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Deep Sky Imager™ Technical Trivia
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What
is the benefit of stacking many images
to obtain a single final image?
• How does
Meade’s Deep Sky Imager™ cool
itself?
• How does
turning off part of the camera during
long exposures benefit the final image?
• How is electronic
noise reduced in Meade’s Deep Sky
Imager?
• How
have you increased the sensitivity of
the CCD camera?
•
Why
is dark frame subtraction important to
CCD imaging? |
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What
is the benefit of stacking many images
to obtain a single final image?
On September 17, 2004, scientists
in Mauna Kea, Hawaii made the following
news release, “The Laser Guide
Star Adaptive Optics system at the
W. M. Keck Observatory is exceeding
performance expectations and is poised
to revolutionize many fields of astronomy.
The new guide star system, the only
one of its kind on a very large telescope,
allows astronomers to use adaptive
optics to study astronomical objects
with unprecedented resolution anywhere
in the night sky. ‘We are just
thunderstruck with this new capability,’
said Dr. Frederic Chaffee, director
of the W. M. Keck Observatory in Hawaii.
The dramatic boost in telescope performance
is due to a new laser system that
allows adaptive optics (AO) to make
precise atmospheric corrections to
a scientific target."
Image
blurring has plagued astronomy since
the invention of the telescope nearly
400 years ago, this blurring is due
to the turbulence in the atmosphere
which bends the light and distorts
the image. The scientists in Hawaii
have spent millions of dollars and
found a very cleaver way to correct
for these random disturbances in the
air and produce very clear images.
The
Meade Autostar Suite software and
DSI camera has also found a very clever
way to eliminate this same blurring
in a much less expensive way. It is
done by stacking images which reinforces
the actual image and diminishes the
noise. Engineers call this increasing
the signal to noise ratio. This is
the same principle you use when you
turn up the radio volume in a car
with the windows open.
Look
at the results below. In picture 1,
the DSI camera took a single image
of a nearby building in Irvine. Notice
how blurred and distorted the image
appears. This is due to the heat induced
turbulence in the air between the
camera and the building. To your eye
this same scene would appear as a
shimmering moving building. Picture
2 was created by the DSI version of
adaptive optics using overlaid images.
Notice how clear and sharp this image
looks. This same technique also works
on the celestial images captured by
the DSI. We just turn up the volume
and out comes a clear and sharp image. |
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| Picture
1 |
Picture
2 |
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How
does Meade’s Deep Sky Imager™
cool itself?
The Boy Scouts have a saying, “If
your feet are cold, put on a hat”.
This is because your body goes to
extreme measures to protect (i.e.
keep warm) vital parts of the body
(for most people this includes the
head) at the expense of more disposable
parts such as the hands and feet.
Blood flow is directed such that the
head is warmed while extremities get
cold. We use a reverse strategy in
our Deep Sky Imager to cool the image
sensor by directing heat away from
it. In solid-state electronic devices,
heat means electronic noise and noise
means poor picture quality. For every
7º C rise in temperature the
noise doubles. Here’s how we
cool the sensor. First we connect
the sensor directly to the camera’s
large metal case, which features cooling
fins to bring it to almost ambient
temperature. Next, we make a little
island for the sensor by routing-out
the circuit board area surrounding
it and leaving only enough material
to support the electrical connections
and hold the sensor in place. We then
connect the rest of the circuit board
(the source of most of the heat) to
the front metal cover which is attached
to the telescope. A rubber ring thermally
isolates the front cover from the
back so the heat flows forward into
the telescope and away from the sensor.
So the next time your feet are cold,
remember, “our Deep Sky Imager
does not wear a hat” and its
cold little feet (image sensor pixels)
are making nice clear pictures. [back
to top] |
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| Globular
Star Cluster – M13 - Image
by Jack Newton with Deep Sky
Imager |
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| How
does turning off part of the camera
during long exposures benefit the
final image? |
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Have
you ever sat around a campfire as
the fire dies and watched the coals
glow? If you were to shine a bright
flashlight on the coals, the glow
would no longer be visible. However,
if you turn off the flashlight you
will see that in the dark even the
smallest glowing coal becomes visible.
The Sony® sensor in our Deep Sky
Imager has a glowing ember - we call
it the read-out amplifier. It is located
in the upper left corner of the sensor.
It is used to amplify the signals
coming from the light sensitive area
of the circuit. When used in applications
other than astronomical imaging –
remember, this sensor was originally
designed to make T.V. images - this
little glowing coal goes unnoticed
because the light falling on the sensor
is bright and the images are being
read out at 30 frames per second.
We don’t want to use this sensor
at 30 frames per second because our
light is very dim and we are trying
to soak up every little particle of
light. |
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Irregular Galaxy –
M82 - Image by Chuck Reese
with Deep Sky Imager |
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In order to collect all of this
light, we slow the exposure
time so it takes many seconds
or even minutes. If allowed
to glow, this little ember in
the upper left corner would
fill that part of the picture
with a visible and very undesirable
feature we call amplifier glow.
