Introduction to Spectroscopy

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.

Solar Spectrum

NOAO Solar Spectral Atlas

Solar Spectrum

Stellar Classification

• Stellar spectral classification forms the bedrock of our understanding of stars.

Wavelength

Exoplanet Detections

51 Peg

• 51 Pegasi b was the first exoplanet discovered around a Main Sequence star.

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.

Galaxy Dynamics

• Evidence for Dark Matter.

Galaxy Properties

SDSS Spectra

Large Scale Structure

• Large Scale Structure of the Universe Revealed by the Sloan Digital Sky Survey.

2D Galaxy Spectroscopy

Most Distant Galaxies

Second Most Distant Galaxy…

z~8.7 (13.2 billion years ago)

Most Distant in 2015

Lyα

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)

Anatomy of a Spectrograph

Collimator

Camera

Sensor

Disperser

Slit or

Fibre Feed

Δx = fΘ (For object/image placed at lens focal length)

Prism Dispersion• Snells’ Law: n1 sinΘ1 = n2 sinΘ2.

• Refractive index varies with λ.

Geometric Effect

L>>D (far-field)

m = Diffraction Order

Transmission Grating

α = Incident Angleβ = Diffracted Angle

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

GratingsTransmission Gratings

Reflective Gratings

Newport Gratings

If you know pixel size (Δx), you can calculate Δλ/pixel.

Linear Dispersion

Angular Dispersion

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.

MEGARA at GTC

Real Astronomical Spectrograph

Lab 2

• Lab Handout and Supporting Material are now on class webpage.

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