Wavelength calibration in physical model based calibration pipelines. Astronomical Data Analysis III S. Agata sui due Golfi, Naples, April 2004

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Wavelength calibration in physical model based
calibration pipelines. Astronomical Data
Analysis III S. Agata sui due Golfi,
Naples, April 2004
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Overview

• IPMG at ST-ECF - Who we are.
• HST Spectrographs traditional pipelines.
• Predictive calibration based on
• physical model of the instrument
• simulated annealing technique for optimization
• Show how we implement this into the science data
  pipeline.

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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
• Wavelength Calibration
• Calibration lamp line list - measurements at NIST
• Detector Model repairing the Charge Transfer
  (CTE)

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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

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STIS optical layout
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STIS Pipeline calstis

• calstis for spectra - series of modules that
• Control the data flow through the pipeline
• Basic 2-D image reduction (e.g. bias subtraction)
• Reject cosmic rays from CCD data
• Process the contemporaneously obtained wavecal
  data to ascertain zero point shifts in the
  spectral and spatial directions
• Extract 1 dimensional spectra need to know
  geometry
• Perform spectroscopic wavelength and flux
  calibration
• Sum any CR-SPLIT and REPEATOBS exposures.

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Pipeline Flow for Spectroscopic Data
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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.

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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).

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Short and Long Wavecal
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Short and long wavecal (detail)
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Traditional Pipelines 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.

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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

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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.

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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 dont 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.

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SA Data Flow
Start
Yes
Min Temperature reached ?
NO
Yes
No
Exit
Yes
No
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Fitlines
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STIS Anneal
If config file is good store it
If not good
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Reference Files Data Flow
Mode, CenWave, SlitPos, Config File What else?
Fitlines
Wavecal exposure
Not possible to anneal all parameters at the same
time therefore needs to identify set of them to
be annealed.
STISAnneal
If good store it
Store it or not Store it ?
If not good
Learning curve for a new instrument.
New Master ?
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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.

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SA into the Science Pipeline
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Discover Dependencies
Relation T, Focal Length ?
N Wavecals extracted.
Enhance the model
N Config files
Analyze config files against environmental
conditions.
Number of config files reduced.
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Modeling Echelle Spectrographs

• At the ST-ECF we are currently implementing
• a STIS model based on first optical principles.
  It
• incorporates off-plane grating equations and 3D
  rotations in
• order to account for line tilt and order
  curvature.
• Similar formalism had already been partially
  implemented
• and applied for FOS(HST), UVES, CASPEC pipelines
  with
• significant science improvement.
• See Ballester and Rosa AAS 126, 563-571 (1997).
• www.stecf.org/poa/pcrel/scicase.html
• www.eso.org/observing/dfo/quality/Messenger/UVES_M
  essenger_101.html

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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

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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).

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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_M
  essenger_101.html

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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 1115407, 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.

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Standard Calfos dispersion solution
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Improved dispersion solution.
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STIS Spectroscopic Capabilities

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Traditional Pipelines 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.

04/30/04
ADA III - Napoli