Notes for instruments, J. Hedberg © 2018 · Adaptive Optics Make a star using a laser. The idea of adaptive optics has improved observing tremendously. If you create an artificial

Instruments1. Basic Optics

1. Rays of Light2. Waves of light3. Basic Imaging Systems4. A Basic Telescope5. Aberrations6. Mirrors

2. Some Real Instruments1. Keplerian Optics2. Astronomical seeing3. Detectors

3. Other Wavelengths1. Radio Telescopes2. Orbiting Astronomical Observatory3. Hubble

4. Other improvements1. Adaptive Optics

Basic Optics

Rays of LightThe pin-hole camera allows a smallamount of light to pass through anopening. However, because the wholeis small, very little light passes, whichmakes measurements difficult. If thehole's diameter is increased, then theimage becomes blurred.

Converging LensBy using a converging lens, we can allow more light to pass through the hole,which increases our 'photon count.' A consequence of using a lens however, isthat now the focal length, , is fixed.

Fig.1A pinhole will project an invertedimage on a plane.

Fig.2The image will be in focuseverywhere. It's size changes based onthe position of the focal plane.

F

PHY 454 - instruments - J. Hedberg - 2018

updated on 2018-09-27 Page 1

Images

Light from stars

Waves of lightCoherent, mono-chromatic light passing through a circular aperture will bediffracted.

Airy Disc

Fig.3

Fig.4Changing the position of thescreen will result in a blurry image.

θ θ

F

lens focal plane

stars

d

Fig.5Two stars separated by an angle in the sky, will create images

separated by a distance on thedetection screen.

θ

d

D

circularaperture

screen

light intensity

incident light

L

θ1

Fig.6



There is a limitUnresolved: if the central maximum falls inside the location of the firstminimum.

Unresolved point sources

Rayleigh Criterion

Basic Imaging Systems

The eyeNature has made many different eyeballs. Most operate on the principles of lenses we’ve just looked at.

Fig.7

Fig.8Two point sources gettingcloser.

-2 -1 0 1 2

Fig.9

= 1.22θminλ

D



focusing the eyeFocusing on objects: We cannot adjust the position of the lens with respect tothe retina. So, the muscles around the eye change the shape of the lens, whichthen changes its focal length.

A Basic Telescope

Magnification:

AberrationsLenses aren't perfect.

Chromatic Aberration

retina

lens

cornea

iris

aqueoushumor

optic nerve

Fig.10The human eye

Fig.11Focusing the eye

objectlens 1

(objective)

real image

Fig.12

objectlens 1 lens 2

(objective) (eyepiece)

virtual image

Fig.13

M = =f1

f2

fobjective

feyepiece



Since different colors will refract atdifferent angles, the focal point will beslightly different for differentwavelengths. This leads to ChromaticAberration.

Spherical AberrationEarlier, our thin lens approximation ignored the fact that the thickness of thelens changed as a function of distance away from the central axis.

This leads to rays having slightly different focal points depending on wherethey are incident on the lens. The further the rays are away from the centralaxis, the worse the SphericalAberration effect is.

Fixing aberrationsFortunately, by using a multi lens setup, we can correct these aberrations. Forexample, to correct the chromatic aberration caused by a converging lens wecan insert a diverging lens after the converging lens to refocus the differentcolors back to the same point.

Mirrors

Some Real InstrumentsGalileo's Telescope

Fig.14Chromatic Aberrations Fig.15Different Colors with havedifferent focal points

blurry focal point

Fig.16Spherical Aberrations

Fig.17Fixing Aberrations

object

realimage

pi

C

f

F

object at a distance outside the focal point

Fig.18A mirror ray diagram



Galileo's Telescope

Keplerian Optics

Fig.19Galileo's Telescope

Fig.20Saturn as viewed throughGalileo's telescope

https://www.astromatic.net/2009/05/23/see-saturn-as-galileo-did

Fig.21 Fig.22

Fig.23



A reflecting Telescope

Fig.24Yerkes Observatory 40 inchRefractor Telescope ( It is the largestrefracting telescope used forastronomical research.) Williams Bay,Wisconsin, US

https://commons.wikimedia.org/wiki/File:Yerkes_40_inch_Refractor_Telescope-2006.jpg

Fig.25The Great Paris ExhibitionTelescope of 1900, with an objectivelens of 1.25 m (49 in) in diameter, wasthe largest refracting telescope everconstructed. It was built as thecenterpiece of the Paris UniversalExhibition of 1900. 200 ft long. Too bigto use.

