660 research outputs found
Atmospheric refractivity effects on mid-infrared ELT adaptive optics
We discuss the effect of atmospheric dispersion on the performance of a
mid-infrared adaptive optics assisted instrument on an extremely large
telescope (ELT). Dispersion and atmospheric chromaticity is generally
considered to be negligible in this wavelength regime. It is shown here,
however, that with the much-reduced diffraction limit size on an ELT and the
need for diffraction-limited performance, refractivity phenomena should be
carefully considered in the design and operation of such an instrument. We
include an overview of the theory of refractivity, and the influence of
infrared resonances caused by the presence of water vapour and other
constituents in the atmosphere. `Traditional' atmospheric dispersion is likely
to cause a loss of Strehl only at the shortest wavelengths (L-band). A more
likely source of error is the difference in wavelengths at which the wavefront
is sensed and corrected, leading to pointing offsets between wavefront sensor
and science instrument that evolve with time over a long exposure. Infrared
radiation is also subject to additional turbulence caused by the presence of
water vapour in the atmosphere not seen by visible wavefront sensors, whose
effect is poorly understood. We make use of information obtained at radio
wavelengths to make a first-order estimate of its effect on the performance of
a mid-IR ground-based instrument. The calculations in this paper are performed
using parameters from two different sites, one `standard good site' and one
`high and dry site' to illustrate the importance of the choice of site for an
ELT.Comment: 11 pages, to be published in SPIE Proceedings vol. 7015, Adaptive
Optics Systems, eds. N. Hubin, C.E. Max and P.L. Wizinowich, 200
Discovery of a stellar companion to the nearby solar-analogue HD 104304
Sun-like stars are promising candidates to host exoplanets and are often
included in exoplanet surveys by radial velocity (RV) and direct imaging. In
this paper we report on the detection of a stellar companion to the nearby
solar-analogue star HD 104304, which previously was considered to host a
planetary mass or brown dwarf companion. We searched for close stellar and
substellar companions around extrasolar planet host stars with high angular
resolution imaging to characterize planet formation environments. The detection
of the stellar companion was achieved by high angular resolution measurements,
using the "Lucky Imaging" technique at the ESO NTT 3.5m with the AstraLux Sur
instrument. We combined the results with VLT/NACO archive data, where the
companion could also be detected. The results were compared to precise RV
measurements of HD 104304, obtained at the Lick and Keck observatories from
2001-2010.
We confirmed common proper motion of the binary system. A spectral type of
M4V of the companion and a mass of 0.21 M_Sun was derived. Due to comparison of
the data with RV measurements of the unconfirmed planet candidate listed in the
Extrasolar Planets Encyclopaedia, we suggest that the discovered companion is
the origin of the RV trend and that the inclination of the orbit of
approximately 35 degrees explains the relatively small RV signal.Comment: 4 pages, 4 PNG figures, use aa.cls, accepted for publication in
Astronomy & Astrophysic
PYRAMIR: Calibration and operation of a pyramid near-infrared wavefront sensor
The concept of pyramid wavefront sensors (PWFS) has been around about a
decade by now. However, there is still a great lack of characterizing
measurements that allow the best operation of such a system under real life
conditions at an astronomical telescope. In this article we, therefore,
investigate the behavior and robustness of the pyramid infrared wavefront
sensor PYRAMIR mounted at the 3.5 m telescope at the Calar Alto Observatory
under the influence of different error sources both intrinsic to the sensor,
and arising in the preceding optical system. The intrinsic errors include
diffraction effects on the pyramid edges and detector read out noise. The
external imperfections consist of a Gaussian profile in the intensity
distribution in the pupil plane during calibration, the effect of an optically
resolved reference source, and noncommon-path aberrations. We investigated the
effect of three differently sized reference sources on the calibration of the
PWFS. For the noncommon-path aberrations the quality of the response of the
system is quantified in terms of modal cross talk and aliasing. We investigate
the special behavior of the system regarding tip-tilt control. From our
measurements we derive the method to optimize the calibration procedure and the
setup of a PWFS adaptive optics (AO) system. We also calculate the total
wavefront error arising from aliasing, modal cross talk, measurement error, and
fitting error in order to optimize the number of calibrated modes for on-sky
operations. These measurements result in a prediction of on-sky performance for
various conditions
Shack-Hartmann wave front measurements in cortical tissue for deconvolution of large three-dimensional mosaic transmitted light brightfield micrographs
We present a novel approach for deconvolution of 3D image stacks of cortical tissue taken by mosaic/optical-sectioning technology, using a transmitted light brightfield microscope. Mosaic/optical-sectioning offers the possibility of imaging large volumes (e.g. from cortical sections) on a millimetre scale at sub-micrometre resolution. However, a blurred contribution from out-of-focus light results in an image quality that usually prohibits 3D quantitative analysis. Such quantitative analysis is only possible after deblurring by deconvolution. The resulting image quality is strongly dependent on how accurate the point spread function used for deconvolution resembles the properties of the imaging system. Since direct measurement of the true point spread function is laborious and modelled point spread functions usually deviate from measured ones, we present a method of optimizing the microscope until it meets almost ideal imaging conditions. These conditions are validated by measuring the aberration function of the microscope and tissue using a Shack-Hartmann sensor. The analysis shows that cortical tissue from rat brains embedded in Mowiol and imaged by an oil-immersion objective can be regarded as having a homogeneous index of refraction. In addition, the amount of spherical aberration that is caused by the optics or the specimen is relatively low. Consequently the image formation is simplified to refraction between the embedding and immersion medium and to 3D diffraction at the finite entrance pupil of the objective. The resulting model point spread function is applied to the image stacks by linear or iterative deconvolution algorithms. For the presented dataset of large 3D images the linear approach proves to be superior. The linear deconvolution yields a significant improvement in signal-to-noise ratio and resolution. This novel approach allows a quantitative analysis of the cortical image stacks such as the reconstruction of biocytin-stained neuronal dendrites and axons
Surface Brightness Fluctuations: A Case for Extremely Large Telescopes
The Surface Brightness Fluctuations (SBF) Method for distance determinations
of elliptical galaxies is been modeled in order to investigate the effect of
the Point Spread Function (PSF). We developed a method to simulate observations
of SBF of galaxies having various properties and located at different
distances. We will use this method in order to test the accuracy on the
estimates of the extra-galactic distances for PSFs representing typical seeing
conditions, Adaptive Optics (AO) systems and for future observations with
Extremely Large Telescopes (ELTs) close to the diffraction limit.Comment: 3 pages, 2 figures, to appear in the proceedings of the ESO Workshop
"Science with Adaptive Optics", eds. W. Brandner & M. Kasper, Springer-Verlag
200
The Impact of Adaptive Optics on Star Formation Research
In this paper, we discuss the benefits of ground-based, adaptive optics (AO)
aided observations for star formation research. After outlining the general
advantages, we present results obtained during the ALFA science demonstration
programme in 1999. These results underline the absolute necessity of AO
assistance for almost any kind of observations regarding star formation
regions.Comment: 10 pages, 8 figures (partially converted to bitmap), to appear in
SPIE Proceedings 4007, Astronomical Telescopes and Instrumentation 2000,
Munich, German
- …