Sky & Telescope - June 2022 - 56

JUNE 2022 OBSERVING
First Exposure
Another approach to correct this
noise is to simply capture what are
called dark frames. These are images
captured with the lens cap in place that
are the same length as your photos of
the sky so as only to record the camera's
noise signal. You subtract these frames
from your nightscape images to remove
the noise from your images later when
you process your shots.
Another common technique of
A star-tracking mount, such as this SkyWatcher
Star Adventurer Mini, allows you to
take long-exposure images without the stars
turning into little streaks because of Earth's
rotation. Your camera and lens attach to the
mount via a standard ball head.
nomical use. What does that mean?
Off-the-shelf digital cameras have
internal filters in front of their sensors
to make them suitable for daytime use
but limit their ability to record the part
of the spectrum where many nebulae
glow with their characteristic pinkish
hues. However, some cameras can be
professionally astromodified to let more
of this red light through.
Cutting Through the Noise
Digital photos taken in low light also
record an unavoidable amount of
noise that needs to be subtracted from
the image to produce clean and clear
images. This noise arises mostly from
heat building up in the sensor during a
long exposure, manifesting as random,
false-colored pixels scattered throughout
an image. Many modern cameras
have a feature called long exposure noise
reduction (LENR) designed to automatically
subtract these image artifacts.
Unfortunately, your exposure times are
doubled with LENR enabled, but you'll
save time later on when you process
your photos.
56 JUNE 2022 * SKY & TELESCOPE
reducing random electronic noise
(often in conjunction with the previously
mentioned methods) is to digitally
combine a large number of exposures of
a single target. These are known as subframes
and are combined into a single
photo by utilizing a process known as
stacking. The more images you stack,
the higher the quality of the final result
since you increase the amount of signal
(the light from your galaxy, for example)
while at the same time minimizing
unwanted noise.
The essence of stacking is to combine
several short exposures to produce
results roughly equivalent to a single
long one. In other words, a stack of ten,
2-minute subframes would yield a photo
similar to a single 20-minute exposure.
(Image stacking was discussed in detail
in the April issue, starting on page 54.)
Astro Accessories
Unlike landscape photographers, astroimagers
have to contend with subjects
that are in constant motion thanks to
Earth's rotation. A stationary, tripodmounted
camera is limited to exposures
of mere seconds. This is the reason for
the so-called 500 rule, which states that
the longest exposure you can make with
a full-frame camera before stars start to
trail is 500 divided by the focal length
of your lens. So, a 24-mm lens is limited
to just a 21-second exposure (500 ÷ 24
= 20.8). For exposures of longer duration,
your camera and lens must accurately
follow stars at the rate of Earth's
rotation. The device that enables us to
do that is called a star tracker. I covered
portable star trackers in a recent First
Exposure (S&T: Feb. 2022, p. 54), but
you can also use a motorized equatorial
An inexpensive intervalometer/remote cable
release enables you to control the number of
shots you take and their duration. With such a
device, you can program your DSLR to execute
time-lapse sequences automatically.
telescope mount. Having your camera
and lens ride along with your telescope
on its mount is called piggyback astrophotography,
but it's the same thing as
using a star tracker, usually just bigger.
Galaxies and nebulae can require
cumulative exposures of tens of minutes
- sometimes even hours! Sitting
next to your camera to trigger the
shutter for each subframe can quickly
become tedious. Thankfully, there's a
device called an intervalometer. It's an
inexpensive accessory that plugs into
your camera and allows you automatically
shoot a sequence of frames after
you've preset the number of shots and
their duration. If you control your camera
via computer software, an intervalometer
feature is normally part of the
package.
Taking images of the night sky is
both challenging and deeply rewarding
- and you'll learn something new
each time you attempt it. We've only
scratched the surface of imaging terminology;
indeed many of the topics we've
discussed could be (and will be) entire
articles unto themselves. But starting
with a firm foundation as you set out
to capture the wonders of the night sky
should make that learning process more
enjoyable.
Happy imaging!
¢ In five decades of observing and 30
years of imaging around the world, ADE
ASHFORD is still discovering new and
exciting celestial vistas to enjoy.
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Sky & Telescope - June 2022

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