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Observational Astronomy TELESCOPES, Active and adaptive optics Kitchin pp.51-129
Optical Schemes Spherical mirrors cannot focus light properly due to spherical aberrations:
Optical Schemes … but a single parabola can produce perfect image on the optical axis:Off-axis images suffer from coma.
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
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: Correction plate Spherical mirror
Alternative to RC: Gregorian system Concave secondary after the primary focus: Courtesy of AIP Potsdam
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
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
Coatings • Mirrors: • Aluminum (forming SiO) on the top • Silver-based coatings. Needs coating to prevent mechanical damage during washing • Lenses: MgF2
EXAMPLES OF OPTICAL COATING Enhanced Silver coating results (from 350 nm to 800 nm) Wavelength (nm) Hard Gold coating results (from 0.7 µm to 25 µm) Wavelength (µm) Reflectivity Reflectivity
Point Spread Function • PSF is the intensity distribution in the focal plane produced by a point source. • Ideal PSF (Bessel function)
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
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).
Why do we need adaptive optics? Atmospheric turbulence distorts the wave front. • Three ways of looking at the focal plane image: • Non-collimated beams (speckles) • Curved wavefront (phase shifts) • Changing intensity distribution
Wavefront sensor • Shack-Hartmann • Curvature sensor • Pyramid WFS Fast steering mirror is neededto get all pixels in focus
Sensor chip on a printed-circuit board. A corner of the 1cmx1cm wavefront-sensor chip implemented in standard CMOS. The green elements are the position-sensitive detectors. Sensor implementation Wavefronts must measured many at 100 kHz rate!
Deformable mirror Various shapes of a deformable mirror with 37 actuators
Calibrations Looking for a zero-point of DM:
a b Laser Guide Star
Final result VLT NACO:PSF and resolutionimprovements
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?
Next time… Direct Imaging and Photometry