Wie konstruiert man ein Sonnenteleskop?€¦ · • Desired resolution at 550 nm : 0.1 arcsec – D = 1.4 m • Pixel size = 10 µm – image scale = 0.1 arcsec/20 µm = 5 arcsec/mm

GREGORTelescope, AO, BBI

Dirk SoltauKiepenheuer-Institut für Sonnenphysik

CASSDA School, April 20th, 2015

The demand for high spatial resolution

Show the simulations the truth?

Courtesy Oskar Steiner, KIS

Show the simulations the truth?

Solar Tornado

Credits: Wedemeyer-Böhm (2012). Image produced with VAPOR

High Resolution

Diffraction Limited Resolution

http://www.olympusmicro.com/primer/anatomy/numaperture.html

Dλθ 22.1=

Example:

λ = 550 nmD = 1 m

θ = 0.67 µrad = 0.14 arcsec

[rad]

By the way: There is no free lunch

• Number of photons per arcsec2 ∝ T × D2

• Size of the resolution element in arcsec2 ∝ 1/ D2

Strehl

Strehl > 0.6 „good“Strehl > 0.8 „diffraction limited“

GREGOR Modulation Transfer Function(MTF)

1“ 0.5“ 0.1“

Aperture, f-ratio, field of view• Desired resolution at 550 nm : 0.1 arcsec

– D = 1.4 m• Pixel size = 10 µm

– image scale = 0.1 arcsec/20 µm = 5 arcsec/mm– f = 41.2m– f-ratio = f/30

• Number of pixels = 4096– FOV = 200 arcsec

f

1 mm5 arcsec

image scale [arcsec/mm] = ( f[mm] * tan(1“) )-1

= 206270/ f[mm]

Three types of mirror telescopesNewton Cassegrain Gregory

Optical Design 1The Simple Solution

McMath-Pierce, f/54

f=86 m

McMath-Pierce (Lazy Seven, f/54)

D = 0.7 mf = 45 m

f/64

Vacuum Tower Telescope (VTT)

VTT opticallayout

SOLIS: A solar Cassegrain telescope

The Gregory Coudé telescope

Picture taken fromhttp://www.irsol.ch/Poster/ao-poster.pdf

D = 0.45 mf = 25 m

f/55

From Gregory to GREGOR• Has to be shorter than 4 m to fit into dome• Diameter : 1.5 m

• f/# of primary appr. 2• Diffraction limited resolution = 0.08 arcsec

• Central obscuration < 0.3 f/# M2 > 1• Secondary focus F2

FF1F2

Heat load in a solar telescope focus

• Solar power density (max) 0.1 W/cm2

2

10000ionmagnificatdensity power

≈

fD

Power density in GREGOR F1 ≈ 300 W/cm2 !!

In any solar telescope focus:

Optical Design

D = 1.5 mf/40

Power mirrorsRadius of curv. Conic const f/#

M1 -5013 mm -1 f/1.8

M2 -1039 mm -0.306 f/1.2

M3 -2797 mm -0.538 f/4

Foci (FOV = 150 arcsec)f/# Image scale Magn. FOV Power Power dens.

F1 1.8 82 arsec/mm 1.8 mm 1600 W 300 W/cm2

F2 6 23.6 arcsec/mm

3.5 6.3 mm 10 W 30 W/cm2

F3 40 3.6 arcsec/mm 6.6 41.6 mm 4 W 0.3 W/cm2

Scatter diameter:150 µm = 12 arcsec

Optical quality in F1 (FOV = 200“)

Scatter diameter:40 µm = 1 arcsec

30 W/cm2

Optical quality in F2 (FOV = 200“)

Scatter diameter:20 µm = 0.07 arcsec

0.7 W/cm2

Optical quality in F3 (FOV = 200“)

Overview of telescope structure and optical

path

Dr. Reiner Volkmer, Kiepenheuer Institut für Sonnenphysik, Freiburg

Dr. Reiner Volkmer, Kiepenheuer Institut für Sonnenphysik, Freiburg 28

Telescope structure

• Sun heats structure and mirrors open telescope and completely foldable dome (telescope in free air flow). Telescopes with 1.5 m free aperture and evacuated light path are very difficult to build

