ISO settings, and see what
results you get. But be warned:
You’ll look at a high-ISO image
and think, “Look at all of those
stars!” Unfortunately, most of
the points will be noise.
Manual or bulb setting:
Your DSLR features a pre-
determined number of expo-
sure lengths, with the longest
usually 30 seconds. With the
manual setting, you set the
exposure length. On the bulb
setting, the shutter stays open
while you depress the shutter
button, and it closes when you
release it. Most cameras also
allow you to manually set your
lens aperture from either of
these settings.
File size: This lets you
choose the exposure’s final size.
I always recommend capturing
RAW exposures, the largest file
possible. This will use more
space on your memory card,
but it will also give you more
data for better results when
processing your images.
Noise reduction: Electronic
noise can be an issue with digi-
tal cameras. With this feature,
you can reduce the amount of
noise (which usually looks like
faint stars), an important con-
sideration when many of your
backgrounds will be black and
your subjects will be faint
points of light. The drawback is
a decrease in detail. Experiment
with this feature, and see what
works best for you.
Lens aperture: This is spe-
cific to the lens, though usually
controlled by the DSLR.
Adjusting the aperture changes
the amount of light that reaches
the camera’s sensor. The pur-
pose of an adjustable lens aper-
ture is to change the depth of
field. As the aperture decreases,
the f-number increases, bring-
ing more objects in the picture
into focus. Because we are deal-
ing with astronomical objects
(with the exception of light-
scape imaging) this is not an
issue. Set the aperture at the
lowest number so your lens will
allow the maximum amount of
light through.
Dew control: I bring along a
plastic garbage bag to cover my
equipment in case of sudden
inclement weather. Depending
on where you observe, conden-
sation might also be a problem.
A variety of accessories — for
example, a dew heater — can
help you combat this poten-
tially session-ending issue.
As you complete your
checklist, keep in mind where
you will be doing your imag-
ing. You won’t take many good
images with a lot of light pollu-
tion around. Yet some imaging
from a light-polluted area can
make for interesting lightscape
photography. I am fortunate to
live far enough from bright city
lights that I can easily see the
Milky Way without optics.
However, you might have to
travel some distance to find a
dark sky.
Under the stars
Now that you’ve completed
your preparation, you’re ready
for some imaging. But where
do you begin? If it’s your first
session, start with a bright
constellation. This will help
you see what the sky conditions
are. Take some initial test shots
with different exposures and a
couple of different lenses if you
have them.
Say you’re going to take
some exposures, and it’s winter.
The constellation Orion is
above the horizon, a great place
to start. First, check the focus
carefully. Do not trust the
infinity mark on the lens. Over
the years, I’ve gotten to know
where infinity is on my lenses,
yet I always check at the start of
(and several times during) each
imaging session.
You can approach exposure
length a couple of different
ways, depending on whether
you plan to process your
images, a whole new factor in
astrophotography. Or do you
want to keep it simple for now
and see what immediate results
you can get? The maximum
length you can expose depends
on factors like sky conditions,
Shooting comets is popular with astroimagers. Here, the photographer
attached a Canon XTi to a Zeiss 80mm refractor and mounted them
on a tracking mount. This image, shot April 8, 2013, between 4:45 A . M .
and 4:55 A . M . MST, combines ten 1-minute exposures at ISO 800. CHRIS SCHUR
FULL VS. CROPPED SENSOR LENS
EFFECTIVE FOCAL LENGTHS
Full Sensor Canon APS-C 1.6X Nikon APS-C 1.5X
16mm 25.6mm 24mm
24mm 36mm 38.4mm
50mm 80mm 75mm
85mm 136mm 127.5mm
135mm 216mm 202.5mm
105mm 168mm 157.5mm
400mm 640mm 600mm
the lens and its aperture, ISO
setting, noise reduction factor,
how well your mount is polar
aligned, and more. I also exper-
iment. Try a range of exposures
from short to long, and pick the
one that looks best to you.
Wide-field astronomical
photography will let you image
the Milky Way, meteors and
meteor showers, and aurorae
dancing in the starry sky. You
might even capture a satellite
or the International Space
Station passing through one of
your images.
I cannot encourage you
enough to give DSLR-tracked
astronomical imaging a try,
especially if you already have
the equipment. You’ll find it
fun and relaxing to create
images. You’ll also have lots of
chances to experiment, some-
times with the placement of a
pine tree in the foreground,
and at other times by trying a
range of lens focal lengths.
Hopefully, your sessions will
lead to high-quality pictures
you’ll be proud to share.
Mike Reynolds is a contributing
editor of Astronomy and a
passionate celestial imager —
especially of total solar eclipses.
W W W.ASTR ONOMY.COM
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