1. University of Manitoba Astronomy Club
Special Club!
Contact:
Wonder about the origins of the Universe? Curious about black holes? Come out and help design the club activities! No need to be a geek 
Meeting Wednesday at 5:30 pm in Allen 330

2. Today 3pm practice math and prepare for test.
Phys 1830: Lecture 9
500° K
1000° K
2000° K
5000° K
10,000° K
20,000° K
X-Ray Ultraviolet Visible Infrared Microwave Radio
Intensity
Tutorial/Office hour
Today 3pm practice math and prepare for test.
Test THIS Friday Jan 30th
In class. See online handout.
Includes telescopes
(Wednesday’s material)
Previous class:
Kirchhoff’s Laws
This class:
Spectra
How the interaction of light & matter produce spectra.
Optical Telescopes
Upcoming Topics
Radio Telescopes
B&W imaging workshop – scheduled Monday Feb 2
Test tip: make sure you know all the rules for the powers of 10 that are on the handout on the supplemental resource page.
Change of password
No more planetarium events.
Contact Mr. Cameron.

3. Lecture 9: Telescopes -- Extending Vision
Next topics:
Optical Telescopes and detectors
Light gathering power
resolution
surface brightness
Radio Telescopes
21 cm emission
Multiwavelength observations

4. Kirchhoff’s Laws Spectra 3 empirical laws
summary
Recall column
Kirchhoff’s Laws
3 empirical laws
Hot opaque body  continuous spectrum
Cooler transparent gas between source & observer  absorption line spectrum
Diffuse, transparent gas  emission line spectrum
 means “gives”
Astronomical objects have spectral “finger prints”.
Also called “Continuum emission” can be produced by a hot dense gas. (Like a light bulb – a rainbow of colours.) If we plot this as intensity versus wavelength this produces a blackbody curve.
c) Emission lines are produced by diffuse (low density) gas. (Like a neon sign.)
b) Place the diffuse gas in front of the hot, gas and absorption-lines are created. There is the continuum (rainbow) with dark absorption lines where the emission lines would be.
(see animation)

5. Spectral Finger Prints
Solar Spectrum
Note that the emission lines for the lab spectrum of iron are at the same wavelengths of the absorption lines of iron in the sun.
We can use line spectra to determine the chemical elements in an object.

6. Interaction of Light and Matter
Creating spectral lines at visible wavelengths
Absorption:
If the photon’s energy is not matched to any energy level then the photon passes by the atom. The atom is unchanged.
Three cases associated with absorption.

7. Interaction of Light and Matter
Creating spectral lines at visible wavelengths
Absorption:
2. If the photon’s energy matches the energy needed to cause an electron to jump to a larger energy level, then the atom absorbs the photon (i.e. absorbs energy) and the electron jumps to that energy level. The atom is now in an excited state.

8. Interaction of Light and Matter
Creating spectral lines at visible wavelengths
Absorption:
3. If the photon’s energy is larger than any jump within the atom, then the atom absorbs energy, the photon disappears, and an electron (or more) are kicked out of the atom creating an ion. (In an ion the charge is not balanced.) We say that the atom is ionized.
Some students may think of the difference between the ground state and highest energy level as a jump within the atom. Actually the highest energy level is calculated for an electron is at an infinite distance. Therefore a photon with an energy matching the jump between a specific state and the highest energy level will cause the atom to become ionized.

9. Interaction of Light and Matter
Eg. Iron absorption lines in the atmosphere of the sun.
Photons from the sun’s surface are absorbed by the gaseous iron atoms in the sun’s atmosphere.
The electrons at originally at those energy levels are kicked into excited or ionized states.

10. Interaction of Light and Matter
Creating spectral lines at visible wavelengths
Emission:
The photon matches the energy of the next energy level so the electron is kicked up to that level. However atoms wish to be in the lowest energy configuration so in a very short period of time (e.g. 10 to the power -8 sec) the electron drops back to the lower level. When it does so it emits a photon of the same energy.
Photons can also be emitted spontaneously when an electron falls back down to lower energy levels.
An atom can be excited or ionized. An ejected electron can subsequently be recaptured.
(animation)

11. Interaction of Light and Matter
Creating spectral lines at visible wavelengths
Emission:
Note that there are 2 possible paths in the diagram. The bottom path generates the H_alpha photon.
These electrons can cascade through different energy levels, generating photons that have wavelengths in the visible regime.
The energy level in this example is called “H ” where is “alpha” and glows red – nm.

12. Interaction of Light and Matter
Creating spectral lines at visible wavelengths
Emission:
The Orion Nebula
David Malin
H II regions (said “H two”, since ionized hydrogen is H II and neutrally charged hydrogen is H I (said “H one”).
Add bright white to red, and you get pink.
Recall for David Malin’s images – this is what your eye would see if it was as sensitive as photographic film.
Clouds of gas that glow due to this process have a few names:
Emission nebulae
H II regions
H regions
If they are very bright, they are pinker.
The ionizing photons come from hot stars.

