1. History of Telescopes Lecture: 29 September 2010home.strw.leidenuniv.nl › ~brandl ›...
Transcript of 1. History of Telescopes Lecture: 29 September 2010home.strw.leidenuniv.nl › ~brandl ›...
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
).