introduction to spectroscopyastrolab/files/lecture4_spectroscopy.pdf · astronomical spectroscopy...
TRANSCRIPT
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Introduction to Spectroscopy
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Astronomical Spectroscopy• Spectroscopy determines the intensity of light from a
source as a function of wavelength (or frequency).
• I am mainly going to focus on techniques used in optical/infrared spectroscopy.
• Spectroscopy has been crucial in helping us understand the physical processes that govern distant objects.
• Spectroscopy often reveals the energetics, dynamics and also the composition of an object.
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Solar Spectrum
NOAO Solar Spectral Atlas
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Solar Spectrum
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Stellar Classification
• Stellar spectral classification forms the bedrock of our understanding of stars.
Wavelength
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Exoplanet Detections
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51 Peg
• 51 Pegasi b was the first exoplanet discovered around a Main Sequence star.
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Galaxy Dynamics
Wavelength
Posi
tion
alon
g sl
it
• Spectra of galaxies have revealed their bulk motion from us (redshifts) as well as their internal motion.
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Galaxy Dynamics
• Evidence for Dark Matter.
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Galaxy Properties
SDSS Spectra
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Large Scale Structure
• Large Scale Structure of the Universe Revealed by the Sloan Digital Sky Survey.
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2D Galaxy Spectroscopy
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Most Distant Galaxies
Second Most Distant Galaxy…
z~8.7 (13.2 billion years ago)
Most Distant in 2015
Lyα
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So how do we obtain spectra?
• Need some way of collecting light from only the source we are interested in. (Slit or Fibre Feed)
• Require a method to disperse the light as a function of wavelength. (Prism, Diffraction Grating)
• When the light is dispersed, we need a method that can identify different wavelengths. (Camera + Sensor)
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Anatomy of a Spectrograph
Collimator
Camera
Sensor
Disperser
Slit or
Fibre Feed
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Δx = fΘ (For object/image placed at lens focal length)
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Prism Dispersion• Snells’ Law: n1 sinΘ1 = n2 sinΘ2.
• Refractive index varies with λ.
Geometric Effect
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L>>D (far-field)
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m = Diffraction Order
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Transmission Grating
α = Incident Angleβ = Diffracted Angle
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Diffraction Gratings
• For m = 0, all wavelengths fall at the same place, and the grating acts as a simple mirror.
• Typically higher orders will give weaker spectra: most of the power is put into the lower orders.
• The separation of wavelengths is greater at higher orders: spectral resolving power increases at higher orders.
l2l1
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GratingsTransmission Gratings
Reflective Gratings
Newport Gratings
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If you know pixel size (Δx), you can calculate Δλ/pixel.
Linear Dispersion
Angular Dispersion
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Prism versus Grating Spectrographs
★ Prism spectrographs
• Very efficient (~95% throughput!) over broad wavelength range. • Low spectral resolution (R ~ a few 10s). • Very non-linear dispersion. Need complicated multi-prism
system to improve this.
★Grating spectrographs
• Moderate efficiency (~50-70% throughput) within design wavelength range.
• Broad range of spectral resolution (R ~ 100 - 100,000!). • Linear dispersion.
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MEGARA at GTC
Real Astronomical Spectrograph
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Lab 2
• Lab Handout and Supporting Material are now on class webpage.
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Lab 2 Goals1.Obtain spectra of different sources.
• Analyze with Python.
2.Determine the wavelength calibration of spectrometer.
• Use linear least squares to determine polynomial fit to data to derive wavelength solution.
3.Characterize the CCD detector.
4.Use campus telescope/spectrometer to obtain spectra of astronomical objects.
Due: November 3rd @ 5pm after tutorial/AB220
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