Sky & Telescope - December 2021 - 55

an image file. Both the collecting of the
light and the readout process have their
noise contributions.
When it comes to discussing ISO
and digital photography, there are three
sources of noise that need to be clearly
understood. The first is shot noise, which
is the speckling you see in astro-images
that's quite simply the result of there
not being enough light - the " signal "
- to make a clean, smooth image. Shot
noise is actually the absence of light.
Think of it as a drizzle of rain leaving
splotches on the sidewalk, rather
than a downpour that really soaks the
pavement. A wide aperture or a long
exposure gathers lots of light (rain) and
makes for a well-exposed (wet) image.
Images taken with short exposures,
small apertures, and dim light sources
are going to have more shot noise -
more gaps between the raindrops.
Two other noise sources are read
noise and dark-pattern noise, which
come from the image sensor itself and
are often lumped together. Read noise
is a result of small fluctuations that
arise when the image data are read
from the sensor. Dark-pattern noise
is due to small variations in an image
sensor's pixel array and shows up as
horizontal or vertical lines and banding
in your astrophotos.
Digitally scaling (by changing the
ISO setting) sometimes occurs after the
data are read out from the sensor and
therefore affects both image information
and the noise equivalently. This can
lead to the false conclusion that higher
ISOs produce noisier images. In fact,
with many cameras, increasing the ISO
actually decreases the apparent noise!
The reason is that some portion of
the ISO range (usually the lower end) is
accomplished not with a simple digital
multiplier, but by altering the sensor gain.
This signal amplification is performed
before the readout occurs, so the darkpattern
noise or read noise don't have as
much impact. You could argue that this
is the same as increasing the sensitivity
of the sensor since it now records fainter
objects, but the amount of signal collected
isn't actually increased - it's just
competing against less noise.
sk yand tele scope .o r g * DECEMBER 2021 55
The Horsehead Nebula in Orion is a notoriously difficult visual target but can be captured readily
with a digital camera and telescope (or telephoto lens). When imaging deep-sky targets such as
this, keep the ISO low enough to preserve as much dynamic range as possible to allow for aggressive
processing techniques later.
qThe vertical structures you see in the noise pattern are a sign that this camera's ISO setting is
too low, because an artifact called dark-pattern noise is showing through.

http://Skyandtelescope.org

Sky & Telescope - December 2021

Table of Contents for the Digital Edition of Sky & Telescope - December 2021

Contents
Sky & Telescope - December 2021 - Cover1
Sky & Telescope - December 2021 - Cover2
Sky & Telescope - December 2021 - 1
Sky & Telescope - December 2021 - Contents
Sky & Telescope - December 2021 - 3
Sky & Telescope - December 2021 - 4
Sky & Telescope - December 2021 - 5
Sky & Telescope - December 2021 - 6
Sky & Telescope - December 2021 - 7
Sky & Telescope - December 2021 - 8
Sky & Telescope - December 2021 - 9
Sky & Telescope - December 2021 - 10
Sky & Telescope - December 2021 - 11
Sky & Telescope - December 2021 - 12
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Sky & Telescope - December 2021 - Cover3
Sky & Telescope - December 2021 - Cover4
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