Astronomical Instruments The Formation of Images Suppose we want to apply some of the properties of light discussed in the last session Recall the refraction of light through a prism Forming Images ID: 384196
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Slide1
General Astronomy
Astronomical InstrumentsSlide2
The Formation of Images
Suppose we want to apply some of the properties of light discussed in the last session?Recall the refraction of light through a prism?Slide3
Forming Images
Let's only consider
monochromatic
light for now.Slide4
Forming Images
Let's put a couple of prisms together and bring the beams of light to a point…Slide5
Forming Images
If we smooth the rough edges, we can form a lens
Parallel light rays are focused into a point by the lens.Slide6
Simple Lenses
Diameter
Focal Length
D
f
Parallel light from a distant star
Focal PointSlide7
Simple Lenses
Simple Lenses are characterized by several properties:DiameterFocal lengthIndex of refraction of the material
This relates to how much they will bend the light rays
Shape
Concave
ConvexSlide8
A Convex Lens concentrates light
Simple Lenses
A Concave Lens spreads out lightSlide9
The inverse of the focal length, measured in meters, is the Diopter
For example, a +2 diopter prescription lens has a focal length of ½ meter. Positive is convex; Negative is concave.The eye itself has a refractive power of +60 diopters
(1.7 cm focal length)
Optical Power
We need a concave lens to spread the image to reach the retina, a “-” correction
We need a convex lens to reduce the image location to bring it back to the retina, a “+” correction.Slide10
Forming Images
Focal Point
This image is:
Real
InvertedSlide11
Forming Images
New Focal Point
Increasing the focal length, increases the size of the image
Old Focal PointSlide12
Forming Images
Mirrors, using reflection instead of refraction, also form images
Real Object
The image is:
Virtual
Erect
Same SizeSlide13
Concave Mirror
Focal Point
The image is…
Real
Inverted
SmallerSlide14
IllusionSlide15
Ray Tracing
Following the Photon’s path
The ray tracing technique just used to show how mirrors and lenses form images is very powerful. The image below is created without an artist or (clearly) a photo, it is formed by a set of instructions telling where the light source is, the object(s) are located and where the point-of-view is looking at the scene.Slide16
Mirrors versus Lenses
Mirrors have several advantages over lensesGenerally they are lighter in weightThere is no problems with colorRefraction affects different colored light so that a for a given lens, red light will focus at a different point than blue light
It is easier to produce a large diameter mirror than a large diameter lensSlide17
Telescopes
There are several important considerations in choosing a telescopeLight Gathering PowerResolving PowerMagnification
Type
Mounting
First some definitions:
Objective
Eyepiece
The eyepiece magnifies the image formed by the objective
(Main Lens/Mirror)Slide18
Light Gathering Power
The Diameter of the objective determines the amount of light an optical system can gather; It is proportional to the area of the objective
Brightness
Diameter
2
For example, many amateurs have 2" telescopes;
Stockton's scope is a 16"
16
2
2
2
= 64
An object seen in Stockton's scope is
64 times brighter than through a 2"Slide19
Light Gathering PowerSlide20
The Andromeda galaxy seen through a small telescope…
The resolution (or clarity) of an image also depends on the size of the telescope aperture.
Resolution
…and through a telescope with a larger aperture.Slide21
Resolving Power
As objects get farther away, it becomes harder and harder to tell them apart. Your eye cannot see all of the craters on the moon – they blend together into the background.
In fact the circular shape of the telescope objective produces a circular diffraction pattern as an image.
[Actually so does the circular pupil of your eye and the two straight edges of your eyelids]
The central disk of this diffraction pattern is what we think of as the "star" when we look at it; it is known as the Airy Disk.
It is smeared out – so, two close stars could have their Airy disks overlap.
A telescope's Resolving power measures how close together objects can be and still be seen to be separate.Slide22
Resolving Power
Resolving Power is measured in terms of the angular
separation
1
2
3
4
Image
#
3 is "just resolved. The angle of separation when
Images are just resolved is the Resolving PowerSlide23
Resolving Power
Resolving Power =
180
Π
D
(in degrees)
Notice it depends on both the wavelength and the diameter of the objective. Small numbers are 'good', so redder light (longer wavelengths) are harder to resolve than bluer light. The bigger the objective, the better the resolving power.
