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Image Calibration Using PixInsight
Duncan Kitchin
Oregon Star Party 2017
duncan@kitchinonline.com
Outline
• Introduction
• A tour of the PixInsight user interface
• Calibration & pre-processing walk through
• Calibration & pre-processing demo
• Wrap-up
Introduction
Spectrum of Tools
DeepSkyStacker
MaximDL
ImagesPlus
PixInsight
Complexity
Control
PhotoShop
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!
Directory Structure
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
Maxim Versus PixInsight
PixInsightMaximDL
Here it is
A Tour of the PixInsight User Interface(Live Demo)
Overview of Calibration and Pre-Processing
Calibration and Pre-Processing Outline
• Calibration
– Bias subtraction
– Dark subtraction
– Flat frame correction
• Cosmetic Correction
• Registration
• Integration
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
Hot Pixels, Bad Columns & Cosmic Rays
Expanded View
Uneven Illumination
Example Flat Frame
Satellites & Boosters
Satellites & Boosters
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
Components of Pixel Values
Bias Frame Dark Frame(Long)
Flat Frame Light Frame
Calibration Requires Separating These Components
BiasComponent
DarkComponent
FlatComponent
LightComponent
Flat Correction
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
Outlier Rejection
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
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
Rejection Settings
Set Algorithm
Yes
No
Rejection Settings
Typically 2.500 to 3.000Lower value rejects more
Creating a Master Bias Frame
Integration
M
Creating a Master Dark Frame
Integration
M
Calibration
M
Creating a Master Dark Frame
MCalibration
MIntegration
M
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!
“Post-Calibrate” Workflow
“Pre-Calibrate” Workflow
The Next Slide is Important
Calibration Tool
Calibrate optionsfor calibration frames
Only check these forpost-calibration flow
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
Integration for Flats
Scale flats tosame brightness
Take this into accountin rejection algorithm
Final Calibration Workflow
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
Calibration & Pre-Processing(Live Demo)
Wrap-Up
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