astro img fun planning notes

8/17/2019 Astro Img Fun Planning Notes

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FUNDAMENTALS OF ASTRONOMICAL IMAGING (IMGS-112-01)

COURSE LECTURE PLANS & NOTESSPRING 2016

INSTRUCTOR: JOEL KASTNER

TA: KRISTINA PUNZI

Week 1

Preliminaries.

• meet and greet...– name, major, interests, hobbies, etc.

– taken prereq.? any astronomy knowledge/background?

• course goals, expectations, evaluation

– goals/expectations:

∗ gain understanding of how astronomy relies on, uses, pushes limits of imaging systems

∗ gain understanding of demands put on imaging systems by astronomers(e.g., wavelength coverage; sizes of telescopes, their resolution & FOV;

immense astronomical distance/size scales; etc)...

∗ The joy of scientific notation! All students will be expected to learn howto think in powers of ten.

∗ Math is the language of science, so we will show (and use) the basicequations that describe how the universe works (e.g., c = λν , E = hν ).But (usually) we will only expect students to be able to think/work interms of proportions and ratios (e.g., if I make an object 10 times hotter,it will emit light at a characteristic wavelength 10 times shorter).

– evaluation: HW, tests, labs, class participation.

Lab exercises: drawn from:∗ Google Sky: https://www.google.com/sky/

∗ AladinLite: http://aladin.u-strasbg.fr/AladinLite/1


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2 INSTRUCTOR: JOEL KASTNER TA: KRISTINA PUNZI

∗ ds9: http://ds9.si.edu/site/Home.html

Class participation: daily in-class student presentations of a recent As-tronomy Picture of the Day (APOD):

http://apod.nasa.gov/apod/astropix.html

The Imaging Chain (review). There are many “imaging chain” variants possible; hereare two I personally find most useful:

• Source (plus intervening medium)

• Object

• Collection

• Detection

•

Processing• Transmission (plus further processing)

then either (essential for non-science applications; desirable/important for science applica-tions):

• Display

• Perception

—or— (essential for most science applications, i.e., for astronomy/astrophysics)

• reduction (aka further processing)

• analysis

• interpretation

The Imaging Chain: an example. We will run through a detailed example, usingimaging system of the day’s (or a recent) APOD.

Discuss/set up HW 1.

• Show Powers of Ten movies (on YouTube):https://www.youtube.com/watch?v=0fKBhvDjuy0

https://www.youtube.com/watch?v=qxXf7AJZ73A

• Examples of use of scientific notation; introduce concept of angular diameter usingMoon and/or Sun


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– concept of lens focal length, f , resulting from refraction of rays entering &leaving lens

– how astronomical imaging (almost) always uses the lens equation

1

s +

1

s =

1

f

at its s → ∞ limit (i.e., the source lies “at infinity”); hence s → f , i.e., theimage forms at the lens’s focal point.EXAMPLE: if the focal length of a lens (or mirror; see below) is 1 meter, thenthe image of a distant star, galaxy, etc. forms approximately 1 meter from thelens. It’s that simple.

– concept of lens (or mirror) f -number: f # = f /D where D is lens (or mirror)diameterEXAMPLE: If a lens has a focal length of f = 1 meter and a diameter of

D = 10 cm, then f /D = 10, and we say the lens in question is an “f /10 lens”.

– concept of telescope for visual use: a telescope for viewing w/ human eye is just is a system of 2 lenses w/ common focal point; yields system magnificationM = f 1/f 2 where f 1, f 2 are focal lengths of objective & eyepieceEXAMPLE: If you use a lens with focal length f 1 = 1 m as the objective (firstlens) and a second lens with a focal length of f 2 = 10 cm as the second lens(eyepiece), then M = f 1/f 2 = 10. So a distant object would appear 10 timesbigger — its angular size (i.e., apparent size) would increase by a factor of 10— when viewed through such a system.

• basics of mirrors

– can construct paraboloid (parabolic) mirror to bring parallel rays to focus;concept of f for a mirror is perfectly identical to that of a lens (so can use lensequation; sign of f changes though)

– why use a mirror instead of a lens?

(1) lenses suffer from aberrations ; mirrors inherently immune from chromaticaberration; and, if ground to parabolic form, are also free from sphericalaberration (well, then there’s the sad Hubble Space Telescope mirrorstory...)

(2) mirrors can be supported from behind, so can be made much bigger than

lenses

– why make telescope collection element (be it a lens or mirror) as big as possi-ble?


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FUNDAMENTALS OF ASTRONOMICAL IMAGING (IMGS-112-01)COURSE LECTURE PLANS & NOTESSPRING 20165

(1) light collecting power : goes like D2 for collecting element of diameter D;specifically, collecting area A = πR2 = π(D/2)2 (where A is typicallyexpressed as m2 or cm2)

(2) angular resolution : angular resolution limit (“blur” or “smear” angle) θis given by

θ ≈ λ/D

for θ in radians . The smaller θ, the better the image quality. So onewants D as big as possible for a given λ.