A Rule of Thumb: ArcSec = 10/D, where D is in cm.Slide24
Resolving Power
ArcSec
Inch
:
1
4
5
8
14
16
Angstroms
Cm:
2.54
10.16
12.7
20.32
35.56
40.64
3500
2.84
0.71
0.57
0.36
0.20
0.18
5600
4.55
1.14
0.91
0.57
0.32
0.28
7000
5.68
1.42
1.14
0.71
0.41
0.36
Resolving power in arcsec for a given objective diameter
In order to subtend the same angle with a dime, you
would to be these many miles away:
0.8
3.1
3.9
6.3
11.0
12.6
0.5
2.0
2.5
3.9
6.9
7.9
0.4
1.6
2.0
3.1
5.5
6.3Slide25
Resolving Power
Twinkle, Twinkle Little Star…
Because the light from the star follows a winding path through the (sometimes turbulent) atmosphere, the star appears to move around a bit.
The motion varies between 1" and 2" (best case is 0.25")Slide26
Resolving Power
What angles do the planets subtend?
Mercury 6.4"
Venus 16.0"
Mars 6.1"
Jupiter 37.9"
Saturn 17.3" Moon 31' 05"
This means that the movement (twinkle) is smaller than the planet – so any motion is invisible; the motion is within the bounds of the planet. This leads to a rule:
Stars twinkle, Planets don'tSlide27
Magnification
Magnification is the least important of the telescope properties (although the one touted by TV sales shows). Unless you are looking only at planets, the moon or other extended objects, magnifying the point of light that is a star does nothing.
Magnification is the ration of the focal lengths of the objective and the eyepiece:
M =
f
o
f
e
The Stockton 16" has a focal length of 4064 mm
32mm eyepiece
M = 4064/32 = 127x
24mm eyepiece
M = 4064/24 = 170xSlide28
Mounting Systems
A telescope mount has two functions
provide a system for smooth controlled movement to point and guide the instrument
support the telescope firmly so that you can view and photograph objects without having the image disturbed by movement.
There are two major types of mounts for astronomical telescopes:
Altazimuth
EquatorialSlide29
Altazimuth
The simplest type of mount with two motions, altitude
(up and down/vertical) and
azimuth
(side-to-side/horizontal).
Good altazimuth mounts will have slow-motion knobs to make precise adjustments, which aid in keeping tracking motion smooth.
These type mounts are good for terrestrial observing and for scanning the sky at lower power, but are not for deep sky photography.
Certain altazimuth mounts are now computer driven and allow a telescope to track the sky accurately enough for visual use, but not for long exposure photography. Slide30
Equatorial
Superior to non-computerized altazimuth mounts for astronomical observing over long periods of time and absolutely necessary for astrophotography. As the earth rotates around its axis, the stationary stars appear to move across the sky. If you are observing them using an altazimuth mount, they will quickly float out of view in both axes.
A telescope on an equatorial mount can be aimed at a celestial object and easily guided either by manual slow-motion controls or by an electric clock drive to follow the object easily across the sky and keep it in the view of the telescope.
The equatorial mount is rotated on one axis (polar/right ascension) adjusted to your latitude and that axis is aligned to make it parallel to the Earth's axis, so that if that axis is turned at the same rate of speed as the Earth, but in the opposite direction, objects will appear to sit still when viewed through the telescope.Slide31
Equatorial
German Mount
Both reflector and refractor telescopes normally use this type mount
.
A large counterweight extending on the opposite side of the optical tube is its distinguishing feature.
The counterweight is needed to balance the weight of the optical tube.Slide32
Equatorial
Fork Mount
Most catadioptric and other shorter optical tubes use this style mount which is generally more convenient to use than the German mount, especially for astrophotography.
A more recent state-of-the-art computer controlled telescope allows fully automatic operation making it easy to locate objects while saving the observer considerable time and effort.Slide33
Types of Telescopes
Refractor versus ReflectorThe 'classic' telescope most of us think of when we imagine one is the refractor. In practice, however, there are some significant drawbacks to refractors – especially those of large size.
As usual, the telescope is measured by its objective diameter. The largest refractor is 40".