• full performance at wind speeds up to 20 m/s with relative pointing error (rms): 0.5“ (open loop)

• temperature difference to ambient :+0.2 K < ∆T < -0.5 K

Structure

Dr. Reiner Volkmer, Kiepenheuer Institut fürSonnenphysik, Freiburg 30

Why Silicon Carbide main mirrors?• solar radiation on surface of 1.5 m main mirror: 2000 W• ∆T mirror – ambient temperature should be < 1K (internal seeing)• heating of mirrors is critical !!!• silicon carbide has high thermal conductivity:

K(silicon carbide) ~ 100 x K(Zerodur) • silicon carbide mirrrors can be "thinned" and structured (high stiffness)

– ► light weight mirror (M1 ~ 90 kg) faster reaction on temperature changes

– ► removal of heat on back side• GREGOR: Primary, secondary and tertiary are silicon carbide mirrors

M336 cm2.5 kg

raw

finished

Mirror seeing - Mirror Cooling

• Heat transfer bythermal conductivity:

TAd

Q ∆⋅= λ.

Material λ[W/(mK)]

Air 0.03

Glass 0.76

Cesic 121

Zerodur 1.46

Copper 403

d = thicknessA = area

Example M1:ZerodurD = 30 mmA = 1.7 sqm∆T = 3 K dQ/dt = 250 W

Example F1 heat stop:CopperD = 10 mmA = 0.0005 sqm∆T = 10 K dQ/dt = 200 W

Primary mirror and field stop cooling

Air flowM1 mirror

Nozzle

Heatexchanger

FanHousing

Thermal control system

The enemy

Some atmospheric optics(index of refraction for air)

62 10)22.16.272(1)( −++=

λλn

6106.771)( −=−T

Prn

Wave length dependence of n

T and P –dependence of n

Example : P=1000 mbar, T=293 K → ∆n=10-6 per degree

Some atmospheric optics(turbulence)

viscosity:scale spatial :d

velocity:vdensity:

η

ρη

ρ dvRe⋅⋅

=

3/11)( −∝ kkE

Reynolds number Re: 105, i.e. . Atmosphere is always turbulent and never laminar

Kolmogorov spectrum

Kolmogorov’s theory of turbuklence

van Gogh, „Starry Night“

Some atmospheric optics: structure function

[ ] 3/22211 )()( −=−+= rCrnrrnD nrrn

Dn = Structure function Cn2 = structure constant (unit m-2/3 )

Cn2 is a function of height

Cn2 = 10-17 m-2/3

Cn2 = 10-14 m-2/3 37

D. Soltau, Mai 2000 38

Cn2 profile (Example Hufnagel model)

( )2/120

5

2

),(1500/161000/2

10532

)(15

1

1027

210.2)(

=

+

=

∫

−−−−

km

km

thrhhn

dhhvkm

W

eeeWhhC

D. Soltau, Mai 2000 39

Fried’s Parameter r0

λπ

β

2

)()sec(423.05/3

0

220

=

=

−

∫

k

dhhCkrL

n

5/60 λ∝r

[ ]223/5

0

2 364.0 radDr

Dtilt

=

λσÜbrigens: Allein die Korrektur für tip/tilt halbiert die durch die Turbulenz verursachte Varianz

r0 is a kind of atmospheric “aperture”

Measuring r0 (example)

[ ]223/5

0

2 364.0 radDr

Dtilt

=

λσ

DIMM = „Differential image motion monitor“

Adaptive Optics: The Idea

• Babcock (1953): The possibility of compensating astronomical seeing

Horace Welcome Babcock, 1912 - 2003

Adaptive Optics:An Additional Requirement for a

Telescope Design

We need a pupil image

http://lyot.org/background/adaptive_optics.html

Relay Optics

fast

GREGOR AO System: GAOS

GAOS: How many actuators do weneed?

