1. 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

2. 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.

3. 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

4. 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?

5. Quick S/N Ratio Estimates

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

7. 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!

8. 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?

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

10. 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)