Spectroscopy - Strategies, Reduction, and Analysis
Feeding your spectrograph
1. Longslit (what we've been talking
about)
- Slit width - typically match seeing.
- wide slit: lots of
light, but degrades spectral resolution and gives more sky
contamination. also need to worry about non-uniform slit illumination
on extended sources.
- narrow slight:
highest resolution, but capture less light.
- Slit Orientation
- point sources: sworry about atmospheric dispersion --
rotate to parallactic angle (ie perpendicular to horizon)
- extended sources: choice based on science
2. Fiber Fed multiobject
- Fibers are fixed size. What do you need to worry about?
- Fibers can only be packed so tightly
- Machining and plugging plates can be labor intensive.
3. Integral Field Units
Closely packed fiber bundles, used for spectroscopy of extended
objects. ie Sparsepak:
4. Objective Prism/Grism Spectroscopy
No slit, no fiber, just let the light in!
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Direct and slitless grism spectroscopy images of a field of
galaxies using the HST Advanced Camera System. Note three emission line
galaxies appearing in the right image. Originally from
http://www.astro.spbu.ru/staff/dio/ACS_G800L/, via Steve Majewski's Astronomical
Techniques lecture notes
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Spectroscopic Reduction
Basic CCD reduction: bias, flat field, etc
Wavelength Calibration:
For a pixel x, what is the
associated wavelength? ie, a polynomial: lambda(x) = C0 + C1*x + C2*x2
+ ...
Calibration changes over time: run-to-run, night-to-night,
hour-to-hour. Why? How do we calibrate it?
Arc lamps (before and
after exposure, or during exposure)
Night sky (during exposure)
Absorption cell (I2 or
HF)
In reality, spectra aren't
rectilinear: lambda(x) --> lambda(x,y)
Raw
calibration arc
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Rectified
image
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Sky subtraction
Need slit to see sky. Then can
extract the sky spectrum and subtract.
This is often very difficult near
bright sky lines, rendering the data in this region useless.
Spectrophotometric Calibration
Ask yourself: is
it necessary?
Spectrophotometric standards: typically hot, spectrally featureless
stars
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Spectroscopic Analysis
Radial Velocities:
Emission lines: Pretty straightforward
Absorption lines: more complicated.
- Observe "radial velocity standards" - star of known velocity
- Use cross-correlation between RV standard and your object to
get velocity
- Complication: line broadening
Line Strengths:
Definition of equivalent width
Must define continuum level. This can be hard!
