Dr David Buscher, Astrophysics Group

Room 918, Rutherford Building, x37302

E-mail: dfb@mrao.cam.ac.uk

Meet to discuss 2:15pm on Tuesday October 29th in rm 918

1. Lasers for artificial guide star adaptive optics systems

Adaptive optics systems are used on large astronomical telescopes to
correct the distortions introduced by the Earth's atmosphere into images
of astronomical objects. A bright reference source close (in angular
terms) to the object being studied is required in order to derive the
wavefront corrections needed, but the density of bright natural sources
(i.e. stars) is insufficient to allow adaptive optics imaging of the whole
sky. One solution to this problem is to generate artificial "stars"  by
projecting a laser spot high in the Earth's atmosphere. This requires
lasers with a number of unusual characteristics, and so in many cases the
relevant laser technology is still under development. This review will
summarise the current state of the art in the lasers being used to
generate artificial beacons for adaptive optics.


2. Spots on stars

The surfaces of many stars are not uniform bright discs but are "spotty".
These spots can range from small dark features occupying less than a
percent of the surface area, as on our own Sun, to giant bright convection
cells which can cover the entire visible surface. What are the physical
processes which lead to this great variety of spots? What experimental
constraints are there on the properties of these spots on other stars?
What links are there between these spots and mass ejection from the
surface?


3. Practical applications of the geometric phase

The geometric phase is a phase shift which is induced by applying a
particular series of polarisation changes to a light beam or other
wavelike phenomenon. Since its discovery by Pancharatnam in 1956 it has
become increasingly important in experimental applications because of its
lack of wavelength dependence. The aim of this review is to summarise the
current range of practical applications of this unusual effect.


4. Adaptive optics on the human eye

The technology of adaptive optics, originally developed for astronomical
purposes, is based on the idea of compensating for optical aberrations
using a "rubber mirror" which dynamically takes up a shape which is
opposite to the aberrations caused by some distorting medium.  This
technique has recently been used to compensate for imperfections in the
lens of the human eye with some remarkable results. The aim of this review
is to summarise the technology involved, the applications of adaptive
optics and wavefront sensing in the human eye, and the latest results from
this technique.


5. Wavefront sensing using curvature and phase-diversity techniques

A critical component of adaptive optics systems is the device which senses
the aberrations in an optical wavefront. Two related techniques for
wavefront sensing are curvature sensing and phase-diversity sensing, both
of which rely on comparing images taken at two different focus positions.
You should review the physics behind these wavefront sensors and the
mathematical methods used to derive the aberrations, with the aim of
explaining the differences between the two sensing techniques and their
respective areas of applicability.