April 20, 2023 1

Observational Astronomy

TELESCOPES,Active and adaptive

opticsKitchin pp.51-129

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Optical Schemes

Spherical mirrors cannot focus light properly due to spherical aberrations:

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Optical Schemes

… but a single parabola can produce perfect image on the optical axis:

Off-axis images suffer from coma.

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Ritchey-Chrétien telescope

Hyperbolic primary and hyperbolic secondary solve main aberration problems (sphericaland coma) in a rather large field of view(tens of arcminutes) in Cassegrain focus

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Schmidt-Cassegrain RC provides very good image quality in

a relatively small field (1º - 2º) When large FoV (up to 5º) is need

Schmidt-Cassegrain is the preferred design:

Spherical mirror

Correction plate

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Alternative to RC:Gregorian

system

Concave secondary after the primary focus:

Court

esy

of

AIP

Pots

dam

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Materials

Low thermal expansion: zerodur & sitall

mean linear coefficient of thermal expansion within temperature range-60º to +60º С is <10-8 cm ºC-1

Astro-sitall blank at LZOS(VST, VISTA, SALT, LAMOST, OWL?) Zerodur VLT primary at

REOSC

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More materials

Silicon Carbide Low thermal expansion

(not as good as glass) Very light Very hard, keeps the shape well Hard to make in large pieces Fragile, difficult to process

SiC 60 cm X-ray mirrorWeight: 6.2 kg

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Coatings

Mirrors: Aluminum (forming SiO) on the

top Silver-based coatings. Needs

coating to prevent mechanical damage during washing

Lenses: MgF2

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Hard Gold coating results (from 0.7 µm to 25 µm)

Wavelength (µm)

EXAMPLES OF OPTICAL COATINGEnhanced Silver coating results (from 350 nm to 800 nm)

Wavelength (nm)

Ref

lect

ivit

yR

efle

ctiv

ity

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Point Spread Function

PSF is the intensity distribution in the focal plane produced by a point source.

Ideal PSF (Bessel function)

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Active optics

Large thin mirrors are shaped bysupport system: VLT mirror is8.2m in diameterand only 18 cm thick! •Compensate for

thermal and orientation distortions

•Close loop operation during adjustment

•Low frequency:30 s cycle

•VLT: 150 actuators

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Adaptive optics

Seeing corrections (PSF):

The Strehl ratio is the ratio of peak intensities in the aberrated and ideal point spread functions in the focal plane (Born and Wolf 1999).

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Why do we need adaptive optics?

Atmospheric turbulence distorts the wave front.

Three ways of looking at the focal plane image:

1. Non-collimated beams (speckles)

2. Curved wavefront (phase shifts)

3. Changing intensity distribution

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Wavefront sensor

Shack-Hartmann Curvature sensor Pyramid WFS

Fast steering mirror is neededto get all pixels in focus

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Sensor implementation

A corner of the 1cmx1cm wavefront-sensor chip implemented in standard CMOS. The green elements are the position-sensitive detectors.

Sensor chip on a printed-circuit board.

Wavefronts must measured many at 100 kHz rate!

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Deformable mirror

Various shapes of a deformable mirror with 37 actuators

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Calibrations

Looking for a zero-point of DM:

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Closing the loop

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Laser Guide Star

a b

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Final result

VLT NACO:PSF and resolutionimprovements

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Home work Find a telescope with Silicon Carbide mirror. Why this

material was used? When using adaptive optics what are the pluses and

minuses of using natural and laser guide star?

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Next time…

Direct Imaging and Photometry