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Considerations for Precision Spectroscopy at STScI: Calibration and Measurement Techniques

This document discusses key aspects of precision spectroscopy, emphasizing calibration and measurement requirements based on scientific goals. It covers topics such as radial velocities, stellar atmosphere analysis, and the impact of spectrophotometric precision on astronomical observations. The calibration process involves understanding various elements, including the line-spread function, wavelength determination, and the choice of light sources. The aim is to achieve the highest possible accuracy in spectral data, crucial for advancing astrophysics research. It also highlights the importance of stable, high-density line sources for precise measurements.

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Considerations for Precision Spectroscopy at STScI: Calibration and Measurement Techniques

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  1. Precision Spectroscopy: some considerations S. Deustua STSCI 2014 STSCI Calibration Workshop

  2. Calibrate these spectra!

  3. As you wish

  4. Spectroscopic Measurements Precision Spectroscopy • Requirements depend on the science goals • Precision radial velocities ~m/s (cm /s?) • Stellar atmosphere composition ~1 Å/mm • High redshift galaxies ~103 km/s Precision Spectrophotometry • As above plus • Photometric precision • Photometric accuracy (absolute flux) R ~ 104 –106 (107?) R ~ 102–103

  5. Calibration The general problem S(λ) = R(λ) x D(λ) Ajk( fk+ sk) = pj+ nj+ bj Ajk: Calibration matrix Given: fk: source flux vectorsk: background vector pj: detectorpixel counts vector nj: Pixel noise vectorbj: Internal background vector Ajk: Calibration matrix • Wavelength solution • Spectral trace solution • Cross-sectional profile • Relative pixel response • Line-spread function • Relative fiber response • Flux calibration • Camera aberrations Adapted from A. Bolton, 2010

  6. Calibration Considerations • Known wavelength as a function of slit widths • Shape of the line spread function (LSF) • Wings of the LSF • Shape of the point spread function (PSF) • Echelleshave significant issues with ghosts and scattered light, need to characterize properly • How well is the dispersion known( nm/pix ) • resolution of the instrument (R=λ/Δλ) • Spectral region - UV, VIS, NIR, MIR • Stability • NIST traceable standards (wavelength, flux) • Wavelength lamps, frequency combs, monochromators, tunable lasers

  7. Spectrophometry Considerations Flat Fields • Light source has significant slope in spectral energy distribution – different than the target • No such thing as a flat continuum slope (sadly) • lamps • laser driven light sources • xenon plasma (between 300-400 almost has a flat continuum!) • Faint targets • Flux Standards

  8. NplexSpectroscopy Monoplex: • Single slit • One or two objects in slit Slitless Multiplex • Objective prism • Grisms • Multiple objects on array • Overlapping spectra • Low resolution Slit or Pseudo slit Multiplex • multishutter arrays, • integral field units, • multi fiber spectroscopy • ‘Slits’ • Slit masks • Fibers • Micro shutters • Multiple objects • Minimal overlap • High Resolution

  9. Multiplex Modes

  10. CANDELS field – WFC3 IR Grism Slitless Spectroscopy

  11. Wavelength Calibration • R~100 000 doable • R~1 000 000 harder • R~200 (slitless) harder to calibrate precisely in wavelength • Telluric Features • Astrophysical Sources (e.g Planetary Nebulae) • Hollow Cathode Lamps • Laser driven light sources • Line density must match resolution

  12. Hollow Cathode Lamps • Where astronomy needs hitchhiking on industry • Elements: Neon, Argon, Xenon, Deuterium, Thorium, Uranium • Purity of spectrum is important • Line width ~0.005 nm • Good for years, but do degrade.

  13. HCL Line density

  14. Comparing HCL in NIR Redman et al

  15. Thorium – Argon HCL, 5 microns - VIS http://physics.nist.gov/PhysRefData/ThArLampAtlas/ThArLampAtlas.html

  16. Astrophysical Sources Bright enough, compact enough, sufficient line density e.g. PN IC 5117, Vy 2-2 Telluric lines from the ground e.g. OH. Rudy et al

  17. Laser Combs • Checking on fundamentals in physics • UV, optical, NIR • Tailored for high resolution only, • though some are being designs for low resolution work • Stablity over decades/years • Excellent frequency standard • 10-11 for the system • System performance depend on the quality of the components • Are not turnkey systems • Expensive ~$1 000 000

  18. Summary • Era of precision astrophysics • Interesting astrophysics requires precision • Definiton of precision depends on science goal • To move beyond 0.1 pixel calibration need high line density sources • Deep understanding of instrument characterization Post script good models of instrument behavior, data analysis algorithms are important to extract maximum science

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