Astronom

ische Waarneem

technieken(A

stronomical O

bserving T

echniques)

4thLecture: 2

9 Septem

ber 2

010

•Hans Lipperh

ey1608 –first patent for “spy glasses”

•Galileo G

alilei1609 –first use in astronom

y

1. H

istory of Telescopes

•Galileo G

alilei1609 –first use in astronom

y

•New

ton 1668 –first refractor

•Kepler

–improves reflector

•Hersch

el 1789 –4 ft refractor

•...

Two m

ain types:1.

Equatorial m

ounting2.

Azim

uthalm

ounting

2. Telescope M

ounts

Azim

uthal:

+ light and

symmetric

-requires com

puter control

Equatorial:

+ follows th

e Earth

rotation -typically m

uch larger and

massive

Telescope

Mounts (2

)Variations of equatorial (or parallactic) m

ounts:

•Germ

an mount

•English

mount

•Fork m

ount

3. Telescope F

oci

2 fund

amental ch

oices:

•Refractor �

Reflector

•Location of ex

it pupil

Telescope

Foci –

where to put th

e instrum

ents

Prime focus –

wide field

, fast beam

but

difficult to access and

not suitable for h

eavy Prim

e focus –wide field

, fast beam

but

difficult to access and

not suitable for h

eavy instrum

ents

Cassegrain

focus–moves w

ith th

e telescope, no im

age rotation, but flex

ure may b

e a prob

lem

Telescope

Foci –

where to put instrum

ents (2

)

Nasm

yth–ideal for h

eavy instruments

to put on a stable platform

, but field

to put on a stab

le platform, b

ut field

rotates

Coud

é–very slow

beam

, usually for large spectrograph

s in the “b

asement”

4. Basic T

elescope Optics

Image Scale and Magnifica

tion

Scale:

fl

f lω

ωω

01

75

.0

:

sm

all

for

an

d

tan

≈=

Magnification:

1 2

2 1

2 1

ω ω=

==

D D

f fV

Geom

etrically:

Practically, the F

OV is lim

ited by ab

errations:The b

igger the m

irror the b

igger the d

ifference [parabola –

Ca

mera

f D

=

ta

nm

ax

ω The Field of V

iew

Practically, the F

OV is lim

ited by ab

errations:The b

igger the m

irror the b

igger the d

ifference [parabola –

sphere] near th

e edge. �

bigger telescopes h

ave smaller F

OVs

(~<1 d

eg).

Parabolic prim

ary:

v

The Schmidt T

elescope

The S

chmidt telescope uses a sph

erical primary

mirror to get th

e max

imum

field of view

(>5 deg)

�no off-ax

is asymmetry b

ut spherical

aberrations:

�Sch

midt telescopes require a corrector lens.

Two m

eter Alfred

-Jensch

-Telescope in T

autenburg,

the largest S

chmidt cam

era in th

e world

.

Light Gathering Pow

er and Resolution

Light gath

ering power

For ex

tended

objects:

(see lecture on S/N

)

For point sources:

2

/

∝f D

NS

2/

DN

S∝

For point sources:

Angular resolution

(given by th

e Rayleigh

criterion)

/D

NS

∝

D f

D

λλ

22

.1

lo

r

22

.1

sin

=∆

=Θ

Parameters of a

Ritch

ey-Chrétien Configura

tion

12 2

2 2

=−

b y

a x

02

=−

ax

y

RC telescopes use tw

o hyperb

olic mirrors, instead

of

a parabolic

mirror.

Choice of O

rbits:

•com

munications

•therm

al background

radiation

5. Space T

elescopes: Orbits

•space w

eather

•sky coverage

•access (servicing)

Two E

xam

ples:Two E

xam

ples:

HST : low

Earth

orbit ~

96 m

inutes

Spitzer: E

arth-trailing solar orb

it ~60 yr

The L2 Orbit

Joseph-Louis Lagrange (18

thcentury m

athematician) :

search for a stab

le configuration in which

three b

odies could

orbit each

other

yet stay in the sam

e position relative to each oth

er �

five solutions, the five L

agrange points.An ob

ject placed at any one of th

ese 5 points w

ill stay in place relative to the

other tw

o.oth

er two.

E.g., J

WST and

Hersch

el are in orbits around

the L

2 point �

orbit w

ith Earth

The Grow

th of T

elescope

Colle

cting Area

6. Telescope S

izes

Mass L

imitations

Most im

portant innovations: 1.

faster mirrors �

smaller telescopes �

smaller d

omes

2.

faster mirrors

new polish

ing techniques

3.

bigger m

irrors thinner / segm

ented mirrors

active support

Polishing T

echnique

s

(OS

A, 1

98

0)

Polishing a 6

.5-m

mirror on th

e Large Optical

Polishing a 6

.5-m

mirror on th

e Large Optical

Generator (LO

G) using th

e stressed-lap

polishing tool. T

he lap ch

anges shape

dynam

ically as it moves rad

iallyfrom

center-to-ed

ge of the m

irror to produce a parab

oloid.

Our 6

.5-m

mirrors are typically figured

to a focal ratio of f/1.2

5 with

a finished

precision of ±

15-20 nanom

eters.http://m

irrorlab.as.arizona.ed

u/TECH.ph

p?navi=poli

Segmented, Thin a

nd Hone

ycom

b Mirrors

Active

Optics (M

irror Support)

Liquid

Mirror T

elescope

s•First suggestion b

y Ernesto C

apocciin 18

50

•First m

ercurytelescope b

uilt in 1872 with

a diam

eter of 350 m

m•Largest m

irror: diam

eter 3.7 m

Optica

l Telescope

s in Comparison

Palomar

Keck

JWST

Telescope aperture

5 m

10 m

6.5 m

Telescope m

ass600 t

300 t

6.5 t

# of segm

ents1

36

18

Segm

ent size5 m

1.8 m

1.3 m

Mass / segm

ent14.5

t400 kg

20 kg

Arecib

o, Puerto Rico –

the largest

(305m) single-aperture telescope

Dish

es similar to optical telescopes

but w

ith much

lower surface accuracy

7. Sub-mm & Radio T

elescopes

Effelsb

erg, Germ

any –100m fully steerab

le telescope

Greenb

ank, USA –after

structural collapse (now reb

uilt)

VLA

in New

Mex

ico –27 antennae

(each 25m) in a Y

-shape (up to 3

6

km baseline)

Arrays and Inte

rferom

eters

WSRT (W

esterbork)

in Drenth

e–14

antennae along 2.7 km

line

ALM

A in C

hile –

50 dish

es (12m each

) at 5000m altitud

e5000m altitud

e400µm

–3mm (7

20 GHz –

84GHz)

LOFAR in th

e Netherlands

Arecib

oThe LO

wFrequency A

Rray

uses two types of low

-cost antennas:

•Low

Band

Antenna (10

-90 M

Hz)

•High

Band

Antenna (110

-250 M

Hz).

Antennae are organized

in 36 stations

Antennae are organized

in 36 stations

over ~100 km

. Each

station contains 96 LB

As and

48 H

BAs

Baselines: 10

0m –1500km

Main LO

FAR sub

systems:

•sensor field

s•sensor field

s•wide area netw

orks•central processing system

s•user interfaces

•X-rays im

pinging perpendicular on any m

aterial are largely absorb

edrath

er than reflected

.

•�telescope optics is b

ased on glancing angle reflection (rath

er than

refraction or large angle reflection)

8. X-ray T

elescopes

refraction or large angle reflection)

•typical reflecting m

aterials for X-ray m

irrors are gold and

iridium

(gold

has a critical reflection angle of 3

.7 deg at 1 keV

).