both amateurs and professionals.
From dazzling pictures of the Milky Way arching
over an equally majestic landscape, to glittering portraits of
spiral galaxies exploding with starbirth, we all know a great
image when we see one.
So what actually makes for a supurb astrophoto? This
question is surprisingly difficult, if not impossible, to answer.
One reason is that our personal tastes are often as unique as
individual snowflakes — beauty is in the eye of the beholder.
What one person finds attractive might not make the grade
in someone else’s eyes. Another reason is that often it is a
combination of things that takes an image to a higher level.
Still another consideration is the subject — what makes for
an outstanding nightscape image will be completely different
than, say, what makes an outstanding planetary picture. So let’s
look closely at elements found in images that most can agree are
top-notch, and incorporate those into our own photographs.Focus
One of the first things we notice in any astrophoto is
whether it’s in focus. To be fair, it can be quite tricky to
nail down focus with most astrophotographs. With the
exception of images recorded from space-based observatories
and planetary exploration missions, every astro-image you
see is shot through Earth’s usually churning atmosphere,
bad ‘seeing’ that often results in fuzzy, blurry photos. And
while some detail can be rescued using sharpening filters
or complex 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 or smartphone apps 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.Tracking
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 is a simple formula known as the ‘500 rule’ that
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, enabling
you to capture subjects like the Milky Way over a picturesque
landscape, plus meteor showers and aurorae. 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.Colour, contrast and saturation
Unless you’re shooting the Moon or concentrating on a
specific monochromatic wavelength of light such as hydrogen-
alpha, pleasing colour is an extremely important aspect in the
best astrophotos. Note that I say pleasing rather than accurate
— colour perception is somewhat subjective, and most people
experience colour slightly differently. False-colour narrowband
imaging has become quite popular as well.
That said, a natural-colour image of the night sky
taken from a dark site should generally have a neutral sky
background. The reddish-brown cast recorded in nightscape