jwst calibration pipeline software requirements walkthrough june 10, 2010 robert jedrzejewski...

JWST Calibration Pipeline SoftwareRequirements Walkthrough

June 10, 2010

Robert Jedrzejewski

Science Software Branch/STScI

Calibration Software Requirements Walkthrough 2

Purpose of this Walkthrough

Demonstrate to the JWST project that STScI can specify and architect a system to calibrate all JWST data

Specify:

- WHAT the calibration pipeline software system shall do

Architect:

- What pieces the system will need and how will they fit in with the rest of the Data Management Subsystem


Purpose…

Calibrate:

Calibration Calibrateddata

Rawdata


JWST Mission System Tree

JWSTMission

ObservatorySegment

GroundSegment

LaunchSegment

Spacecraft

Integrated Science Instrument Module Science & Operations

Center

Common Systems

Institutional Systems

Launch Site Services

Payload Adaptor

Launch VehicleOptical Telescope Element

Data Management Subsystem (DMS)

Proposal Planning Subsystem (PPS)Flight Operations

Subsystem (FOS)Project Reference

Database Subsystem (PRDS)

Mission

Subsystem

Element

Segment

WFS&C Software Subsystem (WSS)

Data Processing

Calibration Pipeline Data Ingest

Archive

Component


Calibration Overview

Calibration is the process whereby the imperfect data from the JWST detectors is improved by the application of algorithms and reference files that quantify our understanding of the detectors and instruments:

Raw JWST data suffers from many effects that are introduced everywhere along the observatory chain

- Science target photons are accompanied by unwanted background signals

- OTE does not reflect all the photons, shapes the Point-Spread Function (PSF), introduces geometric distortion and adds to the unwanted background

- Instruments do not propagate all the desired photons to the detector, introduce their own geometric distortion and adds their own background signal

- Detectors do not detect all photons, add background, sub-sample the data, and add both Poisson and cosmetic noise, as well as efficiently detecting cosmic rays

SI Teams and Instrument Scientists characterize the instruments by analyzing data taken under specific conditions

- This allows them to model the behavior of the instruments

- They develop algorithms for correcting each of the unwanted effects

- They generate Calibration Reference Files that quantify the unwanted effects

The combination of algorithms and reference files provides a way to ‘undo’ many of the undesired effects introduced in the observatory

It is the job of the calibration software to apply the calibration algorithms using the necessary calibration reference files


Motivation for Calibrating the Data

Most astronomers are not experts on instruments or detectors Nor should it be necessary for them to be

Doing the calibration in one place ensures consistent, well-documented products Also avoids duplication of effort

Having STScI scientists involved in the calibration process ensures that the required expertise is developed to enable the support of the user community

A well-developed and tested calibration pipeline available when the first science data are collected allows for quick turnaround of scientific results, and improves the quality of science proposed for in later cycles The limited duration of the mission means that we cannot afford to

spend a significant fraction of the early mission years figuring out how to calibrate the data


More Calibration Discussion

Calibration is not ‘all or nothing’ – it is a process of continuous improvement of data quality that starts before the instruments are designed, continues through fabrication, testing, launch, commissioning, science operations and post-Mission

Calibration does not directly affect Mission Operations, it primarily interacts with the Science Requirements

Although the use of calibrated products in the Wavefront Sensing and Control element is a Mission Critical aspect

Understanding the calibration needs can help STScI ensure that adequate calibration data is collected

If we are to meet the Science Requirements, it presupposes that the necessary level of calibration can be achieved

Can we calibrate the data to achieve the consistency and predictability of results for standard targets to ensure that data for science targets can be held to the same level of confidence?

Do our algorithms model the observatory and does the calibration software execute the algorithms?


What is Calibrated Data?

Working definition:

Calibrated data is what would have been received if an ideal JWST with ideal instruments and ideal detectors had observed the chosen field in the chosen observing mode.

Using calibrated data it will be possible to

- Determine the flux incident on each pixel, along with its uncertainty (relative and absolute flux determinations limited by different effects)

- Determine the relative locations of each pixel to very high accuracy (limited by knowledge of the geometric distortions in the telescope + instruments)

- Determine the absolute locations of the observed field to high accuracy (limited by absolute Guide Star accuracies)

- Determine what effects may have contributed to degraded calibration quality for each pixel


What are “Requirements”?

