Sky and Telescope - October 2015 - 69

programmable gadget that will ﬁre your camera's shutter
automatically for as many images as you like, as long
as you want, and spaced at the time interval you desire.
Intervalometers are readily available from your camera
manufacturer and third-party suppliers.

Expose to the Right
Once you've got your equipment and begin shooting the
night sky, nothing will improve your images more than
following this edict: expose to the right. This means to
use the histogram function in your camera as a gauge to
adequately expose your image. You want the histogram
to spread across the entire graph - not peak far to the
left (underexposed) side. Although you can boost the
brightness of your photo in processing later, doing so
will also increase noise. If you are seeing artifacts such
as banding and a "glow" at the edge of the frame, your
images are underexposed.
On dark, moonless nights, don't be afraid to shoot at
ISO 6400 or "wide open" at f/2 if that's what it takes to
get a well-exposed image.

WIDE LENS Fast
"prime" lenses -
ones with ﬁxed focal
lengths - are the
favorites of nightscape
photographers. Nofrills manual lenses,
such as the Rokinon
14-mm f/2.8, provide
excellent optical quality and fast speed at
aﬀordable prices.

The 500 Rule
Shooting longer exposures can also help avoid underexposed, noisy images. But camera-on-tripod nightscapes
are usually limited by "the 500 rule." This is the longest
reasonable exposure before sky rotation introduces
noticeable star trails, and it can be roughly determined
by dividing 500 by the focal length of your lens.
For example, to avoid star trails when shooting with a
24-mm lens, an exposure should be no longer than about
500/24 = 20 seconds. Keeping exposures under that limit
while still exposing to the right on dark nights often
demands fast f/ratios and high ISO speeds (1600 and up).

Shoot RAW
To produce the best photography of any kind, always
shoot in RAW format. Only RAW images preserve the
full range of 14-bit data recorded by the camera's sensor. Saving in Jpeg (JPG) format converts these images
into 8-bit ﬁles, which greatly reduces tonal range. This
throws away half your image quality, information that
can never be retrieved later. Don't shoot JPGs just to
save space - memory cards and storage space are now
relatively inexpensive.

To Calibrate or Not?
Most high-end digital cameras oﬀer a menu option
called Long Exposure Noise Reduction (LENR). Turning
LENR on forces the camera to take a "dark frame" immediately after the main exposure with the shutter closed.
A dark frame is an image of the noise generated by your
camera that is then subtracted from the previous image,
which reduces speckling in the result.
I recommend using LENR, particularly on warm

PROCESS RAW Adobe Camera Raw in Photoshop and Adobe Photoshop
Lightroom oﬀer many powerful tools to help process your nightscape photos, including vignette reduction, noise reduction, and the ability to recover
details in the shadows and highlights independently.

summer nights, because a large component of the noise
in a dark frame is thermal signal. However, using LENR
isn't practical when shooting a sequence of images to be
stacked into star trails or turned into a time-lapse movie.
The time the camera takes to shoot the dark frames will
introduce large gaps in star trails or jumps in the motion
of the stars.
A way around this is to take dark frames separately
at the end of a shoot. They can then be subtracted from
your images later when processing your images.

Digital Darkroom
The most important step in processing nightscape
images is to ﬁrst "develop" your RAW ﬁles. I use the
Adobe Camera Raw plug-in within Photoshop to process
all my RAW ﬁles. An alternative is Adobe Photoshop
Lightroom. Its Develop module is identical to Adobe
Camera Raw.
Camera Raw and Lightroom include many tools speciﬁcally designed to correct common optical problems in
DSLR images. For example, both have tools that correct
vignetting in wide-angle images. The latest versions
include powerful tools that can reduce chromatic aberSk yandTelescope.com October 2015

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Sky and Telescope - October 2015

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