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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
1.1A
0.5A
0.3A
0.18A
0.17A
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
Noise
S/N
ratio
high
30
90
225
• 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)