PRIMA/PACMAN is scheduled for commissioning on Paranal in late 2008 as part of the VLTI. In this paper, we
discuss the important aspects of its astrometric data-reduction software. For example, the top-level requirements,
interfaces to existing ESO software, data types, data levels, and data flow among the recipes dictate the overall
design of any software package. In addition, the complexity of the PACMAN instrument, the long-term nature
of astrometric observations, and the need to improve algorithms as the understanding of the hardware improves,
impose additional requirements on the astrometric data-reduction software.
The PRIMA (Phase-Referenced Imaging and Microarcsecond Astrometry) instrument at ESO/VLTI is scheduled
for commissioning in late 2008. It is designed for phased-referenced imaging and narrow-angle astrometry. The
latter, which is the focus of this paper, may be used for exoplanet detection.
A key PRIMA subsystem consists of two fringe sensing units. They employ polarized and dispersive optics
to measure cross fluxes and differential phases in five narrow K band channels without the need of delay-line
dithering. The differential phases are used to correct the differential delays, which are the primary observables
used to determine relative proper motions, relative parallaxes, and planetary orbits. Real optical components
are imperfect, which means that systematics will appear in the differential phases.
In this paper, we 1) present a closed mathematical form for the differential phase, including small systematic
offsets and random errors; 2) perform Monte Carlo simulations to understand how the small systematic offsets
and random errors affect the differential phases; and 3) show that delay-line stepping can be used to eliminate
the effects of small systematic offsets and random errors.
Previous experimental measurements and theoretical modelling indicate that atmospheric turbulence is expected
to show intermittency (or "clumpiness"). The impact that this intermittency has on simulated adaptive
optics (AO) and Lucky Imaging (LI) performance is assessed in this SPIE contribution using simulated phase
screens with Von Karman power spectra which incorporate turbulent intermittency. The statistics of the intermittency
model used are based as closely as possible on astronomical seeing measurements at real observatories.
The performance of realistic AO correction of large Taylor screens with intermittent turbulence is compared
directly with the performance given with traditional Gaussian random Taylor screens having the same Von
Karman power spectrum. Also discussed is the improvement in performance which can be obtained by modifying
the AO control parameters in response to the changing seeing conditions. Lucky Imaging simulations
indicate that the performance of this method can be significantly improved under intermittent seeing.
The fundamental task of AO system calibration is the acquisition of the Interaction Matrix (IM). This task is usually performed in a laboratory or at the telescope using a reference fiber illuminating both deformable mirror and wavefront sensor. The problem of measuring the IM on a bright reference star has been attacked by some authors. The principal problem of this measurement is to achieve a high SNR when atmospheric turbulence is present. This is very difficult if sensor signals are simply time averaged to get rid of the turbulence effects. The paper presents a new technique to perform an on sky measurement of the IM with high SNR and reducing the overall measurement time by an order of magnitude. This technique can be very useful for AO systems using large size DMs like MMT, LBT and possibly VLT and OWL. In these cases fiber-based IM measurements require challenging optical set-up that in some cases, like for OWL, are unpractical to build. The technique is still relevant for classical small DM AO systems that could be calibrated on sky avoiding misregistration errors. Finally this technique is valuable for laboratory measurements when the IM of an AO system has to be measured with great accuracy against external disturbances like bench vibrations, local turbulence effects and so on. Again IM measurement SNR is increased and the overall measurement time can be significantly reduced. The paper will introduce and detail the technique physical principle and quantify with numerical simulations the SNR improvement achieved using this technique. Finally laboratory results obtained during the test of the LBT AO system prototype are given and compared to simulations.
We present VLTI-MIDI (the Mid-Infrared Interferometric Instrument at ESO's Very Large Telescope Interferometer)
observations of MWC349 A, which are a prime example of the power of combined spatial and spectral
resolution for addressing complex astrophysical phenomena.
Previous observations of the peculiar emission line star MWC349A suggest that it is a young massive star in
the short-lived phase of already having dissipated its parent cloud, but still being surrounded by the accretion
disk, which is seen nearly edge-on. It is believed that the unique hydrogen recombination line maser / laser
activity of MWC349A from mm to infrared wavelengths is also a consequence of this viewing geometry.
