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