13. Each chemical element has its own “finger print” of lines.
Spectral Finger Print
Hydrogen Atom Energy Levels
Each chemical element has its own “finger print” of lines.
The number of lines for one element depends only on the number of energy levels in its atom.
The more elements in a star, the more lines in the star’s spectrum.

14. Spectral Finger Print
The strength of the absorption lines gives the number of atoms of that element in the gas.
Comparison of strengths of absorption lines of different elements in the gas gives
Density
Temperature
Can get these characteristics for the outer layers of a star from its absorption line spectrum.
How dark a line is, is its strength.
Draw a plot of Intensity vs Wavelength... It is a black body curve with dips in it. The width of the absorption line (dip) and depth of it give density and temperature.

15. What can we do with this information?
Study activity on the sun!
Isolating the light in the UV associated just with an emission line of iron, the sun looks very different than in optical light.
the Sun in extreme ultraviolet light (Solar Dynamics Observatory.)
false-color image shows emission from highly ionized iron atoms.
Loops and arcs trace the glowing plasma suspended in magnetic fields above solar active regions.

16. What can we do with this information?
(note asteroid trail in upper right corner)
Consider stars...

17. What can we do with this spectral information?
Spectral Finger Print
What can we do with this spectral information?
If 2 stars have the same elements, same density, and same temperature then they have the same intrinsic luminosity.
If they have the same intrinsic luminosity we can use their apparent brightnesses to derive their relative distances using the Inverse Square Brightness Law!

18. The density of the gas in the stellar atmosphere.
Spectral Finger Print
The uniqueness of the spectral line pattern of any element is caused by
The density of the gas in the stellar atmosphere.
The temperature of the gas in the stellar atmosphere.
The energy level structure of the atom.
How dark a line is, is its strength.

19. Instrumentation for observing spectra:
To view spectra we use a spectrograph on a telescope.
Rather than using a prism, modern instruments use a diffraction grating to disperse the light.
Credit: Adapted from a diagram by James B. Kaler, in "Stars and their Spectra," Cambridge University Press, 1989.
A diffraction grating is a glass surface with narrow lines ruled into the surface. These reflect the light in such a way that the incoming “white light” is dispersed (spread out).

20. Telescopes and Detectors:

21. What does a telescope really do?
gather light to see
faint objects
focus the light to form
an image or spectrum
resolves the image to see
detail in the image

22. Light Gathering Power (LGP)
How does it do that?
2.4m
150mm
Gathers light – depends on the size of the primary (main) mirror or lens in optical telescopes.
Light Gathering Power (LGP)
Area of a circle is pi * radius squared and the radius is ½ diameter.
Therefore

23. Light Gathering Power
Colour scale is inverted so that bright stars are black. Your eye discerns black on white better.

24. Light Gathering Power - detector used is no longer the eye
LGP increases with the square of diameter of mirror/lens.
Light Gathering Power
summary
Text
Recall column
Text
- detector used is no longer the eye
- early on replaced by the glass photographic plate
- more recently with the CCD
smaller diameter
larger diameter
Digital Photography
Photographic Plate

25. Charge-Coupled Device (CCD)
summary
Recall column
CCD replaced the Photographic Glass Plate as a recording medium (which in turn had replaced the eye as a detector)
similar to chip used in digital cameras but in astronomy we record in black and white. (More colour images coming up.)
CCD has a grid of pixels. Each pixel captures photons – allowing them to be counted in each pixel. Each photon generates a specified number of electrons and these generate our digital datasets which are displayed by computers.

26. Image Formation with optical telescopes
double
convex
lens
Refraction
Two common methods of manipulating light
Refraction: light bends due to lens - curve the surfaces appropriately to direct the light
Reflection: light bends due to mirror - curve the surface appropriately to direct the light
Both methods focus the light. However lens are heavier than mirrors. If you want to have a large diameter for higher resolution or collecting light from faint stars, then it is economical to use a mirror rather than a lens.
More in the lab.
Reflection

27. Resolution – depends on the diameter of the mirror:
summary
Recall column
Low Resolution
High Resolution
Recall... lambda == wavelength.
The larger the mirror, the finer the *angular* resolution.
The smaller the wavelength, the finer the angular resolution.
High resolution: you can see things closer together as distinct objects
Low resolution: things close together appear as one object
The implications are
- the larger the diameter of the telescope, the higher (finer) the resolution.
- the shorter the wavelength, the higher the resolution.
If 2 stars exist and the angular resolution is low then they look like 1 star. If the angular resolution is good enough then they are distinguishable as 2 stars.
-- If the angular resolution is small (i.e. fine), the resolution is high
 of mirror

28. If the angular resolution in an image is large then by definition the resolution of an image is high and one sees a lot of detail.
True
False

29. Talk to your neighbour about:
which are harder to build support structures for, mirrors or lenses? Why?
how does the diameter of the primary mirror effect the light gathering power and the resolution?
what is the relationship between angular resolution and resolution?