Refractors suffer from two main problems
Chromatic Aberration
WeightSlide34
Chromatic Aberration
Recall that refraction bends light differently depending on its wavelength. This means that different colors will have differing focal lengths:
Resulting in an image with "color halos"Slide35
Chromatic Aberration
Chromatic aberration
CorrectedSlide36
Weight
Generally, the bigger the objective diameter, the longer the focal length and therefore the higher in the air the lens of the refractor will be when mounted in the telescope tube.Since large refractors could have an objective lens weighing in the tons, moving it about is a definite problem.Slide37
Reflectors
Reflectors on the other hand have no chromatic aberration - reflection acts the same no matter what the wavelength of the light.Second, mirrors are generally placed closer to the ground and with a lower center of gravity are easier to move.
Mirrors are usually spherical rather than parabolic – leading to spherical aberation
(because it's cheaper & easier to make a spherical mirror)Slide38
The 40-inch refractor at Yerkes Observatory:
The world’s largest refractor.Slide39
Yerkes' 40"Slide40
Spherical Aberration
One property of a parabolic shape is the fact that any incoming parallel rays will be focused to a single point:
A spherical shape does not have this property:Slide41
Spherical AberrationSlide42
Types of Reflectors
Prime FocusHerschellNewtonianCassegrainCoudéSchmidtSlide43
Types of Reflectors: Prime Focus
Observer rides in a 'basket' inside the telescopeBrightest image
Yes, it blocks some light.
No, it doesn't change the
image, just dims it a bitSlide44
The 4-meter reflecting telescope at Kitt
Peak National Observatory.Slide45
Types of Reflectors: Herschell
The eyepiece is set at the top of the tube and the mirror canted so that the light will be focused into the eyepiece
Drawback: You might be very far off the ground on a ladder trying to see some objects – this is especially thrilling when trying to move the scope to follow the motion of a planetSlide46
Herschell's Telescope
"This wonderful instrument, though gigantic in its size, is moved with great facility in all directions, by means of rollers, ropes, and pullies. The ascent to the uppermost end is by means of steps or rather a ladder; and to this end there is a seat attached, on which the astronomer is placed to make his observations on the starry world. Of course he looks in, and not through the tube; in the lower end of which, near the ground, is placed the mirror which reflects the light through a small tube, upon his eyes. The mirror weighs two thousand five hundred pounds, and is worth, according to the doctor's valuation, ten thousand pounds.
While he views the firmament with its glittering orbs, he communicates his observations to his sister, Miss
Herschell
, who is his amanuensis, and who has her station in a small lodge built in the lower framework of the machinery. This he does by a speaking trumpet, one end of which is applied to his mouth, and the other to her ear; thus they are recorded without either having to leave their seats…"
--Description of
Herschell's
telescope at Slough from Joshua White's Letters on England, written in 1810.Slide47
Types of Reflectors: Newtonian
A Newtonian reflector allows you to "keep your feet on the ground"It does this by placing a diagonal mirror in the tube so that the eyepiece may be lower.
The small amount of light blocked by the mirror is minimal in return for the convenience and usefulness of the lower placement of the eyepiece
Newton’s 1
st
Reflector, 1688Slide48
Types of Reflectors: Cassegrain
This clever arrangement puts a small convex mirror in front of the objective and bores a hole in the objective.The small mirror reflects the incoming light through the hole and into the eyepiece. This arrangement allows the focal length to be increased dependent on the placement of the small mirror.Slide49
Types of Reflectors: Coudé
Suppose you want to put a camera, or even heavier equipment, in line with the telescope optics.Even the cassegrain focus can be hard-pressed to handle several hundred pounds of analysis equipment hanging on the back of the scope.
The arrangement makes use of the fork method of the equatorial mount. Light can be directed into the point where the scope tube is gripped by the fork and then directed down through the mounting (conveniently hollow or with fiber optics) to a room below the telescope where the equipment is located.Slide50
Types of Reflectors: Schmidt
Sometimes called a Schmidt Camera, this design allows the use of a spherical mirror and a Corrector Plate
These usually have wide fields of vision and "fast optics" allowing for photography.
Many of these are also Cassegrain – leading to the designation "Schmidt-Cassegrain"Slide51
Other Instrumentation
InterferometersDetectorsCameras and filmPhotoelectric photometersCharge-Coupled Devices (CCD)Slide52
Instead of using photographic plates to take pictures, we use sensitive solid-state light detectors known as
Charge Coupled Devices (CCDs).
CCDs can detect light with an efficiency of greater than 90%.
