For applications like direct imaging detections of Exo-Planets from the ground e.g. in the CHEOPS project, extreme adaptive optics (XAO) systems using DMs with > 1000 actuators and correction frequencies of ~2kHz are proposed to be used in combination with coronographic devices. If the XAO and science channel work at the same wavelength it is a natural idea to combine the coronograph with the XAO's beam splitter (BS) to make use of the light that would otherwise just be lost. However, the location of the BS in the focal plane and the severe field limitation of the AO by a small (~0.3'') aperture in the focal plane imposes a spatial filtering on the wavefront sensor signal. In this paper, we examine the effect of the spatial filter on the "AO control radius" and the Strehl ratio provided by the system in a semi-analytical way, numerical simulations for various wavefront sensor types and a laboratory verification experiment.
A new wavefront sensor based on the pyramid principle is being built at MPIA, with the objective of integration in the Calar Alto adaptive optics system ALFA. This sensor will work in the near-infrared wavelength range (J, H and K bands). We present here an update of this project, named PYRAMIR, which will have its first light in some months. Along with the description of the optical design, we discuss issues like the image quality and chromatic effects due to band sensing. We will show the characterization of the tested pyramidal components as well as refer to the difficulties found in the manufacturing process to meet our requirements. Most of the PYRAMIR instrument parts are kept inside a liquid nitrogen cooled vacuum dewar to reduce thermic radiation. The mechanical design of the cold parts is described here. To gain experience, a laboratory pyramid wavefront sensor was set up, with its optical design adapted to PYRAMIR. Different tests were already performed. The electronic and control systems were designed to integrate in the existing ALFA system. We give a description of the new components. An update on the future work is presented.
The use of a pyramid wavefront sensor without any kind of
modulating device, dynamical or statical, is a tempting idea that
is being considered in the actual design of some wavefront sensing
systems. However, such a system has not yet been fully studied, as
for the effect of static non-common path aberrations, which in an
extreme case would leave the system working in a saturated regime.
Here we analyze the performance of a sensor, with and without
modulation, working under these conditions, with two approaches:
In laboratory experiments with a pyramid wavefront sensor system
working in open- and closed compensation and through numerical
We are currently investigating the possibilities for a high-contrast, adaptive optics assisted instrument to be placed as a 2nd-generation instrument on ESO's VLT. This instrument will consist of an 'extreme-ao' system capable of producing very high Strehl ratios, a contrast-enhancing device and two differential imaging detection systems. It will be designed to collect photons directly coming from the surface of substellar companions - ideally down to planetary masses - to bright, nearby stars and disentangle them from the stellar photons. We will present our current design study for such an instrument and
discuss the various ways to tell stellar from companion photons. These ways include the use of polarimetric and/or spectroscopic
information as well as making use of knowledge about photon statistics. Results of our latest simulations regarding the instrument will be presented and the expected performance discussed.
Derived from the simulated performance we will also give details
about the expected science impact of the planet finder. This will
comprise the chances of finding different types of exo-planets -
notably the dilemma of going for hot planets marginally separated
from their parent stars or cold, far-away plamnets delivering very
little radiation, the scientific return of such detections and
follow-up examinations, as well as other topics like star-formation,
debris disks, and planetary nebulae where a high-resolution,
high-contrast system will trigger new break-throughs.
Detection of faint companions near bright stars requires the usage of high dynamic range instrumentation. The four quadrant phase mask is a quite efficient nulling device for the light of on-axis stars as shown by simulations. We conducted a test of the true performance of this concept starting with the manufacturing of the optical element, continuing with the installation in the telescope and the usage of the Adaptive Optics system. A four quadrant phase mask was installed in the 3.5m telescope on Calar Alto and several tests with both an artificial source and natural stars were conducted. Tests in order to detect faint companions around HD 140913, TRN 1 and HD 161797 were successful for the last target and also, although almost serendipitously, in the case of HD144004. The main limitations found for the phase mask cancelling effect at relatively low Strehl ratios (16%-63%) were the residual tip-tilt of our system and the control of placement of the mask in the optical train.
In the pyramid wavefront sensor some dynamic range is accomplished by modulating the optical signal across the four faces of the pyramid before the dissection and detection of the light. Although this can be realized in different ways, including systems which do not require any moving part, we question and discuss the real needs for such a modulation. In fact, when the closed-loop performance is not perfect, some residual errors on the wavefront sensor are expected and one should take care to allow for enough dynamic range to get a linear response within such a residual range. However, the non-corrected aberrations themselves can be considered as a form of modulation. Higher order uncompensated residuals are equivalent to a modulation for the lower compensated modes.
We present a preliminary study showing that this sort of 'natural' modulation could be, at least under certain conditions, enough to reach comparable results with respect to dynamical modulation during correction, hence rising the question of the need of a modulation in the realization of the pyramid wavefront sensor.
The objective of the PYRAMIR project is to complement the Calar Alto Adaptive Optics System - ALFA - with a new pyramid wavefront sensor working in the near IR, replacing the previous tip-tilt tracker arm. Here we describe the Science as well as the Technical motivation for such a system. The optical design will be presented, discussing the particular requirements posed by sensing the wavefronts in the infrared like a cooling system for the opto-mechanical components, etc. We will also talk about the components, like the IR detector we plan to use - PICNIC, as one option, the sucessor of NICMOS3 from Rockwell, together with the AO-Multiplexer. It is described how we expect to integrate the system into the optical, machanical, electronical and control architecture of ALFA.