image calibration using pixinsight - duncan...
TRANSCRIPT
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Image Calibration Using PixInsight
Duncan Kitchin
Oregon Star Party 2017
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Outline
• Introduction
• A tour of the PixInsight user interface
• Calibration & pre-processing walk through
• Calibration & pre-processing demo
• Wrap-up
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Introduction
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Spectrum of Tools
DeepSkyStacker
MaximDL
ImagesPlus
PixInsight
Complexity
Control
PhotoShop
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Workflow Document
• PixInsight workflows can be very complex
– The tools have many options
– Much experimentation can be required to fully understand all of the capabilities
– The return on work put in to understand all of the features can be high
• Writing everything down is key
– Get a copy of my current workflow document
– Most important recommendation: write your own!
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Directory Structure
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Calibration & Pre-Processing Options
• PixInsight provides a number of tools for calibration & pre-processing– Other tools treat at least calibration as a single black
box
– PixInsight has many tools that can be used to assemble a pre-processing workflow
• PixInsight also includes a “Batch Pre-Processing” (BPP) script– Assembles a single black box for you
– I do not recommend this
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Maxim Versus PixInsight
PixInsightMaximDL
Here it is
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A Tour of the PixInsight User Interface(Live Demo)
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Overview of Calibration and Pre-Processing
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Calibration and Pre-Processing Outline
• Calibration
– Bias subtraction
– Dark subtraction
– Flat frame correction
• Cosmetic Correction
• Registration
• Integration
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Defects That We Are Trying to Fix
• Quasi-Consistent Dark Effects– Offsets in the pixel values due to repeatable bias and
dark current
– Hot pixels and bad columns
• Light dependent effects– Uneven illumination
– Uneven photo-response of individual pixels
• One-Off Outliers– Cosmic rays
– Satellites & rocket boosters
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Hot Pixels, Bad Columns & Cosmic Rays
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Expanded View
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Uneven Illumination
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Example Flat Frame
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Satellites & Boosters
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Satellites & Boosters
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The Mechanics of Calibration
• Start with a set of calibration frames– Bias– Dark– Flat
• Set will contain a large number of each of these– Average* the set of each to create master bias, dark and flat
frames
• Calibrate a set of light frames using the calibration set
• Not obvious from the “black box calibration” method:– Some of the calibration frames themselves have to be calibrated– There is more than one order in which the operations can be
done
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Components of Pixel Values
Bias Frame Dark Frame(Long)
Flat Frame Light Frame
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Calibration Requires Separating These Components
BiasComponent
DarkComponent
FlatComponent
LightComponent
Flat Correction
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Averaging (or “Integration”)
• The same integration tool will be used to combine sets of bias, dark and flat frames, as well as registered light frames
• This is the point at which you can (and should) eliminate one-off effects
• PixInsight’s Integration tool contains many statistical outlier rejection algorithms for this purpose
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Outlier Rejection
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Outlier Rejection Algorithms
• Several options for the rejection algorithm
• Which one is optimal will depend on how many frames you have in the set
• There is also an option for setting an absolute range of values for rejection, which you almost certainly want to turn off
• Experiment using the Blink tool
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Algorithms
Number of Frames Algorithm
1 - 5 Percentile clipping
5 - 10 Averaged sigma clipping
> 5 Linear fit clipping
> 10 Sigma clipping
> 15 Winsorized sigma clipping
• Opinions vary on this
• Note that the ranges overlap
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Rejection Settings
Set Algorithm
Yes
No
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Rejection Settings
Typically 2.500 to 3.000Lower value rejects more
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Creating a Master Bias Frame
Integration
M
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Creating a Master Dark Frame
Integration
M
Calibration
M
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Creating a Master Dark Frame
MCalibration
MIntegration
M
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Two Calibration Methods for Darks(and Flats)
• “Pre-calibrate” or “Post-calibrate”
• Which one does MaximDL do?
– It’s inside the black box
– Probably the second one
• Which one does PixInsight do?
– Whichever one you want
– Be sure not to accidentally do both!
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“Post-Calibrate” Workflow
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“Pre-Calibrate” Workflow
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The Next Slide is Important
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Calibration Tool
Calibrate optionsfor calibration frames
Only check these forpost-calibration flow
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Why Would You Use the More Complex Version?
• Flat frames may have uneven brightness
• This can be fixed in integration of flats by scaling according to brightness
• In order to do this, you need to pre-calibrate flats
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Integration for Flats
Scale flats tosame brightness
Take this into accountin rejection algorithm
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Final Calibration Workflow
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Next Step: Cosmetic Correction
• Final step after calibration
• Removes “escaped” hot pixels & bad columns
• Allows creation of a defect list
– This can take some time, but can be reused later
• Results in extremely clean data going in to post-procesing
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Calibration & Pre-Processing(Live Demo)
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Wrap-Up