Telescopes & Adaptive Optics. Roberto Ragazzoni INAF Astronomical Observatory of Padova

Telescopes & Adaptive Optics Roberto Ragazzoni INAF Astronomical Observatory of Padova

PAST

PAST FUTURE

This is a simmetry line

This object is drawn in a plane but

it acctually reppresent a three dimensional object

So this is a round concave mirror with a central hole

D is the diameter or aperture of the telescope D

D is the diameter or aperture of the telescope D=1.22m the Asiago-Pennar telescope D=1.82m the Asiago-Ekar telescope D=3.58m the TNG, Telescopio Nazionale Galileo D=8.2m one of the VLT, Very Large Telescope D=8.4m one of the mirror of the LBT, Large Binocular Telescope D=30m the TMT, Thirty Mirror Telescope D=39m the E-ELT, European Extremely Large Telescope D

So, let s keep the roundness as a first approx D

An old fashioned conventional 1.22m telescope

An old fashioned conventional 1.22m telescope and a futuristic segmented 39m telescope

This is light coming from a certain unresolved point source located at infinity.

This light is actually collected from the telescope

while this other is lost

This edge defines the pupil

This edge defines the pupil D

ε is the linear obstruction coefficent D ε D

The shaded one is the useful area

The shaded one is the useful area Useful area is πd 2 /4(1-ε 2 ) ε=0.0 100% of the area (Yerkes) ε=0.1 99% ( ) ε=0.2 96% (Very Large Telescope) ε=0.3 91% (Hubble Space Telescope) ε=0.5 75% (Pino Torinese)

So these are the edges of the pupil

And these rays are called marginal

And this is the primary mirror foci

And this is where it focus an off-axis source

The primary mirror focal plane

The mirror conjugates two points

One at infinity (or very long distance) and lying on the simmetry axis

And one in the focus of the concave surface

If the primary mirror is a parabolid the conjugation is perfect in geometric approx and it is said stigmatic

Let s now mount on top of the primary mirror a convex hyperboloidal mirror

This surface is characterized by two conjugated points, the foci of the hyperbola generating the surface

..and co-align with the foci of the concave mirror

The marginal rays now does no longer reach the primary focal plane

But instead reach what is called the Cassegrain foci

This also re-defines the obstruction by the secondary mirror

Some nomenclature

Some nomenclature M1

Some nomenclature M1 M2

Some nomenclature The primary mirror foci M1 M2

Some nomenclature The primary mirror foci M1 M2 The combined Cassegrain- foci

Some off-axis marginal rays

Please note this ray goes just on the edges of M2

All together now

This defines a Field of View FoV/2

..and a magnification factor m FoV/2 x f 1

FoV/2 x f..and a magnification factor m FoV/2 x f 1

FoV/2 x f..and a magnification factor m m=f/f 1 FoV/2 x f 1

What if the observation is in the Thermal InfraRed Thermal InfraRed means that the environment where the telescope is located emits significantly at that wavelength range On the ground (T=300K) this happens at wavelengths larger than about 2.4um K or K-short is the K band truncated below such threeshold K is in Thermal InfraRed This thermal background can significantly increase the background

How can an evilish ground-based Th-IR photon, reach the focal plane?

Let s undersize the secondary mirror

These rays does no longer reach the focal plane

And, in fact, they are not anymore marginal

These are actually the new marginal rays

And this is the bundle of rays that are actually collected

This is actually the new pupil of such a telescope

Making the effective aperture of the telescope smaller than the diameter of the main mirror

Making the effective aperture of the telescope smaller than the diameter of the main mirror D D 1 VLT D 1 =8.2m, D=8.0m LBT: D 1 =8.4m, D=8.2m

The pupil Where the pupil is located can be extremely important It can makes a telescope able to reject ThIR light at the expense of a little loss in effective aperture It can be used to conjugate a small corrector to a large mirror (Arecibo, MacDonald) It can be used in Adaptive Optics