JWST has 4 science instruments, each of which can be used in several ways for science (“Modes”)

NIRSpec

Multi-Object Spectroscopy

Long-slit spectroscopy

Integral Field Spectroscopy

MIRI

Imaging

Coronography

Long-slit Spectroscopy

Integral Field Spectroscopy

FGS-TFI

Imaging

Coronography

Non-redundant Mask Imaging

NIRCam

Imaging

Coronography

Grism Spectroscopy

Pupil Imaging


A Deeper Look at Calibration

All JWST data is a representation of a celestial scene, as processed by the OTE and instrument to create an image on the detector

The detector turns that image into a digital dataset (imperfectly!)

JWSTOTE

InstrumentDetector

Data

CelestialScene

Image formedby telescope

Image/spectrumformed by instrument


Modes

Each Mode is just a different way that a JWST instrument renders the celestial scene onto the detector(s)

Imaging modes transfer the whole scene to the detector through a filter that isolates a specific range of wavelengths

Spectroscopic modes disperse the light from part of the scene and transfer the resulting spectra onto the detector

Coronographic modes are like imaging modes, except that part of the scene is occulted and the point-spread function is changed to increase the contrast between the wings of the PSF and neighboring targets

But all Modes involve reading the detector(s)

There is a natural separation of the calibration into 2 parts: detector calibration (Mode independent) and Instrumental calibration (Mode dependent)


Decomposing the Calibration Pipeline

DetectorCalibration

InstrumentCalibration

ImagingCalibration

SpectroscopicCalibration

CoronographicCalibration


More on Decomposing the Pipeline

The Spectroscopic Calibration can be further decomposed

Multi-slit calibration

Longslit calibration

Integral-field calibration

Grism calibration

Decomposing the pipeline into Mode-specific behavior is preferable to decomposing it into Instrument-specific behavior

Calibrating similar Modes will involve sharing much of the code

Even though each instrument implements the behavior slightly differently, there will be many primitive operations that will be shared

- E.g. Re-mapping a spectrum into linear space, correcting for aperture throughput…


Detector Calibration

The detector calibration is simplified because we will only have 2 types of detectors on JWST 15 Hawaii 2RG HgCdTe detectors on NIRCam, NIRSpec, FGS 3 Ga:As detectors on MIRI

Even so, the description of the calibration steps will be the same E.g., Both types of detectors have a Reference Pixel Correction

step

But the best algorithm for doing the correction will often be different It’s unlikely that the Reference Pixel Correction will be the same for

Ga:As and HgCdTe detectors

And often the same The Nonlinearity Correction algorithm may well be the same for all

detectors – we’ll see


Calibration Software Architecture

The JWST Calibration Pipeline will inherit many of the successful architectural features from the HST Calibration Pipelines, while dropping some of those that are less useful or problematic

The good:

Existence!

Data driven, using keywords in the data and reference files pointed to by keywords

Flexible: users can choose at runtime what steps to run and which reference files to use

Open source

Distributed so users can run locally

The not-so-good:

‘Stovepipe’ architecture, separate pipelines for each instrument

Duplication of code

Mixed languages (spp, C, Python)

Dependence on IRAF


The Fundamental Concept of the JWST Calibration Pipeline

The JWST Calibration Pipeline will be set up as a sequence of independent Calibration Steps, where the output from each step becomes the input for the following step:

Input (from previous step)

Reference File/Table Calibration Step

Output (to next step)

May include control parameters


Calibration Steps and Observing Modes

Each calibration step will correct a single imperfection in the data

Calibrating a given Mode will be done by chaining together a sequence of these calibration steps

The order of the calibration steps will be the reverse of the order in which the imperfections occur

Calibration is the process of transforming the data to how it would have looked if the detectors, instruments and telescope worked ‘perfectly’

Detector Readout Detection Instrument

Each calibration step can be applied and tested independently of the others

Although it will be necessary to make sure the appropriate data is fed into the calibration step


Where do we start?