30. They can collect more photons and hence show faint objects.
Review Question
Large reflector telescopes are better than medium-sized refractor telescopes because
They can collect more photons and hence show faint objects.
Telescopes with larger diameter mirrors show more detail within the field of view.
Mirrors are lighter than lenses and so reflecting telescopes need less support structure than refractors.
All of the above.
Discuss with neighbours.

31. Telescopes: World’s largest
Recall column
Text
notice the last column! Build observatories at high altitude. Why?
So is bigger, better?
Notice that they are all reflectors and built at high altitude. Why high altitude?

32. Atmospheric Transmission
ionized gases
N2,O2
H2O, CO2
The atmosphere filters electromagnetic radiation to us ground based folk. The molecules in the atmosphere that are doing the absorption are listed above the diagram.
Notice IR band.

33. Mountaintop Sites build high to get above most of the atmosphere
summary
Recall column
build high to get above most of the atmosphere

34. Atmospheric Seeing – affects resolution!
>1"
κ Peg
Atmosphere limits what you can see in several ways

35. Adaptive Optics
summary
Recall column
Keck 1I 10m
1"
Individually control the smaller mirrors to compensate for the atmospheric motion.
Astronomers can use an artificial star – laser. They know the characteristics of this “star” well and can deform the mirror until the laser “star” appears undistorted by the atmosphere.
0.3"
1.8m hex
Keck 1 10m
Can achieve a mirror’s resolution in the near infra-red.

36. Why SpaceborneTelescopes?
summary
Recall column
Relatively small telescope – 2.5m diameter mirror - but very effective instrument by virtue of being above the atmosphere.
HST 2.4m

37. Orion transformed?

38. Can observe at wavelengths that the Earth’s atmosphere blocks.
Above the atmosphere
far infrared view quite different
Optical
Far Infrared
Can observe at wavelengths that the Earth’s atmosphere blocks.

39. Planck/Hershel and IRAS satellites: Far-Infrared (FIR)
False colour of course. Check APOD for colour coding. Dust in our Milky Way.

40. Summary:
summary
Recall column
- Resolution is affected by atmospheric seeing. - Atmospheric transmission filters electromagnetic radiation.
Seeing is less affected at longer wavelengths. Near IR ground-based observations can be as high resolution as optical observations on the HST.
The atmosphere filters electromagnetic radiation to us ground based folk

41. Resolution Revisited: When one doesn’t want high resolution.
Kepler Mission’s “first light” image.
This is a space-based satellite so it isn’t impacted by atmospheric seeing.
Globular Cluster NGC 6791 – about a million stars in this system.
Why is this image of a globular cluster from NASA’s Kepler Mission fuzzy?

42. Resolution Revisited: When one doesn’t want high resolution.
What categories (e.g. x-ray through radio) of the EM spectrum does this wavelength range cover?
Kepler Mission is specifically is designed to survey our region of the Milky Way galaxy to discover hundreds of Earth-size and smaller planets.
Primary mirror: 1.4 meter diameter
Wavelength range: nm

43. Resolution Revisited: When one doesn’t want high resolution.
Recall light curves are plots of brightness versus time. Kepler’s plots are of the total light of a system consisting of a star and it’s planet.
The large dip in brightness occurs when some of the star’s light is blocked by a planet as it passes in front - transit.
The smaller dip in brightness occurs when the planet’s light is blocked by the star as it passes behind - occultation.
(Rising and falling slopes due to phases of planet.)
Spacebased photometry - measurement of amount of light
Light curves for stars to measure:
Transits of planets across the front of the star
Occultations of planets as they pass behind the star

44. Charge-Coupled Device for Optical Images
The number of photons per pixel are converted into electrons
each pixel can hold electrons
if there are too many photons from a bright star then there are too many electrons and they spill over into neighbouring pixels.
Unlike film, a CCD is a straight-forward photon counting device.

45. Resolution Revisited: When one doesn’t want high resolution.
HST exposure of NGC 6791
Saturation occurs when the pixel in the CCD has too much charge due to too many photons falling on that pixel.
The photons are spread over more pixels so that a given pixel will not saturate.
Notice that this exposure is in black and white.
Bleeding or blooming from saturated pixels in bright stars.
Can avoid bleeding in lower resolution images.

46. Resolution Revisited: When one doesn’t want high resolution.
Kepler Mission’s “first light” image.
Avoids errors caused by bleeding. Photometry is the counting of photons in an area covering the star in an image.
Images of stars from NASA’s Kepler Mission are fuzzy so that stellar photometry can be measured accurately.

47. Review Question:
The Kepler Mission’s goals are to take the highest resolution images of stars as is possible in order to image planets encircling those stars.
True
False

48. Review Question:
Transit is when a smaller object passes behind a larger object. Occultation is when the smaller object is between the viewer and the larger object.
True
False