A pupil

Adaptive Optics The atmospheric distorted WF can be corrected using a Deformable Mirror (DM). The wavefront sensor (WFS) measures the WF of a reference star-object The measurement is used to drive the DM to introduce an opposite WF-deformation. A new WF measurement is then performed to apply a differential correction in a closed loop way 15/10/07 MPIA - Heidelberg 96

Syracuse about 2215 years ago

Syracuse about 2215 years ago DM

Syracuse about 2215 years ago DM WCSD (dichroic) Woodden Coated Scattering Device

Syracuse about 2215 years ago DM WFC WFS WCSD (dichroic) Woodden Coated Scattering Device

California about 62 years ago

California about 62 years ago Dichroic DM WFS

California about 62 years ago Resembling a variable Thickness Mangin mirror

California about 62 years ago

Kolmogorov

Kolmogorov Outer scale Telescope size r0 Innerscale

A 2D plot of turbulence

Atmospheric turbulence TIP-TILT: AVERAGE OF ALL THE DEFORMATIONS!! TIP-TILT HIGH ORDERS Movement Integrated in time Size increase Tilt extimation 4-Quadrant Sensor High Orders extimation Specific Wavefront Sensors TIP-TILT CORRECTION: TILTING FLAT MIRROR HIGH-ORDER CORRECTION: DEFORMABLE MIRROR

Fried parameter r0 Our enemy some figures One rad of phase is our unit

Arizona about 44 years ago

Arizona about 44 years ago Wavefront Lenses Spot images

Our enemy some figures Astronomical Object Wavefront Telescope Entrance Pupil Image Plane r FRIED Parameter 6/5 2 3/5 0 [ dhcn ( h) ] 6/5 0 0 Temporal behaviour of the turbulence o 0 6/5 1ms r 0 Perfect Image Deformated Image Strehl Ratio: PSF obs /PSF teo

Isoplanatic angle Guide Star Scientific Object ISOPLANATIC ANGLE: Angle from the reference star where the correction is still effective Telescope pupil projections Turbulent Layers 0 r / 0 h h Telescope pupil

Turbulence parameters Fried parameters (the size at which WF perturbation is statistically more than one radians) Greenwood frequency (the inverse of the time at which perturbation changes more than one radians) Isoplanatic angle (the angular distance between two sources whose wavefront is perturbed differently by more than a radians)

Wavefront Error (nm) 100nm 10nm Narrow field NGS IR AO Narrow field Visible AO Narrow field LGS IR AO # of DM actuators # of WFS sub-apertures Ground-layer AO Multi-Conjugate AO (MCAO) Optics size, optical complexity AO loop speed More photons needed 0 10 1 Field of View

The strategic defense initiative (propagation in the atmosphere declassified in 1991) Kick off: March 23rd, 1983

The strategic defense initiative (propagation in the atmosphere declassified in 1991)

South of France about 26 years ago and then at ESO LaSilla 3.6m With Come-On and Come-On+

Deformable Mirrors

Deformable Mirrors Densità attuatori

Deformable Mirrors Densità attuatori Frequenza temporale

Deformable Mirrors Densità attuatori Frequenza temporale Accoppiamento

Deformable Mirrors

Florence about 22 years ago Active and adaptive optics: ESO Proceedings of the ICO-16 August 2-5, 1993, Garching

Secondary Adaptive Mirrors

Layers for an artificial reference

Problems of LGSs Tip-tilt indetermination problem Conical anisoplanatism Focus at a finite distance Rayleigh fratricide effects Actual distance depends upon altitude and layer local variations Cyrrus can make large scattered light Aircraft and satellite hazards You need a working laser!

Layers for an artificial reference

LGS launch systems Launch mirror Beam expander exit and folding flat Wavefront sensor Laser units Laser platform

Arizona (with some Italian flavor ) about 20 years ago

Canale: PadovAdopt

present to future High order AO with high efficiency mades XAO Wider Field of View achieved with multiple DMs Pushing into the visible and into larger Sky Coverage Using LGSs or even higher efficient Wide Field AO (wait for a couple of days ) Making the telescope fully adaptive Making corrections achievable on a small scale (MOAO)

Wider Field of View