Now this naughty little amplifier
is not really needed while we
are collecting the light because
it is only used after the exposure
is taken to read out the image.
So rather than feed it power
(and heat) during the exposure,
we drop the main voltage to
a point at which it effectively
turns off, thereby greatly diminishing
the amplifier glow. Now you
might think to yourselves that
this is a pretty clever little
trick but to our seasoned camera
designers we would no more think
to leave this little amplifier
running than to leave our car
idling in the parking lot all
day while we work inside.
[back to top] |
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How
is electronic noise reduced in Meade’s
Deep Sky Imager?
In Olympic air rifle competition,
participants wear a sweatshirt --
sometimes two -- under their heavy
leather team jackets. Why would they
wear sweatshirts and jackets during
summer competition? The answer, sweatshirts
are used to prevent the skin from
making contact with the leather jacket,
thereby dampening the transmission
of the heart beat to the gun. |
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| The
Great Orion Nebula –
M42 - Image by Dave Street
with Deep Sky Imager Processed
by Don Waid |
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Our
Deep Sky Imager also has a heartbeat
– a digital heartbeat.
The computer and digital logic
in the Deep Sky Imager has a
heart (a crystal oscillator)
that beats 24 million times
every second to synchronize
the logic. This throbbing source
of noise can corrupt the analog
portion of the camera circuits
that need a very quiet environment
to make precise measurements
of the picture data. To isolate
this "digital heartbeat,"
Meade engineers made the circuit
board larger than required to
place all of the digital logic
on the bottom of the circuit
board while placing analog circuits
and the image sensor on the
top of that same board. These
circuits were further separated
on opposite halves of the circuit
board so they were not directly
above and below. Finally, the
entire camera is surrounded
in a metal case and the circuit
board has two grounded shielding
copper layers sandwiched in
the center. All of this is done
to "lessen the transmission
of the heart beat" to your
astronomical pictures. [back
to top] |
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How
have you increased the sensitivity
of the CCD camera?
A good marksman stops breathing just
before squeezing the trigger to improve
his accuracy. The Deep Sky Imager
holds its breath too (figuratively
speaking) while measuring the intensity
of light captured during an exposure.
That's right; the internal Deep Sky
Imager computer shuts down the internal
power supplies used to operate the
camera and runs on stored energy while
it measures the charge stored during
an exposure. This reduces the surrounding
noise in the circuits and gives a
more accurate reading. After the reading
is done, the power supplies are turned
on until the next measurement, when
they are turned off again. It is this
level of attention that makes this
camera many times more sensitive than
cooled cameras costing thousands of
dollars more. [back
to top]
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| Globular
Star Cluster – M2 - Image
by Jason Ware with Deep Sky
Imager |
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| The
Dumbbell Nebula –
M27 - Image by Mike Sabina
with Deep Sky Imager |
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Why
is dark frame subtraction important
to CCD imaging?
In oil painting, it is important
to prepare the surface of the
canvas to receive the paint.
The unfinished surface of the
canvas is not conducive to the
fine brushstrokes required for
photorealism. The surface may
have fabric strings or grainy
bumps that will distort the
finished picture. Canvas preparation
is accomplished by applying
oil gesso with broad regular
strokes in one direction. After
drying for several days the
artist sands the surface until
smooth and repeats the gessoing
process as needed to achieve
a smooth surface. Only then
is the canvas ready for the
masterpiece. A similar process
is needed for our Deep Sky Imager,
which can be thought of as a
canvas for a fine painting,
only in this case the canvas
is the silicon surface of the
imager. This silicon surface
is uneven in a different way.
If the Deep Sky Imager sensor
were covered to block all light
for a period of time, you would
expect the resulting image to
be uniformly black; but it is
not! This image, which is called
a “dark frame”,
is speckled with a pattern of
dots of varying intensities.
The brightest dots are called
“hot pixels” and
all of these are created by
electrons leaking into the pixels
due to heat or other means,
not from exposure to light.
This pattern is very predictable
and changes only in intensity
with time. If this “dark
noise pattern” is not
removed from the exposure, it
will add distortion to the picture. |
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| Fortunately,
because “dark noise” is
predictable, it can be subtracted
from an exposure leaving a clear image.
The AutoStar Suite software performs
these “dark noise” subtractions
automatically for the LPI™ and
Deep Sky Imager because they transfer
complete and uncompressed images to
the computer. Images from webcams
such as the ToUcam™ and NexImage™
cannot perform “dark noise”
subtraction. Why? Because they compress
their data and are unable to create
a “dark frame” for correcting
the image. So when it comes time to
paint your astronomical masterpiece,
would you use the smooth well prepared
silicon canvas of the Deep Sky Imager
or the gravel roadbed of a webcam?
[back to top] |
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