Transcript Observational Astronomy

Observational Astronomy
TELESCOPES,
Active and
adaptive optics
Kitchin pp.51-129
12 April 2016
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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 (spherical
and coma) in a rather
large field of view
(tens of arcminutes) in
Cassegrain focus
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Schmidt-Cassegrain
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RC provides very good image quality in a
relatively small field (1º - 2º)
When large FoV (up to 5º) is need SchmidtCassegrain is the preferred design:
Correction
plate
Spherical mirror
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Alternative to RC:
Gregorian system
Courtesy of AIP Potsdam
Concave secondary after the primary focus:
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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
SiC 60 cm X-ray mirror
(not as good as glass)
Weight: 6.2 kg
 Very light
 Very hard, keeps the shape well
 Hard to make in large pieces
 Fragile, difficult to process
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Coatings
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Mirrors:
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Aluminum (forming SiO) on the top
Silver-based coatings. Needs coating
to prevent mechanical damage
during washing
Lenses: MgF2
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Reflectivity
EXAMPLES OF OPTICAL COATING
Enhanced Silver coating results (from 350 nm to 800 nm)
Wavelength (nm)
Reflectivity
Hard Gold coating results (from 0.7 µm to 25 µm)
Wavelength (µm)
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Point Spread Function
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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 by
support system: VLT mirror is
8.2m in diameter
and 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
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Shack-Hartmann
Curvature sensor
Pyramid WFS
Fast steering mirror is needed
to get all pixels in focus
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Sensor implementation
Wavefronts must measured many at 100 kHz rate!
Sensor chip on a printedcircuit board.
A corner of the 1cmx1cm wavefrontsensor chip implemented in standard
CMOS. The green elements are the
position-sensitive detectors.
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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 resolution
improvements
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Home work
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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
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