Spectral Line Measurement What can we measure from a spectral line? total absorption width detailed shape asymmetry wavelength polarization Need spectral resolution Low/moderate 10,000-50,000 line shape dominated by instrument profile High resolution >50,000 true shape of line profile can be determined

Instrument Profile How is the true spectral profile changed by the instrument width of entrance slit pixel size optical aberrations The IP is the profile that would result from an infinitely narrow spectra line The observed profile is the convolution of the true line profile with the instrument profile For high S/N ratio data, Fourier techniques can be used to back out the IP from a spectrum to recover some of the resolution.

Arcturus at several resolutions The original spectrum from the Arcturus Atlas has been convolved with different Gaussians Note the increased line blending and the difficulty in finding and measuring weak lines. Also note the depression of the continuum. The 2nd spectrum from the top is comparable to Hydra echelle resolution 1.1A 0.5A 0.3A 0.18A 0.17A

Noise Identifying and measuring weak lines is difficult in noisy data high 30 90 225 S/N ratio Noise Identifying and measuring weak lines is difficult in noisy data Where should the continuum be set?

Quick S/N Ratio Estimates

Resolution vs. S/N Ratio high res, high S/N low res, high S/N low res, low S/N

Scattered Light Two components randomly scattered light (dust on mirrors, in the air) interpolate from areas adjacent to the spectrum what is underneath linearly scattered light from slit and grating (along dispersion direction) much harder to measure!

Continuum Setting continuum is the hardest part Compare to high S/N ratio atlases Find locations with true continuum What’s noise and what’s real?

Setting Continuum with Noise It isn’t always easy to know where to set the continuum!

Line Strength or Equivalent Width Measuring the area of spectral lines integration fit a profile (Gaussian, Voigt) fit a triangle (1/2 base x height)