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Fiorentino & Rosa
Wavelength calibration in physical model based calibration pipelines.
Astronomical Data Analysis III S. Agata sui due Golfi, Naples, April 2004
Fiorentino & Rosa 04/30/04 ADA III - Napoli
Overview
• IPMG at ST-ECF - Who we are. • HST Spectrographs & traditional pipelines. • Predictive calibration based on:
1. physical model of the instrument
2. simulated annealing technique for optimization
1. Show how we implement this into the science data pipeline.
Fiorentino & Rosa 04/30/04 ADA III - Napoli
IPMG at ST-ECF
• Comprehensive empirical calibration pipeline already exists for the HST STIS Spectrograph
• We aim to improve those components which benefit from physically motivated corrections
• Current work includes 1. Wavelength Calibration
2. Calibration lamp line list - measurements at NIST
3. Detector Model repairing the Charge Transfer (CTE)
Fiorentino & Rosa 04/30/04 ADA III - Napoli
What is STIS ?
• STIS is the HST imaging spectrograph.– spatially resolved spectroscopy from 1150 Å to 10,300 Å at
low to medium spectral resolution– echelle spectroscopy (high resolution) in the ultraviolet.– time tagging of photons in the ultraviolet (high time
resolution).• Since 1997 on board HST • Unlikely to be replaced during the remaining HST lifetime
Fiorentino & Rosa 04/30/04 ADA III - Napoli
STIS optical layout
Fiorentino & Rosa 04/30/04 ADA III - Napoli
STIS Pipeline ‘calstis’
• calstis for spectra - series of modules that1. Control the data flow through the pipeline
2. Basic 2-D image reduction (e.g. bias subtraction)
3. Reject cosmic rays from CCD data
4. Process the contemporaneously obtained wavecal data to ascertain zero point shifts in the spectral and spatial directions
5. Extract 1 dimensional spectra – need to know geometry
6. Perform spectroscopic wavelength and flux calibration
7. Sum any CR-SPLIT and REPEATOBS exposures.
Fiorentino & Rosa 04/30/04 ADA III - Napoli
Pipeline Flow for Spectroscopic Data
Fiorentino & Rosa 04/30/04 ADA III - Napoli
Where the empirical wavelength calibration is currently used.
• Determine MSM offset from wavecal. • Its purpose is to find the offset of the spectrum from the
expected location, owing to non repeatability of the MSM.
• Spectroscopic Calibration and Extraction.• 1-D spectral extraction. A spectrum is extracted along a
narrow band, summing over the cross-dispersion direction and subtracting background values to produce a 1-D array of fluxes for each spectral order.
• In order to calculate the offsets and to assign wavelengths the empirical pipeline uses bi-dimensional polynomial dispersion solutions. Therefore it can only apply linear translations (offsets) , but not rotations.
Fiorentino & Rosa 04/30/04 ADA III - Napoli
STIS Auto Wavecals
A standard wavecal is usually only a few seconds long.•X and Y displacements based on a few lines.
• X and Y are not the same on the whole detector because, the differential rotation (“splaying‘”) of individual echelle orders resulting from the combined effects of the echelle and cross-dispersing elements, cause different orders to be differentially rotated (“splayed”).
Fiorentino & Rosa 04/30/04 ADA III - Napoli
Short and Long Wavecal
Fiorentino & Rosa 04/30/04 ADA III - Napoli
Short and long wavecal (detail)
Fiorentino & Rosa 04/30/04 ADA III - Napoli
Traditional Pipeline’s accuracyvs. Enhanced calibration.
•1)Image shift (-3,+3) pixels due to the MSM.
•2)Thermal effects cause the spectrum to drift by about 0.1 pixels up to 0.35 pixels per orbit.
•3)Shift not always precisely determined due to, for instance, a short wavecal.
1)The Absolute Wavelength zero points shifts are not predicted with the traditional calibration (errors in E140H up to 1.3km/s 0.5-1.0 Pixel).
We aim to reach 0.1 pixel precision.
2)We will have an homogenous calibration for each mode and overall the lifetime of STIS.
Fiorentino & Rosa 04/30/04 ADA III - Napoli
The alternative: predictive calibration
“The calibration of astronomical data can be significantly improved by constructing instrument models which incorporate as fully as possible a knowledge of optical and detector physics”
• A typical example is the wavelength calibration – • empirical dispersion relations should be replaced by a physical
model (simple ray trace) of the spectrograph• This usually yields better than 0.1 % accuracy (1 pix in 1000)
straight away• Distortions may be added to go to sub-pixel accuracy
Fiorentino & Rosa 04/30/04 ADA III - Napoli
Predictive Calibration: Echelle model & Simulated Annealing.
•Mathematical model with about 35 parameters which need to be optimized. Derivatives cannot be easily formulated and analytical inversion is impossible. •Simulated Annealing (SA) is one of the technique which cope with such a problem.•Although easy in principle, its implementation may not be trivial.
Fiorentino & Rosa 04/30/04 ADA III - Napoli
Simulated Annealing.
• SA exploits an analogy between the way in which a metal cools and freezes into a minimum energy crystalline structure and the search for a minimum in a more general system.
• SA don’t get trapped at local minima.
• The algorithm accepts also changes that increase objective function f with a probability following the Boltzmann probability distribution.
• Not all sets of parameters which minimize the cost function are physically acceptable therefore our SA algorithm will make those configurations extremely costly.
•
ADA III - Napoli
Randomize according to the T
Better than the current solution ?
Start
Store it. Yes
Accept or reject based on Boltzmann Probability
Distribution.
NO
Random tries > Max_Iteration ?No
Decrease Temperature
Min Temperature reached ?