Instruments and DetectorsSlide53
Instruments and Detectors
Comparison between a photographic plate and a CCD image with the same amount of exposure. The CCD is
much
more sensitive to light!Slide54
Other wavelengths
Radio TelescopesInterferometryVLBISlide55
Radio Astronomy
A radio telescope in Australia. Slide56
Radio Astronomy
The Very Large Array (VLA) in New Mexico is the world’s best radio telescope.Slide57
Radio Astronomy
The largest telescope in the world is the 1000-ft diameter radio telescope of the Arecibo Observatory in Puerto Rico.Slide58
Gamma Ray Observatories
Compton ObservatorySlide59
X-Ray Observatories
Chandra Space TelescopeSlide60
X-ray Astronomy
The
Chandra
X-ray ObservatorySlide61
Observations of the supernova remnant, IC 443
The close-up, shows a neutron star that is spewing out a comet-like wake of high-energy particlesSlide62
Infrared Observatories
Spitzer Space TelescopeSlide63
Elephant’s TrunkSlide64
Hubble Space TelescopeSlide65
By observing objects at different wavelengths we learn different things. This is the Whirlpool Galaxy (Messier 51) observed in:
infrared
radio
visible
X-raysSlide66
Adaptive Optics
Slides adapted from Dr Claire Max, UCSCSlide67
Why is adaptive optics needed?
Turbulence in earth’s atmosphere makes stars twinkle
More importantly, turbulence spreads out light; makes it a blob rather than a pointSlide68
Images of a bright star, Arcturus
Lick Observatory, 1 m telescope
~
l
/ D
Long exposure
image
Short exposureimage
Image with adaptive opticsSlide69
Turbulence arises in several places
stratosphere
tropopause
Heat sources w/in dome
boundary layer
~ 1 km
10-12 km
wind flow over domeSlide70
If there’s no close-by “real” star, create one with a laser
Use a laser beam to create artificial “star” at altitude of 100 km in atmosphereSlide71
Keck Observatory
Laser is operating at Lick Observatory, being commissioned at Keck
Lick ObservatorySlide72
Keck laser guide star AO
Best natural guide star AO
Galactic Center with Keck laser guide starSlide73Slide74
Adaptive optics makes it possible to find faint companions around bright stars
Two images from Palomar of a brown dwarf companion to GL 105
Credit: David Golimowski
200” telescopeSlide75
The new generation: adaptive optics on 8-10 m telescopes
Summit of Mauna Kea volcano in Hawaii:
Subaru
2 Kecks
Gemini NorthSlide76
Neptune in infra-red light (1.65 microns)
Without adaptive optics
With Keck adaptive optics
June 27, 1999
2.3 arc sec
May 24, 1999Slide77
Neptune at 1.6 m: Keck AO exceeds resolution of Hubble Space Telescope
HST - NICMOS Keck AO
(Two different dates and times)
2.4 meter telescope
10 meter telescope
~2 arc secSlide78
Uranus with Hubble Space Telescope and Keck AO
HST, Visible
Keck AO, IR
L.
SromovskySlide79
VLT NAOS AO first light
Cluster NGC 3603: IR AO on 8m ground-based telescope achieves same resolution as HST at 1/3 the wavelength
Hubble Space Telescope WFPC2,
= 800 nm
NAOS AO on VLT
= 2.3 micronsSlide80
Cerro Tololo Inter-American Observatory, Chilean Andes
Kitt Peak National Observatory, Arizona
The National Observatories:Slide81
Mauna Kea
For several reasons, most observatories are built on top of high mountains in remote areas of the world.
This image shows the summit of Mauna Kea, at an altitude of 14,000 ft.Slide82Slide83
The twin 10-meter Keck reflecting telescopes on Mauna Kea, Hawaii, are the world’s largest.Slide84
The Keck primary mirrors consist of 36 1.8-meter mirror segments that fit together precisely to create the 10-meter reflecting surface.Slide85
The Gemini 8-m telescopes:
Gemini South, Chile
Gemini North, Mauna KeaSlide86
The Very Large Telescope(s): Four 8-m telescopes
ChileSlide87
To Infinity, and Beyond!Slide88
Dawn
Dawn
is a space probe launched by NASA in 2007 to study the two most-massive objects of the asteroid belt: the protoplanet Vesta and the dwarf planet Ceres. Slide89
Cassini
Cassini launched in October 1997 with the European Space Agency's Huygens probe. The probe was equipped with six instruments to study Titan, Saturn's largest moon. It landed on Titan's surface on Jan. 14, 2005, and returned spectacular results.
