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Wavelength calibration in physical model based calibration pipelines. Astronomical Data Analysis III S. Agata su

Wavelength calibration in physical model based calibration pipelines. Astronomical Data Analysis III S. Agata sui due Golfi, Naples, April 2004. Overview. IPMG at ST-ECF - Who we are. HST Spectrographs & traditional pipelines. Predictive calibration based on:

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Wavelength calibration in physical model based calibration pipelines. Astronomical Data Analysis III S. Agata su

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

  2. 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. 04/30/04 ADA III - Napoli

  3. 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) 04/30/04 ADA III - Napoli

  4. 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 04/30/04 ADA III - Napoli

  5. STIS optical layout 04/30/04 ADA III - Napoli

  6. 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. 04/30/04 ADA III - Napoli

  7. Pipeline Flow for Spectroscopic Data 04/30/04 ADA III - Napoli

  8. 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. 04/30/04 ADA III - Napoli

  9. 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”). 04/30/04 ADA III - Napoli

  10. Short and Long Wavecal 04/30/04 ADA III - Napoli

  11. Short and long wavecal (detail) 04/30/04 ADA III - Napoli

  12. 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. 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. 04/30/04 ADA III - Napoli

  13. 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 04/30/04 ADA III - Napoli

  14. 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. 04/30/04 ADA III - Napoli

  15. 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 localminima. • 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. 04/30/04 ADA III - Napoli

  16. Decrease Temperature Randomize according to the T Store it. Better than the current solution ? Accept or reject based on Boltzmann Probability Distribution. Random tries > Max_Iteration ? Start SA Data Flow Yes Min Temperature reached ? NO No Yes Exit Yes No ADA III - Napoli

  17. Mode,Slit,Central wavelength, Catalog lines Wavecal (FITS Image) Center a box around the predicted lines positions and estimate the exact observed positions by fitting the line shape with a Gaussoid. Calculates the predicted X,Y lines positions on the detector. Output: observed X,Y lines positions on the detector. Fitlines 04/30/04 ADA III - Napoli

  18. One or more configuration file each with 35 parameters One or more sets of lines positions from Fitlines. Annealing Algorithm to optimize the set of parameters for each configuration. repeat the anneling process. STIS Anneal If config file is good store it If not good 04/30/04 ADA III - Napoli

  19. Master Catalog ExtractSubCatalog SubCatalog (X,Y)measured New Cfg File Reject Bad lines, Change Weights, Intensity thresholds Not Yet good ? 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 ? ADA III - Napoli

  20. 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. 04/30/04 ADA III - Napoli

  21. User selects a mode, Central wavelength … Run Fitlines + Fast-Anneal in order to calculate the the actual MSM position Final set of config files for each mode, Central wavelength, slit, Epoch. Run Calstis Config File extracted but MSM positions may no longer be accurate. SA into the Science Pipeline 04/30/04 ADA III - Napoli

  22. Select a mode and fetch all the wavecals. Run Fitlines +Anneal 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. 04/30/04 ADA III - Napoli

  23. 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 A&AS 126, 563-571 (1997). www.stecf.org/poa/pcrel/scicase.html www.eso.org/observing/dfo/quality/Messenger/UVES_Messenger_101.html 04/30/04 ADA III - Napoli

  24. 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… 04/26/04 ADA III - Napoli

  25. 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). 04/30/04 ADA III - Napoli

  26. 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 04/30/04 ADA III - Napoli

  27. 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. 04/30/04 ADA III - Napoli

  28. Standard Calfos dispersion solution 04/30/04 ADA III - Napoli

  29. Improved dispersion solution. 04/30/04 ADA III - Napoli

  30. STIS Spectroscopic Capabilities 04/30/04 ADA III - Napoli

  31. 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. 04/30/04 ADA III - Napoli

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