www.stecf.org/poa
The CE-STIS CTI Preprocessor- BETA
Paul Bristow TIPS Feb’04
The STIS Calibration
Enhancement Project
According to an ESA/NASA MoU
 A comprehensive empirical calibration pipeline
already exists for STIS. We aim to improve
those components which benefit from physically
motivated corrections.
 Current work includes:

 Wavelength Calibration
 Calibration lamp line list - new lab measurements at NIST
 Optical Model
 Optimal
Spectral Extraction
 Detector Model
Pre-processor: Overview

Readout model:
 Concept
 Implementation
 Validation

Pre-processor script
 Handling

reference files
BETA version ready for testing
 In
order to progress beyond BETA, needs criticism
Readout model: Motivation


A physical solution, CTI effects understood in
terms of the CCD operation and environment
A correction that applies to the entire 2D array:
 Equally
valid for imaging and spectroscopy
 Also applicable to extended objects


Portable to other space based detectors
(WFPC2, ACS, WFC3?, Kepler instruments etc.)
Extendable to model further aspects of CCD
readout
Readout Model: Concept

Forward Simulation:
 Start with
 2D Charge distribution on chip
 Distribution of bulk traps
 Shift the charge under each electrode
as during
readout
 Calculate capture and emission of charge in each
shift

Take into Account:
 Status
of bulk traps
 Dark current
 Trap capture and emission timescales
 Chip clocking frequency, architecture, gain etc.

Currently only deals with parallel CTI
Correction:
Raw
data
=
>> SIMULATION
Simulation
output
=
-
Raw
data

Difference
image
Raw
data
Difference
image
Corrected
image
Further iterations (usually not necessary):
Corrected
image
>> SIMULATION
Readout direction
Cleaning CTE Trails
Raw Data
Quantitative Validation





Matches photometric and spectroscopic empirical
corrections (Goudfrooij & Kimble 2002, Bohlin & Goudfrooij 2003)
well with physically realistic parameters
Charge restored reliably to central isophotes of point
sources => whole image array is corrected reliably.
2 CE-STIS ISRs describing these results
Many parameters still not well confined - there is a lot of
parameter space to search!
Still some outstanding issues regarding the more
precise chip details.
The CTI Pre-Processor Pipeline




Originally developed to automate testing and
comparison to empirical models
Automated application of CTI correction,
CALSTIS pipeline and evaluation of corrected
data
Designed so that the CTI correction is a stand
alone processing step executed before CALSTIS
and completely independent of CALSTIS
CALSTIS is run as for normal raw dataset
CTI Pre-processor Tasks:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Extracts relevant header details
Corrects dark and bias reference files
Updates dark and bias file keywords in header
Prepares raw data for readout simulation
Simulation goes forward (adds CTI effects)
Simulates read out of all sub-exposures in raw data
Compares output to input, difference subtracted directly from raw
science extension [1,4,7…]
Runs CALSTIS on corrected raw and original raw data
Automatically extracts sources/spectra from CTI corrected and
uncorrected calibrated data
Uses empirical corrections as a comparison for extracted
sources/spectra.



Darks also require CTI
correction
Readout simulation
reproduces some of the
signature which is present in
the standard bias files =>
bias files must be updated
The pre-processor:




obtains required reference files
corrects them
updates the raw header
New ISR discussing this
Dark reference image before
and after CTI correction
Readout direction
Dark and Bias Reference Files
Dark and Bias Reference Files
Modify dark
Modify bias
Correct CTI and calibrate
Calibrated
Data
Readout direction
Calibrated
Data
Readout direction
Summary


Modeling concept works
Pipeline implementation developed as preprocessor to CALSTIS
 Available
for testing internally (ST-ECF/STScI)
 Still very much under development




Potential to be exported to WFPC2, ACS
ISRs available from:
www.stecf.org/poa
Software directly from me:
bristowp@eso.org
Criticism very welcome, without it this won’t get
beyond BETA!