Sky and Telescope - April 2015 - 67

Like many imagers, I live under the pervasive glow of
urban light pollution, where the closest dark-sky site
requires a substantial drive. But rather than limit my
imaging to those rare nights I can make the trip, I've
learned to mitigate many of the problems of my location
to take respectable images with my DSLR and a variety of
telescopes of many targets once thought out of my reach.
Whether you're starting out in astrophotography with one
of these readily available cameras or have been shooting
for some time, the tips shared here can help you take
impressive photos of galaxies, nebulae, and star clusters
with a DSLR and telescope from the comfort of your own
neighborhood.
S&T: SEAN WALKER

Settings for Astronomy
Plenty has been written about the strengths and versatility of DSLR cameras. The latest models incorporate
high-sensitivity, low-noise CMOS detectors that rival the
performance of many CCDs used in dedicated astronomical cameras. But unlike CCD cameras, DSLRs are equally
at home taking everyday photos as well as long exposures
of faint galaxies attached to the back of your scope.
To get the most out of your photos taken under any sky,
some changes to the basic camera settings will greatly
increase the quality of your images. The ﬁrst setting
to choose for astrophotography with your DSLR is the
manual mode in your camera. This allows you to control
important features including exposure length and color
balance. For deep-sky imaging, you'll most often be using
the "bulb" exposure, which allows you to keep the shutter
open longer than 30 seconds. This is necessary to record
all but the brightest objects visible to the naked eye.
Next, be sure to set your camera to save images in
"RAW" format. This is a proprietary format (saved with
the extension NEF in Nikon cameras, while Canons use
CR2) in 16-bit format that preserves all the light you
recorded. Other ﬁ le formats (particularly JPEG) compress
your images to save space and introduce unsightly artifacts. You'll need to use the software that came with your
camera or another image-processing program to convert
your RAW ﬁles into other ﬁle formats, but the additional
quality is worth it.
Now examine your camera's ISO setting. This is
similar to the gain setting on video cameras; the higher
the value you use, the more sensitive your camera will be
and the shorter the exposure you'll need. The latest DSLR
models boast extremely high ISO settings - in some this
can be in excess of 50,000! But there is a cost. While it's
tempting to image at these extremely high values, this
approach adds noise and also reduces the dynamic range
of your image. Star colors are washed out, and the image
appears noisy. I recommend using an ISO setting of 400
to 800 as a good compromise to preserve colorful star
clusters and planetary nebulae, while selecting higher
values when capturing galaxies and nebulae.

Besides your camera, you'll need at least two other accessories to take
photos through your telescope. A T-ring adapter replaces the lens on your
camera and connects it to the focuser on your telescope, while a digital
intervalometer (left) allows you to program and shoot multiple exposures.

Calibration: Half the Work
A large part of taking good astrophotos with a DSLR is
using good calibration frames. These are called dark, bias,
and ﬂ at-ﬁeld images, and they're used to reduce unwanted
thermal and optical eﬀects in your images.
Due to the electronic nature of CMOS (and CCD)
sensors, the longer you expose the sensor to light, the
warmer the detector gets. This creates thermal artifacts
that build up in your photo, which appear like static or
noise in photos exposed longer than a few seconds. Dark
frames are photos taken of the same duration and temperature as the images of your target, but with the camera
or telescope covered to record only the thermal signal in
your image. These frames are then subtracted from your

Much light pollution comes from local sources. Porch lights,
streetlights, and headlights all can add to the problems in your
images. The author sets up his equipment where all nearby lighting sources are blocked from shining directly into his telescope.

Sk yandTelescope.com April 2015

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Sky and Telescope - April 2015

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