Transcript Imaging Technique for the Amateur Astronomer

Imaging Technique
for the Amateur Astronomer
by Frank Barrett
Jan 27, 2007
15th Regional Gathering of
Amateur Astronomers
Objectives
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Overview of Astro Imaging Requirements
Emphasis on Imaging Technique
If time permits, A Short Processing Demo
End with a slide show
The Question
What is required to take really good
astronomical images?
???
The Answer
Time
Priority
Patience
Persistence
Equipment
Tools
Software
Acquisition
Technique
Processing
Imaging Technique
Technique: a method of accomplishing a
desired aim
- Merriam-Webster
so when we speak of technique we are
speaking of a methodology or perhaps a
framework of methodologies working
together to accomplish our goal:
Really Good Images!
--> Tools are NOT Technique
Two Phases of Technique
• Acquisition
– How do we use our tools
(equipment + software)
when preparing and
recording images?
– Usually done under the
stars.
• Image Processing
– What can we do with
the image data acquired
to maximize signal and
reduce noise?
– Usually done on the
computer.
Acquisition Techniques
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Polar Alignment
Guiding, PEC
Balance
Focus, focus, focus!
Location
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light pollution
latitude
horizon
weather
• Seeing/Thermal effects
– pavement vs. grass
– cool down time
Acquisition Techniques
• Dew Prevention
• Target Location
– low on horizon?
– over a chimney?
– meridian flip?
• Optical configuration
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focal length/FOV
focal reducer
Barlow
mosaic preparation
Acquisition Techniques
• Imaging
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sub exposure duration
total number of subs
darks, flats
bin factor
filters
• light pollution
• narrow band
• color
– dithering
Processing Technique
Two main phases to image processing:
1) Noise reduction (both random and non random)
2) Signal enhancement
Noise - an unwanted signal or disturbance in an
electronic device or instrument; irrelevant or
meaningless data or output occurring along with
desired information
Signal - a detectable physical quantity or impulse by
which messages or information can be transmitted
(ref: Merriam-Webster, http://m-w.com)
Noise Reduction
Combatting Non Random Noise with Image
Calibration (a.k.a. Reduction):
1) Darks - every electronic imaging device
produces noise due to “dark current”. An image
taken in the “dark” for the time and temperature
of your light frames can be subtracted to remove
this form of noise.
2) Flats - another form of unwanted noise may
come from the optics of our system. Dust, dirt,
and uneven illumination can conspire to produce
unwanted artifacts in our images. A flat is an
evenly illuminated image which can be used as a
baseline for removing these artifacts.
A Sample Dark Frame
A Sample Flat Frame
An Uncalibrated Light Frame
After Dark Subtraction
After Flat Frame
Noise Reduction
Combatting Random Noise by Combining Frames:
• The predominant noise in a calibrated frame is
random in nature...it will be different from one
subframe to the next.
• Note that the signal is constant, but the noise is
random.
• We can leverage this knowledge by mathematically
combining the pixels of a stack of such images.
• Averaging N subframes will decrease noise by
SQRT(N).
• Note: subframes have to be properly calibrated
and aligned before combining.
Noise Reduction
Combination Algorithms:
• Simple Average - Add up all the pixel values from
the stack and divide by N.
– Produces a very smooth image, but may allow effects due
to satellite trails or cosmic ray hits to “bleed” through.
• Median - sort all the pixel values from the stack
and select the one in the middle.
– A bit noisier than average, but does excellent job
removing hot and cold pixels and other unwanted
artifacts such as satellite trails or cosmic rays.
• Sigma Clip - remove out-liers (both high and low)
and average the remaining pixels.
– Gives the best of both worlds. Needs at least 6
subframes to be effective.
Noise Reduction
Combatting Non Random Noise by Dithering Frames:
• Dithering is an acquisition technique where the
guide star is purposefully moved from one
subframe to the next.
• This has the effect of shifting the signal around
on the imaging chip.
• After alignment any non random noise from the
camera will be located at different locations of
the image stack and will be removed nicely by the
combination algorithm.
Noise Reduction
A Final Note:
• All subframes contain noise. This includes darks,
flats, and light frames.
