This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Astronom
ische Waarneem
technieken(Astronom
ical Observing T
echniques)10thLecture
: 23 November 2011
Co
nte
nt:
1.
Atm
osp
he
ric Tu
rbu
len
ce
2.
Wh
y A
O?
3.
Ba
sic Prin
ciple
Based
on:
“Adaptive O
ptics in Astronom
y” (Cambridge U
P) by F. Roddier (ed
.), Claire M
ax’s lecture course on A
O http://w
ww.ucolick.org/~
max/289C/
and ESO: http://w
ww.eso.org/projects/aot/D
SM/AO_modes.h
tml
4.
Ke
y C
om
po
ne
nts
5.
Erro
r Term
s
6.
Lase
r Gu
ide
Sta
rs
7.
Ty
pe
s of A
O C
on
cep
ts
Kolm
ogorov Turb
ulence
Ou
ter s
cale
L0
Inn
er s
cale
l0
so
lar
hννν ν
Win
d s
hear
gro
un
d
hννν ν
co
nve
ctio
nr0 , se
eing, τττ τ
0 , θθθ θ0
The Fried
parameter is th
e radius of th
e spatial coh
erence area.It is th
e average turbulent scale over w
hich the RMS optical ph
ase distortion is 1 rad
ian. Note that r
0increases as λ
6/5.
is called the seeing. A
t good sites r
0(0.5µm) ~10 -30 cm.
()
()
5/3
0
25/
6
01
85
.0
−∞
=∫
dz
zC
rn
λλ
5/1
0
~−
=∆
λλ
θr
The atm
ospheric coh
erence (or Greenw
ood delay)
timeis:
It is the m
aximum tim
e delay for th
e RMS wavefront
error to be less
than 1 rad
(where v
is the m
ean propagation velocity).
The isoplanatic
angleis th
e angle over which the RMS
wavefront
error is smaller th
an 1 rad.
0r
v r0
03
14
.0
=τ
h r0
0co
s3
14
.0
ζθ
=
Im
provem
ent in R
esolution a
nd S
ensitivity
1.Angular resolution:
2.Point source sensitivity:
00
gain
r D
Dr
=⇒
=→
=λ
θλ
θ
4in
t
21
~in
g
ain
D
~/
Dt
NS
⇒
PHARO LGS Ks im
ageWIRO H image
500s inte
g., 40" FOV, 15
0 masFWHM
Kobulnicky
et al. 2
005, AJ 129, 239-250
AO
Principle
1.Maximum scale
of tolerate
d wavefront d
eform
ation is r0
�sub
divid
e the telescope
aperture
into r0 ’s
2.Measure
the wavefront d
eform
ations.
3.Corre
ct the wavefront d
eform
ations by “b
ending b
ack” the patch
es of size
r0 .
The num
ber of sub
apertures is (D/r0 )2at th
e observing w
avelength �
can easily require hund
reds to th
ousands of actuators for very large
telescopes.
AO
Principle (2
)
Wave
front Description: Z
ernike
Polynom
ials
Expansion into a series of orth
ogonal terms:
Units
: Radia
ns o
f phase / (D
/ r0 )
5/6
Tip
-tilt is s
ing
le b
igg
est c
on
tribu
tor
Tip-
Tilt a
nd h
igher ord
er T
erm
s (1)
Refe
rence: N
oll
Fo
cu
s, a
stig
matis
m,
co
ma a
lso
big
Hig
h-o
rder te
rms g
o o
n
an
d o
n…
.
Wave
front Sensors –
Shack H
artm
ann
Most com
mon principle is th
e Shack H
artmann w
avefront sensor measuring sub
-aperture tilts:f
Pu
pil p
lan
eIm
ag
e p
lan
e
WFs: C
urvature
and
Pyra
mid
Sensors
Other com
mon principles are th
e
curvature sensor �
and the pyram
id sensor
�
Deform
able
Mirrors
Basic principle: piece-w
ise linear fit of the m
irror surface to the
wavefront. r
0sets th
e number of d
egrees of freedom.
Two general types: segm
ented mirrors
Two general types: segm
ented mirrors
and continuous face-sh
eet mirrors:
Note th
at the (piezo) actuator stroke is typically only a couple of
microm
eters �requires separate tip-tilt m
irror.
Adaptive
Second
ary
Mirrors
Concept: integrate D
M into th
e telescope �adaptive second
ary mirrors.
Advantages:
•no ad
ditional optical system
needed �
lower em
ission, higher th
roughput
•large surface �
higher actuator d
ensity
•larger stroke �
no tip-tilt mirror need
ed
...but also m
ore difficult to b
uild, control,
DM for M
MT Upgrad
e
...but also m
ore difficult to b
uild, control,
and hand
le.
Typica
l AO
Error T
erm
s•Fitting errors from
insufficient approximation of th
e wavefront (finite actuator spacing, influence function of actuators, etc.).