By Unknown - Le panorama (Paris,1900)., Public Domain,https://commons.wikimedia.org/w/index.php?curid=20083299

eyepiece

parabolic mirror

parallel rays

flat mirror

Fig.26A reflecting Telescope

Fig.27A replica of the Newton -Wickins telescope, Newton's third



Where should we put these telescopes?Early telescopes could be placed near where people lived, since there were not a lot of lights to get in the way. Now,we usually put telescope as far away from people as possible.

Astronomical seeing

As light passes through the atmosphere, small variations in air density, usuallycaused by temperature fluctuations will disturb the incoming waves. There isturbulent mixing of air throughout much of the atmosphere (but noteverywhere)

reflecting telescope that waspresented to the Royal Society in 1766after being restored by Thomas Heath.It is described as the better of theinstruments Newton built

By User:Solipsist (Andrew Dunn) -www.andrewdunnphoto.com, CC BY-SA 2.0,https://commons.wikimedia.org/w/index.php?curid=513483

Fig.28Scintillation of a star, over time

http://spaceweathergallery.com/indiv_upload.php?upload_id=124490

Fig.29Moon Seeing Effect



DetectorsWe can classify most of the standard astronomical instruments as either imaging cameras or spectrometers. Eitherway, we need to collect photons. That used to be done with chemical film, but we use digital methods. They have amuch greater quantumefficiency, which means that it requires fewer photons to trigger a detection by a pixel.

Photographic Plate: 1%Human Eye: 10%CCD: 80-90%

Charge-coupled Device: CCD

Other Wavelengths

Fig.30Mountain in the atmosphere

Fig.31A CCD device

0.2 µm 500 nm 1µm 10µm 100µm 1mm 1 cm 1.0 m

microwaveThermal IRReflected IRVisibleUV

trans

mis

sion

0

100 %

Wavelength

Fig.32The transparency of the atmosphere



Radio TelescopesWhat about for EM radiation with frequencies and wavelengths not in thevisible spectrum?

What if is really big?

When looking at 21 cm wavelengths, a 300 meter dish will have a diffraction limited resolution of what?

Arecibo

Orbiting Astronomical ObservatoryLet's put a telescope above the atmosphere. There, instruments will be able to reach the diffraction limit mentionedabove, rather than the seeing limit of ground based observatories.

Adapted fromhttps://earthobservatory.nasa.gov/Features/RemoteSensing/remote_04.php

Fig.33Clouds linger at twilight overthe Karl G. Jansky Very Large Array inits most compact configuration.

Credit: NRAO/AUI/NSF link

λ

= 1.22θminλ

D

θ = 1.22 = 1.22 × = 0.000854 rad = 2.94 minofarcλ

D

0.21 m300 m

Fig.34

Credit: Author H. Schweiker/WIYN andNOAO/AURA/NSF link



https://public.nrao.edu/gallery/a-cloudy-twilight-at-the-vla/

https://www.flickr.com/photos/nasablueshift/8288406364/

HubbleLaunched in 1990, the Hubble Space Telescope has taken many of the mostrecognizable space images.

It didn't work too well at first, and so a repair mission had to be sent.Astronauts fixed it.

Fig.35The Orbiting AstronomicalObservatory (OAO) satellites were aseries of four American spaceobservatories launched by NASAbetween 1966 and 1972, whichprovided the first high-qualityobservations of many objects inultraviolet light.

Fig.36The Hubble Space Telescope

Fig.37Hubble before and after



Other improvements1. Active Optics - mirrors that move.2. Adaptive Optics - guide stars

Fig.38The telescope that ateastronomy.

Fig.39Inspecting the mirrors on theJames Webb Space Telescope

Credit: NASA/Chris Gunn -https://www.flickr.com/photos/nasawebbtelescope/8047310260

Fig.40The primary mirror of NASA'sJames Webb Space Telescope,consisting of 18 hexagonal mirrors,looks like a giant puzzle piece standingin the massive clean room of NASA'sGoddard Space Flight Center inGreenbelt, Maryland.

Image Credit: NASA/Chris Gunn -https://www.flickr.com/photos/nasawebbtelescope/30116152713/



https://www.flickr.com/photos/nasawebbtelescope/8047310260

https://www.flickr.com/photos/nasawebbtelescope/30116152713

Adaptive OpticsMake a star using a laser.

The idea of adaptive optics has improved observing tremendously. If youcreate an artificial star using a laser, then you should know what is should looklike. Simply adjust your optics in real-time to keep the laser-star looking good!

Fig.41An artificial star

Fig.42Saturn without and withAdaptive Optics

Credit: Heidi B. Hammel and Imke DePater/WMKO Keck



http://www.keckobservatory.org/support/ngao

Notes for instruments, J. Hedberg © 2018 · Adaptive Optics Make a star using a laser. The idea of adaptive optics has improved observing tremendously. If you create an artificial

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