Direct Imagine and wavefront control- Adaptive Optics Flashcards
How do wavefront sensors work?
Measures aberrations of an optical wavefront.
Shack-Hartmann: An image of the exit pupil is projected onto a lenslet array - a collection of small identical lenses. Each lens takes a small part of the aperture, called a sub-pupil, and forms an image of the source. All images are formed on the same detector, typically a CCD. When an incoming wavefront is a plane, all images are located in a regular grid defined by the lenslet array geometry. As soon as the wavefront is distorted, the images become displaced from their nominal positions. Displacements of image centroids in two orthogonal directions x,y are proportional to the average wave-front slopes in x, y over the sub-apertures. The WFS measures the wave-front slopes and the wavefront itself is reconstructed from the arrays of measured slopes
PyWFS: A transparent pyramid splits the wavefront into four beams, each producing an image of the entrance pupil. For a distorted wavefront, the intensity distribution among the pupils will change. The local wavefront gradients can be obtained by recording the distribution of intensity in the pupil images. The wavefront aberrations can be evaluated from the estimated wavefront gradients.
Describe the structure of an uncorrected, AO corrected, and coronagraphically corrected PSF.
An uncorrected PSF scatters light away from the central core of the PSF with complicated structure that evolves in complex ways hundreds to thousands of times a second. If you integrate long enough without AO, the speckles approach a gaussian distribution, much larger than the diffraction limit of large diameter telescopes. On Maunakea, an excellent site, uncorrected spot sizes might be on the order of 1 arcsec to .5 arcsec on a very good night. With AO, much of the complex structure can be taken out and the PSF should approach close to the diffraction limit of the telescope, but still might be limited to around 30-60 mas. With a coronagraph, the light from the center of the field is suppressed by several orders of magnitude, though you are still left with some complex structure in the PSF away from the core. This is still typically much lower than the scattered light of the central source without a coronagraph, though.
What causes diffraction rings?
Because of the way light propagates. Each point on a wavefront is a source of spherical wavelets. So when light passes through a slit, the wavelets from different points on the wavefront interact, and their pattern of interference creates these rings. This can also happen when a light wave travels through a. medium with a varying refractive index. The amount of diffraction depends on the size of the gap. Diffraction is greatest when the size of the gap is similar to the wavelength of the wave.
What is r0? What is t0?
Fried Parameter (r0): As a plane wavefront from a distant star propagates through the atmosphere, it will be randomly distorted by moving cells of air with different indices of refraction. The size of the turbulent cells is characterized by the Fried parameter, r0. The Fried parameter indicates the length over which a wavefront remains unperturbed. Strictly speaking, r0 refers to the length of the wavefront over which the phase changes by 1 radian. The larger the Fried parameter, the better the atmospheric conditions. At a good observing site, the Fried parameter typically has a value of r0 = 10 cm at an optical wavelength of = 500 nm. (Maunakea more like 15cm to 25cm on a good night) For a model of Kolmogorov turbulence, the Fried parameter is theoretically predicted to vary with wavelength as r0 prop to lambda^(6/5).
Coherence time (t0): The turbulent cells responsible for distorting the plane wavefronts from a star generally evolve on longer timescales than the time it takes the wind to move a cell by its own size. Hence it is the wind velocity, v, at the altitude of the turbulence that determines the temporal variation of the wavefronts entering the telescope. A turbulent cell would move its own size in a time t0 = r0/v
Bonus– Isoplanatic angle (theta0): If there are two stars close together on the sky, theta0 is the angle they have to be separated by in order for light from them to pass through approximately the same turbulent region of the atmosphere. This can be estimated from the angle over which the turbulence pattern is shifted by a distance of only r0: theta0 = r0/h
What does “seeing” refer to?
Temperature inhomogeneities by mixing air parcels leading to variation in refractive index and wavefront disturbance
(Ian) Seeing typically refers to the angular resolution you can achieve with current conditions without AO
What is focal-plane wavefront control? Give an example.
A critical approach to reducing non-common path errors between AO wavefront sensor (WFS) detector and science detector. NCPAs, go unsensed as they arise after WFS, so we use the focal plane to make corrections– like using the DM to inject phase corrections in the pupil-plane electric field to null speckles.
Why go to space if you have AO?
The Earth’s atmosphere is nearly opaque to many wavelengths and glows brightly at most of the thermal infrared wavelengths beyond 2µm. To deal with the IR glow, a cold telescope in space is required. And a space-based telescope is certainly more stable
