this lab teaches how to obtain spectra and conduct basic calibration of spectroscopic...

Astronomical Spectroscopy

Lab #2: “Introduction to Spectroscopy”This lab teaches how to obtain spectra and conduct basic calibration of spectroscopic data.

• Due is Nov 6 (start early!!!) • No class on Oct 14 and Nov 4 • Group-led discussion on Oct 21 and 18

Two experimental steps:(1) using the in-lab spectrometer (lab tutorial

sessions on Oct 7 & 8);(2) using the campus telescope spectrometer (late-

night telescope sessions between 4th and 5th

week of October)

Short Introduction to Lab #2

What is spectroscopy and why is it important?

How does spectroscopy work?

What is the equipment the we need for spectroscopy?

How do we reduce spectroscopic data? Specifically how do we know the wavelengths of the photons (= wavelength calibration)?


They are very different … Let’s talk about the difference.

Image (Photometry) Spectrum (Spectroscopy)


Integrated Light Over Wavelength Range

Image (Photometry) Spectrum (Spectroscopy)

Dispersed Light Over the Wavelength


What is the best real life example of spectrum?


Rainbow is a dispersed light (= spectrum) of sunlight by the Earth’s atmosphere (acting as a prism)!


Solar spectrum looks like this – why?


Solar spectrum looks like this – why?

Blackbody-like Continuum Emission (T 6000K)

Line Emission from different atoms (e.g., H, He, C, etc)


The peak wavelength increases as the temperature decreases!

Blackbody Radiation


What is the best real life example of line emission?


Neon Light!


Neon has many bright emission lines in the visible band.

Wavelength ( )


Spectroscopy tells us:

(1) Continuum shape (or temperature if the continuum is blackbody radiation)

(2) Chemical elements/composition for line emission/absorption and physical conditions of the elements

(3) Velocity of the source ( how?)


What do we need to get spectra?

(1) light sources (e.g., stars, galaxies, The Sun, bulb, etc)

(2) ?

(3) detector (e.g., CCD)




(2) dispersing elements (like what?)





(2) dispersing element (prism, grating, etc)



Dispersion of Light by Diffraction Grating

After a dispersion element (e.g., prism, grating), photons of different wavelengths (= colors) travel different paths!


“Photons of different wavelengths arrive at different positions (= pixels) on the detector (e.g., CCD)”

So you can tell the wavelengths of the photons.

Source

Detector

(Grating, Prism, etc)

2

1

Simple Spectrograph Configuration


How do we know the wavelengths of the photons obtained in spectroscopy?

All we have is digitized intensities of detector pixels.


How do we know the wavelengths of the photons obtained in spectroscopy?

Take a spectrum of a known line source (e.g., Ne lamp) with the same instrument.

Establish a relation (“wavelength mapping solution”) between the wavelengths and detector pixels using the wavelengths and detector pixel positions of the known lines. The more line you use, you can obtain the more reliable solution.

Apply the wavelength solution to your spectra.

Astronomical SpectroscopyNeon Calibration Lamp Spectrum

Obtaining a wavelength mapping solution btw. the wavelengths of the calibration lines (e.g. Ne lines) and detector pixel positions using linear least-square fitting is an essential component of Lab #2.


1 (Detector Pixels) 2048

Example: Wavelength calibrated spectrum

All right, let’s look (astronomical) spectroscopy a bit more.

Stellar Spectrum

Example solar spectrum obtained in previous AST325/326

Does this make

sense to you?

Spectrum: Intensity of radiation as a function of wavelength (“dispersed light”)Continuum (e.g., blackbody radiation) and Line Emission

Continuum occurs at all wavelengths: e.g., Bλ(T) as below.

Spectrum: Intensity of radiation as a function of wavelength (“dispersed light”)Continuum (e.g., blackbody radiation) and Line Emission

Line emission occurs at specific wavelength: λ = hc/(E2-E1) E = h = h/c

How are they (left vs. right) different?

Radiation: Continuum & Line Emission

Continuum emission: three types are known


Continuum emission:

Blackbody Radiation (e.g., stellar radiation)

Synchrotron Radiation (electrons around B field, e.g., accelerators, pulsars)

Thermal Free-Free Radiation (= Bremsstrahlung, e.g., ionized gas around hot stars)


Line emission:

Atomic Transition (e.g., H I lines)

Molecular Transition (e.g., OH, CO lines)

Solid-State Feature (e.g., aerosol, …)

Line Emission/Absorption: e.g., Hydrogen (H)

H series (mostly in the visible bands)

e.g. H transition: n = 3 → 2 transition at 656.3 nm.

R: Rydberg constant for hydrogen.

Observed Stellar Spectrum = Continuum + Line Transition (mostly absorption)

Spectrum: intensity of radiation as a function of wavelength (“dispersed light”)Continuum (e.g., blackbody radiation) and Line Emission

Galaxy Spectra: examples

Galaxy: numerous stars and gas clouds

Mixture of continua, absorption, and emission lines


Example Spectrum of a Dusty Galaxy: Complex spectral features

Spectrum provides information for chemical composition!

Star: let’s assume it to be a pure continuum source

Gas cloudContinuum source through gas cloud

Colored bars: gas cloud emission lines

Continuum source

Assumptions:

[1] Star is a pure continuum source.

[2] Gas cloud has no continuum emission.

White bars: gas cloud absorption lines

Depending on the relative positions of the sources and observers, spectra appear differently.

Let’s understand formation of absorption lines.

The Sun radiates continuum emission close to a blackbody radiation of 6000 K. Many elements at its photosphere absorb the continuum emission to create the numerous absorption line features.

Spectra give critical information about the source (e.g., temperature, density, composition, etc)

Stellar Spectra Stars are often classified to be “OBAFGKM”

type depending on their surface temperature.

Finally, spectrum gives velocity information. How?

Photons wavelength changes depending on the relative motion of the source. By measuring the wavelength offset, you can calculate the velocity of the source.

Spectrum gives velocity information.

Doppler Shift

Spectrum gives velocity information.

Doppler Shift

: Offset btw. the measured and intrinsic wavelength.So by measuring the wavelength offset (= Doppler shift), we can obtain the velocity of the source.

this lab teaches how to obtain spectra and conduct basic calibration of spectroscopic...

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