Astronomy

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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,

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

FULL VS. CROPPED SENSOR LENS

EFFECTIVE FOCAL LENGTHS

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.

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