At this time, our understanding of the calibration steps that will be necessary is limited

Most of our experience is with engineering-grade detectors and non-flight readout electronics

Some data taken with instrument Engineering and Test Units

- E.g. MIRI VM1 and VM2 test campaigns

Recently, some data has been taken with flight detectors and flight electronics

The definitive test/characterization data will come from Flight Model tests, scheduled for FY11

We do have NICMOS and WFC3-IR on orbit taking IR data

We can use our experience with these instruments as a starting point in determining what steps we will need


There’s so much we DON’T know…

As testing of the detectors and instruments progresses, we will learn of the existence of new imperfections, that will require new calibration steps to mitigate their effect

The calibration pipeline must be flexible enough to allow the addition of new calibration steps

We will also refine our understanding of the best methods of correcting these imperfections

The calibration pipeline must be flexible enough to allow the swapping in and out of different algorithms to accomplish a given calibration step as the design progresses

Sometimes the ‘best’ algorithm will be different for different detectors (e.g. MIRI vs. HgCdTe)

The calibration pipeline must be flexible enough to support different algorithms for different detectors


JWST Data Primer

Every time a JWST detector is read out, we get a FRAME:


JWST Data Primer…

An exposure is made up of a sequence of frames:


JWST Data Primer

Each frame results from a longer integration time, so the signal-to-noise ratio improves with frame number


MIRI Calibration Software Requirements

Requirements are classified as:

Interface: Specifying the origin and form of inputs, and destination and form of outputs

Functional: Specifying what the software will do

Performance: Specifying execution time, memory constraints, and algorithm faithfulness

Security: Specifying constraints on who may access data at each level of processing


Back To Requirements…

The task:

Calibrate JWST data

- Transform data to ‘how it would look on the sky if observed by a perfect JWST and perfect instrument’

- Linear geometric scale: Each pixel corresponds to a constant number of arcseconds (imaging) or constant number of microns (spectroscopic)

- Linear photometric scale: Counts in each pixel corresponds to a fixed, known flux

- Data Quality Information: Errors associated with count rate presented, pixels known to have calibration issues flagged


Requirements…

To calibrate the data, we will need:

The uncalibrated data (provided by the Data Processing Component of the Data Management Subsystem)

Calibration Reference Files, and a means for determining which reference files to use in each calibration step (provided by the Calibration Reference File System component of the Data Management Subsystem)

A means for determining which exposures may be processed together to provide an improved product (provided by the Data Association component of the Data Management Subsystem)

A means of storing the calibrated data products and associated metadata (provided by the Archive component of the Data Management Subsystem)

A means of delivering calibrated data products to users (provided by the Archive)


Requirements…

Design Issues:

We already know that we don’t know all the steps we will need to do

- We will find out as we go along

- We will need a flexible architecture that allows plugging new calibration steps

- We know some of the steps already from our experience with HST and other IR instruments

We already know that we don’t know what algorithms to use

- We will find out as we go along

- We will need a flexible architecture that allows swapping in and out different algorithms for each step, and provide a means for calibration scientists to test the efficacy of each step

- We know what algorithms worked for HST IR instruments


A starting point

For each of the Science Modes of JWST, we can lay out what calibration steps will be needed

Based on what we know now

Each step will correct for a single data imperfection

Each distinct calibration step will be used to derive a functional requirement for the calibration pipeline

It will state what capability the calibration pipeline software will need to have in order to calibrate the data

The calibration algorithms are not addressed by requirements

They are covered by DESIGN

If the calibration pipeline is able to perform the step, it will meet the requirements


Requirements

REQUIREMENTS map to CALIBRATION STEPS

Requirements describe WHAT the system will do

We expect the requirements to grow in number as new data imperfections are discovered, but we don’t expect individual requirements to change much

DESIGN maps to CALIBRATION ALGORITHMS

Algorithms describe HOW the system will do it

We expect algorithms to change as our understanding of the instruments and detectors improves


How Well Should the Calibration be Performed?

How do we address the QUALITY of the calibration?

Should the requirements state the ACCURACY to which a calibration shall be performed?

I argue NO The calibration pipeline software is a ‘best effort’ endeavor STScI is a stakeholder in the success of the calibration Specifying required accuracies will discourage further exploration of

calibration improvements

There is a place for a process to explore where to place effort in improving the calibration, but the requirements is not it We will almost certainly discover effects that will require our

expertise in calibrating A Working Group/Advisory Board will be much better placed to

make these decisions


Example Proposed Calibration Pipeline

NIRCam Imaging: Data Quality & Error Initialization Coarse Ramp Jump Detection Reference Pixel Correction Detector Dark Current Correction Nonlinearity Correction Fine Ramp Jump Correction Ramp Slope Determination Flatfield Correction Persistence Correction Absolute Calibration Geometric Distortion Correction Image Combination

jwst calibration pipeline software requirements walkthrough june 10, 2010 robert jedrzejewski...

Documents

calibration software

calibration algorithms

calibration process

science data

raw jwst data

imperfect data

jwst detectors

jwst dataspecify