Subapertur: 10 cm x 10 cm

r0 = 5 cm

r0 = 10 cm

How large is the corrected FOV?

Hr0314.0=θ

Example: r0 = 10 cmH = 2 km

θ = 1.6 10-5 rad = 3 arcsec

Deformable mirror

DM Characteristics•stacked-Piezo, made by CILAS•256 actuators, 196 illuminated•48mm illuminated diameter•5μm stroke•3.2mm actuator pitch•new glueing technology•first resonance frequency > 10 kHz

CILAS DM: 256 actuators

Shack Hartmann WFS

Solar AO• Target: Granulation

– Intrinsiccontrast: 14%

– in telescope 2% bis 5%

– structure sizetypical 2“

Observations mostly done in thevisible and near infrared

Minimum size of subaperture?

1/arcsec

taken from ATST archive 52D. Soltau (KIS), Delft Workshop 21./22.02.1321.02.2013

Bottle neck: The WFS camera

1600 fps

GREGOR AO GUI

54D. Soltau (KIS), Delft Workshop 21./22.02.1321.02.2013

Night time AO performance

Night time performance with AO

FWHM = 0.2 arcsec

Multi ConjugateAO for GREGOR

GREGORF1

F2

F3

F4 Pupil

Back end instruments

• GRIS• GFPI• BBI (Broad Band Imager)

Instrument Floor

BBIGFPI

GRIS

Broad Band Imager

F4

F4

WFS

Collim.

Pupil

Pupil

Filter

Cameraf = 260 mm

Detector 1

BS BS

Detector 2

(PCO 4000)PCO Sensicam

Filter(wheel)

f = 400 mm Cameraf = 486 mm

fromtelescope

Broadband Imager

First Results - BBI

• June 2013, parallel with Sunrise flight

• Courtesy A. Lagg, R. Schlichenmaier, M. Franz

Summary

• GREGOR provides world class observingcapabilities. It is:– A large telescope– at an excellent site– fully equipped with AO and backend instruments– beside an excellent Vacuum Tower Telescope

GREGOR�Telescope, AO, BBIThe demand for high spatial resolutionShow the simulations the truth?Show the simulations the truth?High ResolutionDiffraction Limited ResolutionBy the way: There is no free lunchStrehlGREGOR Modulation Transfer Function (MTF)Aperture, f-ratio, field of viewThree types of mirror telescopesOptical Design 1�The Simple SolutionMcMath-Pierce, f/54McMath-Pierce (Lazy Seven, f/54)Vacuum Tower Telescope (VTT)VTT optical layoutSOLIS: A solar Cassegrain telescopeFoliennummer 18The Gregory Coudé telescopeFrom Gregory to GREGORHeat load in a solar telescope focusOptical DesignPower mirrorsOptical quality in F1 (FOV = 200“)Optical quality in F2 (FOV = 200“)Optical quality in F3 (FOV = 200“)Overview of telescope structure and optical pathTelescope structureStructureWhy Silicon Carbide main mirrors?Mirror seeing - Mirror CoolingPrimary mirror and field stop coolingThermal control systemThe enemySome atmospheric optics�(index of refraction for air)Some atmospheric optics�(turbulence)Some atmospheric optics: �structure functionCn2 profile (Example Hufnagel model)Fried’s Parameter r0Measuring r0 (example)DIMM = „Differential image motion monitor“Adaptive Optics: The IdeaAdaptive Optics:�An Additional Requirement for a Telescope DesignGREGOR AO System: GAOSGAOS: How many actuators do we need?How large is the corrected FOV?Deformable mirrorCILAS DM: 256 actuatorsShack Hartmann WFSSolar AOMinimum size of subaperture?Foliennummer 52Bottle neck: The WFS cameraGREGOR AO GUINight time AO performanceNight time performance with AOMulti Conjugate AO for GREGORGREGORBack end instrumentsInstrument FloorBroad Band ImagerBroadband ImagerFoliennummer 63First Results - BBISummary