Sky & Telescope - April 2022 - 55
As this series of images demonstrates, increasing the " signal " improves the quality of the result. The leftmost frame shows that only a little light
yields a dark image with plenty of noise (missing signal). Moving right, additional light fills in and overpowers the noise, resulting in a pleasingly
smooth image. (The complete, final image is presented below.)
The more light we have, the stronger
the signal. For example, if a single pixel
on the camera's sensor records 10,000
photons of light, then we can represent
the signal as 10,000. Note that the
signal is the quantity of light we record
- not the length of the exposure. This
is a crucial point. More time doesn't
produce better images, more light does.
In other words, the signal created by a
short exposure of a bright target can be
the same as the signal generated by a
long exposure of a faint target.
A Shot in the Dark
Shot noise is also quantifiable - it's
simply the square root of the signal
value. For our previous example of a
signal value of 10,000, the shot noise
would be 100, which is 1.0%. The longer
we expose, the more signal we get, but
we also get more shot noise. At first this
may sound counterintuitive. But what
happens is that the signal increases
faster than the shot noise, and eventually
we get a better signal-to-noise ratio
(SNR). So, if our signal value is 20,000
instead of 10,000, the shot noise would
be only 0.7%, which is proportionately
quite a bit less.
In fact, as we collect more light,
we're not reducing the shot noise.
Rather, we're improving the SNR. The
lefthand graph on page 56 shows this
relationship. Starting from the left side,
you can see the noise increases with
signal and early on the noise is overwhelming.
Soon, however, the signal
overtakes the noise and begins to climb
much more rapidly.
The image below serves as a useful
illustration of the key point that
signal is the total light collected at
each pixel, not simply the base exposure
time. In this photo, the Flame
Nebula (at bottom left) is very bright,
so that region of the image has a lot
of signal, and just a little exposure
time will yield a good SNR. The area
around the Horsehead Nebula is fainter
- if you want the image to be smooth
here, you'll need more signal than was
needed for the Flame Nebula, which
means a longer exposure. And to bring
out details in the dark region below the
Horsehead, you'll have to use an even
longer exposure.
Stacking the Deck
All this finally brings us back to the
problem that stacking solves. You might
expect the solution to the challenge of
One of the author's favorite images is this portrait of the notoriously dim Horsehead Nebula in
Orion. To achieve the exceptionally smooth results presented here, he digitally combined some
200 30-second subframes.
sk yand tele scope .o r g * APRIL 2022 55
http://skyandtelescope.org
Sky & Telescope - April 2022
Table of Contents for the Digital Edition of Sky & Telescope - April 2022
Contents
Sky & Telescope - April 2022 - Cover1
Sky & Telescope - April 2022 - Cover2
Sky & Telescope - April 2022 - 1
Sky & Telescope - April 2022 - Contents
Sky & Telescope - April 2022 - 3
Sky & Telescope - April 2022 - 4
Sky & Telescope - April 2022 - 5
Sky & Telescope - April 2022 - 6
Sky & Telescope - April 2022 - 7
Sky & Telescope - April 2022 - 8
Sky & Telescope - April 2022 - 9
Sky & Telescope - April 2022 - 10
Sky & Telescope - April 2022 - 11
Sky & Telescope - April 2022 - 12
Sky & Telescope - April 2022 - 13
Sky & Telescope - April 2022 - 14
Sky & Telescope - April 2022 - 15
Sky & Telescope - April 2022 - 16
Sky & Telescope - April 2022 - 17
Sky & Telescope - April 2022 - 18
Sky & Telescope - April 2022 - 19
Sky & Telescope - April 2022 - 20
Sky & Telescope - April 2022 - 21
Sky & Telescope - April 2022 - 22
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Sky & Telescope - April 2022 - 24
Sky & Telescope - April 2022 - 25
Sky & Telescope - April 2022 - 26
Sky & Telescope - April 2022 - 27
Sky & Telescope - April 2022 - 28
Sky & Telescope - April 2022 - 29
Sky & Telescope - April 2022 - 30
Sky & Telescope - April 2022 - 31
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Sky & Telescope - April 2022 - 33
Sky & Telescope - April 2022 - 34
Sky & Telescope - April 2022 - 35
Sky & Telescope - April 2022 - 36
Sky & Telescope - April 2022 - 37
Sky & Telescope - April 2022 - 38
Sky & Telescope - April 2022 - 39
Sky & Telescope - April 2022 - 40
Sky & Telescope - April 2022 - 41
Sky & Telescope - April 2022 - 42
Sky & Telescope - April 2022 - 43
Sky & Telescope - April 2022 - 44
Sky & Telescope - April 2022 - 45
Sky & Telescope - April 2022 - 46
Sky & Telescope - April 2022 - 47
Sky & Telescope - April 2022 - 48
Sky & Telescope - April 2022 - 49
Sky & Telescope - April 2022 - 50
Sky & Telescope - April 2022 - 51
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Sky & Telescope - April 2022 - 53
Sky & Telescope - April 2022 - 54
Sky & Telescope - April 2022 - 55
Sky & Telescope - April 2022 - 56
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Sky & Telescope - April 2022 - 58
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Sky & Telescope - April 2022 - 63
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Sky & Telescope - April 2022 - 70
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Sky & Telescope - April 2022 - 84
Sky & Telescope - April 2022 - Cover3
Sky & Telescope - April 2022 - Cover4
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