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

Calibrate these spectra! 2

As you wish 3

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~10 3 km/s Precision Spectrophotometry As above plus Photometric precision Photometric accuracy (absolute flux) R ~ 10 4 –10 6 (10 7 ?) R ~ 10 2 –10 3 4

Calibration The general problem S(λ) = R(λ) x D(λ) Ajk ( fk + sk ) = pj + nj + bj Ajk: Calibration matrix Given: fk: source flux vector sk: background vector pj: detector pixel counts vector nj: Pixel noise vector bj: 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,

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) Echelles have 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 6

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 almost has a flat continuum!) Faint targets Flux Standards 7

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 8

Multiplex Modes 9

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CANDELS field – WFC3 IR Grism Slitless Spectroscopy 11

Wavelength Calibration R~ doable R~ 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 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 for the system System performance depend on the quality of the components Are not turnkey systems Expensive ~$

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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 20