Yes
Yes
Exit
No
SA Data Flow
Fiorentino & Rosa 04/30/04 ADA III - Napoli
Fitlines
Mode,Slit,Central wavelength,Catalog lines
Wavecal (FITS Image)
Calculates the predicted X,Y lines positions on the detector.
Output: observed X,Y lines positions on the detector.
Center a box around the predicted lines positionsand estimate the exact observed positions by fittingthe line shape with a Gaussoid.
Fiorentino & Rosa 04/30/04 ADA III - Napoli
STIS Anneal
Annealing Algorithm to optimizethe set of parameters for each configuration.
repeat the annelingprocess.
If config file is good store it
If not good
One or more sets oflines positions from
Fitlines.
One or more configuration
file each with 35 parameters
ADA III - Napoli
Master CatalogMode, CenWave,
SlitPos, Config File What else?
ExtractSubCatalog
Fitlines
SubCatalog
Wavecal exposure
STISAnneal
(X,Y)measured
Store it or not Store it ?
New Cfg File
If good store it
Not Yet good ?
New Master ?
If not good
Reject Bad lines, Change Weights,Intensity thresholds
Not possible to anneal all parameters at the same time therefore needs to identify set of them to be annealed.
Learning curve for a new instrument.
Reference Files Data Flow
Fiorentino & Rosa 04/30/04 ADA III - Napoli
SA into the Science pipeline
• Once all the reference files have been determined we will be able to predict, for a given configuration and for each order and lambda, the position on of the corresponding line on the detector.
• However, in order to cope with the non repeatibility of the MSM, another SA need to be run each time a science exposure is taken.
Fiorentino & Rosa 04/30/04 ADA III - Napoli
SA into the Science Pipeline
Final set of config files for each mode, Central wavelength, slit, Epoch.
User selects a mode, Central wavelength …
Config File extracted but MSM positions may no longer be accurate.
Run Fitlines + Fast-Annealin order to calculate the the actual MSM position
Run Calstis
Fiorentino & Rosa 04/30/04 ADA III - Napoli
Discover Dependencies
Run Fitlines +Anneal
Select a mode and fetch all the wavecals.
N Wavecals extracted.
N Config files
Analyze config files against environmental conditions.
Relation T, Focal Length ?
Enhance the model
Number of config files reduced.
Fiorentino & Rosa 04/30/04 ADA III - Napoli
Modeling Echelle Spectrographs
At the ST-ECF we are currently implementinga STIS model based on first optical principles. Itincorporates off-plane grating equations and 3D rotations inorder to account for line tilt and order curvature.Similar formalism had already been partially implementedand applied for FOS(HST), UVES, CASPEC pipelines withsignificant science improvement.See Ballester and Rosa A&AS 126, 563-571 (1997).
www.stecf.org/poa/pcrel/scicase.htmlwww.eso.org/observing/dfo/quality/Messenger/UVES_Messenger_101.html
Fiorentino & Rosa 04/26/04 ADA III - Napoli
Good only for HST spectrographs ?
• Predictive calibration can be applied to any spectrograph.
• We aim to implement the STIS pipeline such that can be easily re-used for other spectrograph (i.e. Object oriented code).
• Although this is just a part of a pipeline…
Fiorentino & Rosa 04/30/04 ADA III - Napoli
Status of the STIS implementation
• Prototype implementation finished (C++).• Wavelength calibration translated into C in order to import
into the existing IRAF/C STIS pipeline. • Reference files production is in C++ and does not need to
be translated since it is an offline tools. • Future items:• Analyze science cases in order to test the CE_CALSTIS. • Enhance the model (MSM model).
Fiorentino & Rosa 04/30/04 ADA III - Napoli
Referenced articles & URL links
• Ballester and Rosa Astron. & Astrophysic. Suppl.Ser 126, 563-571 (1997).• Ballester & Rosa ADASS XIII, Instrument Modeling in Observational Astronomy. • Kirkpatrick, S., C. D. Gelatt Jr., M. P. Vecchi, "Optimization by Simulated
Annealing",Science, 220, 4598, 671-680, 1983.• Metropolis,N., A. Rosenbluth, M. Rosenbluth, A. Teller, E. Teller, "Equation of
State Calculations by Fast Computing Machines", J. Chem. Phys.,21, 6, 1087-1092, 1953.
• URL links:• www.stecf.org/poa/pcrel/scicase.html• www.stecf.org/poa/index2.html• www.eso.org/observing/dfo/quality/Messenger/UVES_Messenger_101.html
Fiorentino & Rosa 04/30/04 ADA III - Napoli
Science Improved: FOS case.
• Effect of the improved dispersion relation. • We looked at the interstellar absorption lines imprinted on
the spectrum of a low red-shift quasar (PG 1115+407, PI B. Wills).
• There were two separate FOS observations red and black dots. All measurements have been reduced to barycentric velocities.
• The solid line is the weighted average of HI 21 cm line observations with the dashed lines indicating the range of velocities found in the line of sight.
Fiorentino & Rosa 04/30/04 ADA III - Napoli
Standard Calfos dispersion solution
Fiorentino & Rosa 04/30/04 ADA III - Napoli
Improved dispersion solution.
Fiorentino & Rosa 04/30/04 ADA III - Napoli
STIS Spectroscopic Capabilities
•
Fiorentino & Rosa 04/30/04 ADA III - Napoli
Traditional Pipeline’s accuracyvs. enhanced calibration.
•Image shift (-3,+3) pixels due to the MSM.
•Thermal effects cause the spectrum to drift of about 0.1 pixels up to 0.35 pixels per orbit.
•Shift not always precisely determined due to, for instance, a short wavecal.