• For this reason all Frame types should be taken
with multiple subframes and combined!
• Furthermore, Flat frames should be calibrated
with their own darks, and light frames should be
calibrated with darks and flats.
• A typical frame inventory:
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w Flats (one set per filter)
x Darks for Flats
y Lights
z Darks for Lights
Signal Enhancement
Once we have done a good job of removing noise we
can focus our attention on bringing out the signal
of our target.
Our primary goal here is to bring out the detail,
color, and character of the underlying data. We
strive not to distort, but to enhance.
Please Note: It is impossible to remove all the noise
and therefore we need to be very careful that we
enhance the signal and not the remaining noise.
First Step: Histogram Stretch
Most of the signal in a typical deep sky image is in
the low end of the histogram:
• There are many software techniques to perform
histogram stretch. I prefer to use Adobe
PhotoShop's Levels and Curves because it gives
more control over the result.
Histogram Stretch with Levels and Curves
• The idea here is to bring up
the low end of the histogram
while preventing burn out in
the high end.
• After applying an iteration or
two of Curves it is usually
necessary to adjust the black
point with Levels.
Next Step: Cleanup
• Despite our best efforts to remove noise there
will still be unwanted artifacts in the image that
should be removed at this time.
• To remove hot and cold pixels, specks, splotches
or other nasties I prefer to use PhotoShop’s clone
tool.
Next Step: Separating Zones
• Ron Wodaski has written an excellent image
processing book entitled “Zone System for
Astro Imaging” whereby the image is
separated into 3 or more zones based on the
signal to noise ratio of that zone.
• The darkest areas of the image contain
almost no signal and therefore contain the
lowest signal to noise ratio.
• Conversely, the brightest areas of the image
will contain the highest signal to noise ratio.
• The idea is to separate these zone out so
that we can apply different processin
techniques to each of the zones. This is
typically done in PhotoShop with the Color
Range Selection Tool and Layers.
Zone Processing
• The stars. Creating a layer with just the stars allows us to
be aggressive in processing other zones without destroying
any background stars.
• Dark Zone. This zone typically contains almost all noise and
can be defeated by raising the dark point in Levels.
• Dim Zone. These are the faintest areas of our image. We
can best mitigate the noise here by applying a mild blur to
soften the noise.
• Mid Zone. If the image has a wide dynamic range I will
create this zone from tones between the Dim Zone and
Bright Zone. This zone typically does not require any
smoothing, but is not detailed enough to warrant sharpening.
• Bright Zone. This zone will contain the brightest and
perhaps most detailed portions of the image. Here we will
want to apply sharpening. I like to use PhotoShop’s Unsharp
Mask.
So What?
• For most of us our time is limited and we do astronomy on a
budget.
• Therefore we have to fit in with the time available and make
do with the equipment we have.
• The variable with which we have the most control is
therefore Technique.
• The best way to go about developing your Technique is to
focus on problem areas. Continuously ask yourself,
“What could I have done to make this image even better.”
• If you decide to stay in it for the long haul you will never
stop asking that question and your images will continue to
improve!
• When you hit that brick wall (and you probably will) you may
be faced with the fact that you have reached the limits of
what your time and/or tools will allow. Either accept that or
go about making improvements.
See more at:
http://celestialwonders.com
contact me at:
[email protected]
Bibliography
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“The New CCD Astronomy” by Ron Wodaski
“The Zone System for Astro Imaging” by Ron Wodaski
“Photoshop Astronomy” by R. Scott Ireland
“Photoshop for Astrophotographers” by Jerry Lodriguss
CCD Topics by Stan Moore:
http://www.stanmooreastro.com/CCD_topics.html
Technical Notes by John Smith:
http://www.hiddenloft.com/notes.htm
SBIG Yahoo Group:
http://tech.groups.yahoo.com/group/SBIG
Great CCD Software: http://www.ccdware.com/
CCDSoft, TheSky: http://www.bisque.com/Products/
FocusMax, PoleAlignMax:
http://users.bsdwebsolutions.com/~larryweber/
• Cartes du Ciel: http://www.stargazing.net/astropc/