Polarimetry is a particularly powerful technique when imaging circumstellar environments. Currently most telescopes include more or less advanced polarimetric facilities and large telescopes count on it for their planet-finder instruments like SPHERE-ZIMPOL on the VLT or EPICS on the future E-ELT. One of the biggest limitations of this technique is the instrumental polarization (IP) generated in the telescope optical path, which can often be larger than the signal to be measured. In most cases this instrumental polarization changes over time and is dependent on the errors affecting the optical elements of the system. We have modeled the VLT and E-ELT telescope layouts to characterize the instrumental polarization generated on their optical paths using the M&m's code, an error budget and performance simulator for polarimetric systems. In this study we present the realistic Mueller matrices calculated with M&m's for both systems, with and without the setups to correct for the IP, showing that correction can be achieved, allowing for an accurate polarimetric performance.
Control software for adaptive optics systems is mostly custom built and very specific in nature. We have developed
FOAM, a modular adaptive optics framework for controlling and simulating adaptive optics systems in various
environments. Portability is provided both for different control hardware and adaptive optics setups. To achieve
this, FOAM is written in C++ and runs on standard CPUs. Furthermore we use standard Unix libraries
and compilation procedures and implemented a hardware abstraction layer in FOAM. We have successfully
implemented FOAM on the adaptive optics system of ExPo - a high-contrast imaging polarimeter developed at
our institute - in the lab and will test it on-sky late June 2012. We also plan to implement FOAM on adaptive
optics systems for microscopy and solar adaptive optics. FOAM is available* under the GNU GPL license and
is free to be used by anyone.
Wavefront sensorless adaptive optics methodologies are considered in many applications where the deployment
of a dedicated wavefront sensor is inconvenient, such as in fluorescence microscopy. In these methodologies,
aberration correction is achieved by sequentially changing the settings of the adaptive optical element until
a predetermined imaging quality metric is optimised. Reducing the time required for this optimisation is a
challenge. In this paper, a two stage data driven optimisation procedure is presented and validated in a laboratory
environment. In the first stage, known aberrations are introduced by a deformable mirror and the corresponding
intensities are measured by a photodiode masked by a pinhole. A generic quadratic metric is fitted to this
collection of aberrations and intensity measurements. In the second stage, this quadratic metric is used in order
to estimate and correct for optical aberrations. A closed form expression for the optimisation of the quadratic
metric is derived by solving a linear system of equations. This requires a minimum of N +1 pairs of deformable
mirror settings and intensity measurements, where N is the number of modes of the aberrations.
We have implemented a coherence-gated wavefront sensor on a two-photon excitation microscope. We used the backscattered near-infrared light from the sample to interfere with an optically flat reference beam. By applying a known waverfront tilt in the reference beam, a fringe pattern emerged on the camera. The deformmation of the wavefront due to the turbid media under study warps the fring pattern, similar to frequency modulation. Through Fourier transform analysis of the modulated fringe pattern we were able to determine the wave fornt aberrations induced by synthetic and biological samples. By defocussing the microscope objective and measuring the wavefront deformation we established that the errors are reproduceible to within λ/227 for the defocus mode.
Seeing measurements are crucial for the optimum design of (multi-conjugate) adaptive optics systems operating at solar
telescopes. For the design study of the 4-meter European Solar Telescope, to be located in the Canary Islands, several
instruments have been constructed and operated, at the Observatorio del Roque de los Muchachos (La Palma) and at the
Observatorio del Teide (Tenerife), to measure the properties of the ground layer and medium-high altitude turbulence.
Several units of short (42.34 cm) and two long (323.06 cm) scintillometer bars are, or are to be, installed at both
observatories. In addition to them, two wide-field wavefront sensors will be attached to the optical beams of the Swedish
tower, on La Palma, and of the German VTT, on Tenerife, simultaneously used with the normal operation of the
telescopes. These wavefront sensors are of Shack-Hartmann type with ~1 arcminute field of view. In this contribution,
the instruments setup and their performance are described.