Meanwhile, Cassini's 12 instruments have returned a daily stream of data from Saturn's system since arriving at Saturn in 2004.
Cassini completed its initial four-year mission to explore the Saturn System in June 2008 and the first extended mission, called the Cassini Equinox Mission, in September 2010. Now, the healthy spacecraft is seeking to make exciting new discoveries in a second extended mission called the Cassini Solstice Mission. The mission’s extension, which goes through September 2017, is named for the Saturnian summer solstice occurring in May 2017.Slide90
Earth as seen by Cassini at SaturnSlide91
New Horizons
The fastest spacecraft when it was launched, New Horizons lifted off in January 2006.
It awoke from its final hibernation period after a voyage of more than 3 billion miles, and passed close to Pluto, inside the orbits of its five known moons after a 10 year journey.Slide92
At 12:33 a.m. (EST) on January 1, 2019, New Horizons flew just 2,200 miles (3,500) kilometers from the Ultima Thule’s surface, when it was about 4 billion miles (6.6 billion kilometers) from the Sun -- the most distant planetary flyby in history and the first close-up look at a solar system object of this type.
New Horizons: Ultima Thule
Ultima Thule is the first unquestionably primordial contact binary ever explored. Approach pictures hinted at a strange, snowman-like shape, but further analysis of images, taken near closest approach, uncovered just how unusual the KBO's shape really is. At 22 miles (35 kilometers) long, the binary consists of a large, flat lobe (nicknamed "Ultima") connected to a smaller, rounder lobe (nicknamed "Thule").Slide93
Juno
Juno is a NASA space probe orbiting the planet Jupiter after entering orbit on July 5, 2016, 03:53 UTC; the prelude to 20 months of scientific data collection. It took 5 years to reach orbit.
Jupiter’s North PoleSlide94
Messenger
On August 3, 2004, NASA’s MESSENGER spacecraft blasted off from Cape Canaveral, Florida, for a risky mission that would take the small satellite dangerously close to Mercury’s surface, paving the way for an ambitious study of the planet closest to the Sun.
The spacecraft traveled 4.9 billion miles (7.9 billion kilometers) — a journey that included 15 trips around the Sun and flybys of Earth once, Venus twice, and Mercury three times — before it was inserted into orbit around its target planet in 2011.Slide95
Pioneer
The Pioneer Spacecraft Missions are a series of eight spacecraft missions.
Pioneer 6 is the oldest NASA spacecraft extant. There was a successful contact of Pioneer 6 for about two hours on 8 December 2000 to commemorate its 35th anniversary.
Pioneer 7 was launched on 17 August 1966. In 1995,the spacecraft and one of the science instruments were still functioning.
Pioneer 8 was launched on 13 December 1967 it was still functioning in 1996.
Pioneer 10 was launched on 2 March 1972. On 15 July 1972 Pioneer 10 entered the Asteroid Belt. Pioneer 10 passed by Jupiter on December 3, 1973. Contact was lost on 27 April 2002.
Pioneer 11 was launched on 5 April 1973. It reached Saturn on 1 September 1979. Its last transmission was on 30 September 1995.Slide96
The twin Voyager 1 and 2 spacecraft are exploring where nothing from Earth has flown before. Continuing on their more-than-37-year journey since their 1977 launches, they each are much farther away from Earth and the Sun than Pluto.
In August 2012, Voyager 1 made the historic entry into interstellar space.
Voyager 1 and 2Slide97
“That's home. That's us. On it, everyone you ever heard of, every human being who ever lived, lived out their lives. The aggregate of all our joys and sufferings, thousands of confident religions, ideologies and economic doctrines, every hunter and forager, every hero and coward, every creator and destroyer of civilizations, every king and peasant, every young couple in love, every hopeful child, every mother and father, every inventor and explorer, every teacher of morals, every corrupt
politician, every superstar, every supreme leader, every saint and sinner in the history of our species, lived there on a mote of dust, suspended in a sunbeam. “
Carl Sagan, 1994
Voyager I from approx. 6 billion kilometers
The Pale Blue Dot