(3) Basic types of reflector telescopes (use mirrors as main collecting ele-ment):

prime focus: single (primary, concave) mirror; brings light to a fo-cus in front of the mirror, where it can (e.g.) be imaged with acamera

Newtonian: (concave) primary mirror plus diagonal secondary mir-ror to divert focal point to the outside of telescope tube, where itcan be viewed with an eyepiece

Cassegrain: (concave) primary mirror plus (usually convex) secondarymirror to divert focal point through a hole in the primary, where itcan be imaged, viewed with an eyepiece, etc.

• Review of geometrical optics: 2.5–2.7 from Chapter 2 inhttp://www.animations.physics.unsw.edu.au/light

Discuss/set up HW 3. Optics in astronomical imaging.

Week 6

DETECTION...Sensors: turning EM energy into an electric signal.

• CCDs and how they work: powerpoint

Week 7 (8)

Temperature and wavelength (“color”) regimes in astronomy. Highlights fromstar_temperatures.pdf: slides 13, 15–17, 24, 26–28, 31, which cover...

•

Wien’s Law: λ p ∝

1/T • star colors & luminosities as probes of temperatures and radii and (hence) masses

– Sun as“universal standard” (T , R, L, M )


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– L = 4πR2σT

4 and the comparison of Betelgeuse (red supergiant) and Rigel

(“normal” hot star)

• EMR regimes (e.g., IR, X-ray, radio) as probes of the universe’s incredibly wide

range of temperature regimes

Weeks 9 & 10

The optical imaging chain.

• The optical imaging chain: powerpoint [under revision]

• In-class “demo” describing response of a CCD pixel to incoming photons of differ-ent energies (wavelengths):http://www.olympusmicro.com/primer/java/digitalimaging/ccd/quantum/index.ht

Discuss/set up HW 4. Sensors in astronomical imaging; the relationship between tem-perature, color, and wavelength regime.

The Hubble Space Telescope. Overview of the most famous(?) telescope in the world.Selected material from:

• http://www.spacetelescope.org/videos/archive/category/spacecraft/

• http://www.spacetelescope.org/videos/hst15_chapter01/

• http://www.spacetelescope.org/videos/hst15_chapter02/

• http://hubblesite.org/the_telescope/nuts_.and._bolts/

• http://hubblesite.org/gallery/behind_the_pictures/

Week 11

The infrared imaging chain.

• The IR astronomy imaging chain is basically a variant on the optical (visible-wavelength) astronomy imaging chain, with two important differences:

(1) The sensor (detector) array material must be sensitive to lower-energy pho-tons; silicon doesn’t work for photon wavelengths λ > 1.0 µm (λ > 1000 nm).To get a better idea why photon energy matters when detecting light, have a

look at (and play with) this Java applet:http://www.olympusmicro.com/primer/java/digitalimaging/ccd/quantum/indeNote how photons with energies corresponding to wavelengths ∼1 µm passright through the sensor. It turns out that to detect photons with wavelengths


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FUNDAMENTALS OF ASTRONOMICAL IMAGING (IMGS-112-01)COURSE LECTURE PLANS & NOTESSPRING 20167

longer than ∼1 µm, one must turn away from silicon, and instead use materialslike germanium, arsenic, indium, and antimony.

(2) The IR telescope/sensor system must be cooled to a temperature well below

the (blackbody) temperature corresponding to the wavelength of interest. Tounderstand why, recall Wien’s Law: λ p ∝ 1/T ; and remember that the Sun,with a surface temperature T ≈ 6000◦K, puts out most of its light in the visualrange — so, λ ≈ 0.5 µm. This leads to the following table:

Table 1. Relationship between T and λ

object T (◦K) approx. peak λ6000 0.5 µm (=500 nm)300 10 µm30 100 µm3 1000 µm (= 1 mm)

Moral of the story: if you want to detect mid-IR radiation, you need to coolat least the sensors — and preferably your entire telescope/camera system —down way below room temperature...preferably down to temperatures rivalingthe “blank sky” of the cold universe itself (a mere ∼3 ◦K).

• Overview of ground- and space-based IR telescopes & their instrumentation. Se-lected material from:

– http://www.ipac.caltech.edu/missions

– http://www.spitzer.caltech.edu/

– http://wise.ssl.berkeley.edu/

– http://sci.esa.int/herschel/

Week 12

Discuss/set up HW 5. Optical and IR imaging chains.

The X-ray imaging chain and the Chandra X-ray Observatory. Powerpoint andweb-based overview of the world’s most powerful X-ray telescope. Selected material from:

• http://chandra.harvard.edu/


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Week 13

The radio imaging chain. Overview of single dish radio telescope and radio interferom-eter systems. Material from:

• http://www.haystack.mit.edu/edu/undergrad/materials/RA_tutorial.html

Weeks 14/15

New Frontiers: ALMA, Gaia, JWST, ...

• http://www.almaobservatory.org/

• http://sci.esa.int/gaia/

• http://jwst.nasa.gov/

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