General AstronomyAstronomical Instruments

The Formation of ImagesSuppose 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

Let's only consider monochromatic light for now.

Forming ImagesLet's put a couple of prisms together and bring the beams of light to a point…

Forming ImagesIf we smooth the rough edges, we can form a lens

Parallel light rays are focused into a point by the lens.

Simple Lenses

Diameter

Focal Length

D

f

Parallel light from a distant star

Focal Point

Simple Lenses• Simple Lenses are characterized by

several properties:– Diameter– Focal length– Index of refraction of the material

• This relates to how much they will bend the light rays

– ShapeConcaveConvex

A Convex Lens concentrates lightSimple Lenses

A Concave Lens spreads out light

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

fp 1

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.

Forming Images

Focal Point

This image is:RealInverted

Forming Images

New Focal Point

Increasing the focal length, increases the size of the image

Old Focal Point

Forming Images• Mirrors, using reflection instead of

refraction, also form images

Real Object The image is:VirtualErectSame Size

Concave Mirror

Focal Point

The image is…RealInvertedSmaller

Illusion

Ray TracingFollowing 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.

Mirrors versus Lenses• Mirrors have several advantages

over lenses– Generally they are lighter in weight– There is no problems with color

• Refraction 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 lens

TelescopesThere are several important considerations in choosing a

telescope• Light Gathering Power• Resolving Power• Magnification• Type • Mounting

First some definitions:

Objective

Eyepiece

The eyepiece magnifies the image formed by the objective(Main Lens/Mirror)

Light Gathering PowerThe Diameter of the objective

determines the amount of light an optical system can gather; It is proportional to the area of the objective

Brightness Diameter2

For example, many amateurs have 2" telescopes;Stockton's scope is a 16"

162

22 = 64 An object seen in Stockton's scope is64 times brighter than through a 2"

Light Gathering Power

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.

Resolving PowerAs 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.

Resolving Power

Resolving Power is measured in terms of the angularseparation

1 2 3 4

Image #3 is "just resolved. The angle of separation whenImages are just resolved is the Resolving Power

Resolving Power

Resolving Power = 180Π

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.

Resolving PowerArcSec 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, youwould 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 6.3

Resolving PowerTwinkle, 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")

Resolving PowerWhat 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:

MagnificationMagnification 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 = fo

fe

The Stockton 16" has a focal length of 4064 mm32mm eyepiece M = 4064/32 = 127x24mm eyepiece M = 4064/24 = 170x

Mounting SystemsA telescope mount has two functions

1. provide a system for smooth controlled movement to point and guide the instrument

2. 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– Equatorial

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.

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.

EquatorialGerman 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.

EquatorialFork 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.

Types of TelescopesRefractor 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 problems1. Chromatic Aberration2. Weight

Chromatic AberrationRecall 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"

WeightGenerally, 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.

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)

The 40-inch refractor at Yerkes Observatory:

The world’s largest refractor.

Yerkes' 40"

Spherical AberrationOne 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:

Types of Reflectors• Prime Focus• Herschell• Newtonian• Cassegrain• Coudé• Schmidt

Types of Reflectors: Prime Focus

Observer rides in a 'basket' inside the telescope

Brightest imageYes, it blocks some light.No, it doesn't change the image, just dims it a bit

The 4-meter reflecting telescope at Kitt Peak National Observatory.

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 planet

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.

Types of Reflectors: NewtonianA 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 1st Reflector, 1688

Types of Reflectors: CassegrainThis 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.

Types of Reflectors: CoudéSuppose 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.

Types of Reflectors: SchmidtSometimes 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"

Other Instrumentation• Interferometers• Detectors

– Cameras and film– Photoelectric photometers– Charge-Coupled Devices (CCD)

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 Detectors

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!

Other wavelengths• Radio Telescopes

– Interferometry– VLBI

Radio Astronomy

A radio telescope in Australia.

Radio Astronomy

The Very Large Array (VLA) in New Mexico is the world’s best radio telescope.

Radio Astronomy

The largest telescope in the world is the 1000-ft diameter radio telescope of the Arecibo Observatory in Puerto Rico.

Gamma Ray ObservatoriesCompton Observatory

X-Ray Observatories

Chandra Space Telescope

X-ray Astronomy The Chandra X-ray Observatory

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 particles

Infrared ObservatoriesSpitzer Space

Telescope

Elephant’s Trunk

Hubble Space Telescope

By observing objects at different wavelengths we learn different things. This is the Whirlpool Galaxy (Messier 51) observed in:

infrared

radiovisible

X-rays

Adaptive Optics

Slides adapted from Dr Claire Max, UCSC

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 point

Images of a bright star, ArcturusLick Observatory, 1 m telescope

~ / D

Long exposureimage

Short exposureimage

Image with adaptive optics

Turbulence arises in several places

stratosphere

tropopause

Heat sources w/in dome

boundary layer~ 1 km

10-12 km

wind flow over dome

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 atmosphere

Keck Observatory

Laser is operating at Lick Observatory, being commissioned at

Keck

Lick Observatory

Keck laser guide star AO Best natural guide star AO

Galactic Center with Keck laser guide star

Adaptive optics makes it possible to find faint companions around bright

starsTwo images from Palomar of a brown dwarf

companion to GL 105

Credit: David Golimowski

200” telescope

The new generation: adaptive optics on 8-10 m

telescopes

Summit of Mauna Kea volcano in Hawaii:

Subaru

2 Kecks

Gemini North

Neptune in infra-red light (1.65 microns)

Without adaptive opticsWith Keck adaptive optics

June 27, 1999

2.3 arc sec

May 24, 1999

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 sec

Uranus with Hubble Space Telescope and Keck AO

HST, Visible Keck AO, IR

L. Sromovsky

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 microns

Cerro Tololo Inter-American Observatory, Chilean Andes

Kitt Peak National Observatory, Arizona

The National Observatories:

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.

The twin 10-meter Keck reflecting telescopes on Mauna Kea, Hawaii, are the world’s largest.

The Keck primary mirrors consist of 36 1.8-meter mirror segments that fit together precisely to create the 10-meter reflecting surface.

The Gemini 8-m telescopes:

Gemini South, Chile

Gemini North, Mauna Kea

The Very Large Telescope(s): Four 8-m telescopes

Chile

To Infinity, and Beyond!

DawnDawn 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.

CassiniCassini 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.

Earth as seen by Cassini at Saturn

New HorizonsThe 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.

JunoJuno 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 Pole

MessengerOn 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.

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 2

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.