We have taken 13 measurements with MIDI at the VLTI (Very Large Telescope Interferometer) in the GRISM
mode covering the N band (8 to 13 microns) at a spectral resolution R ≈ 230. The wavelength dependence of
the continuum visibility agrees with model calculations for circumstellar dust disks. In addition, the signatures
of at least a dozen emission lines have been identified in the interferometric data.
We present a first analysis of visibility amplitudes and differential phase data. In particular we show that a
simple model can represent the SED and visibility amplitude of the continuum flux. Also the visibilities for the
hydrogen and forbidden lines are discussed.
The PRIMA facility will implement dual-star astrometry at the VLTI. We have formed a consortium that will build the PRIMA differential delay lines, develop an astrometric operation and calibration plan, and deliver astrometric data reduction software. This will enable astrometric planet surveys with a target precision of 10μas. Our scientific goals include determining orbital inclinations and masses for planets already known from radial-velocity surveys, searches for planets around stars that are not amenable to high-precision radial-velocity observations, and a search for large rocky planets around
nearby low-mass stars.
Knowledge of the dispersion due to (humid) air in the light path of the Very Large Telescope Interferometer (VLTI) is crucial to obtaining good science data from MIDI, PRIMA and GENIE. To calculate the refraction due to air at infra red wavelengths in the ducts and delay line tunnel, the temperature and humidity has to be monitored during observations. To accomplish these measurements an easy to use and reliable system was assembled, based on commercially available components. In-house calibration of four humidity and temperature sensors of the system was done in Leiden. A test and calibration program was carried out to make sure that they work reliably and accurately and to determine the sensor characteristics. For this purpose a calibration box was designed which isolates the sensors from the environment so that there is no exchange of air with the outside environment. Using constant humidity salt solutions, the humidity in the box can be controlled. This allows the calibrations to be carried out for typical values of relative humidity and temperature at Cerro Paranal. Calibration of the sensors includes: 1. Reducing the systematic relative humidity differences between the sensors to less than 0.1 % and 2. Reducing the systematic temperature differences between the sensors to less than 0.01 K. In this paper we will present the outcome of the calibrations and the future of the sensors at Paranal.
This investigation focuses on observational measurements of the differential interferometric phase between spectral channels in the VLTI/MIDI instrument. Measurements of target stars are compared with theoretical predictions in order to investigate the effects of dispersion in humid air on differential phase measurements at N band (~10 micron wavelength). An accuracy of 1 degree RMS phase error is achieved after calibration during stable environmental conditions, but this accuracy is degraded if there are fluctuations in humidity between observations. Stabilisation and/or monitoring of the environmental conditions in the VLTI ducts and tunnels will be required in order to achieve the best differential phase performance with MIDI. The measured differential phases are found to be consistent with a model for the refractive index of air based on the HITRAN database.
We discuss methods of interferometric data reduction using coherent integration of fringe visibility. Unlike incoherent estimation techniques which discard the phase of interference, coherent integration retains as a complex quantity the contribution from each frame (or scan). In order to integrate these coherently, one must apply an OPD correction (or "phase reference") to compensate for random atmospheric pathlength fluctuations.
In an instrument with substantial bandwidth, it is also necessary to correct for fixed and random dispersion. The integrity of phase functions obtained is dependent on correct modelling of fixed optical phase functions (obtained from a calibrator observation), dispersion from air filled delay-lines (calibratable in principle), and averaging over time to reduce the effect of random atmospheric water vapor dispersion.
To achieve the best performance, it is necessary to include a dispersion tracker as well as tracking achromatic OPD, applying each as a phase correction as a function of time and of optical frequency.
Using MIDI, the N band instrument of the VLTI, which has a wide bandwidth, it is often possible to uninterruptedly track random dispersion fluctuations over an observation. Plots of dispersion fluctuations due to water vapor above the VLTI are shown, which are used (along with OPD tracking) to coherently integrate raw frames from that instrument.
The resulting complex visibility includes a unique phase delay signature reflecting the source structure. A residual "water-vapor-like" phase may be present due to unmonitored humidity in the delay line paths, and to incomplete averaging of (nominally zero-mean) atmospheric water vapor fluctuations. Nevertheless, the use of visibility phase results corrupted by random dispersion is possible.
Numerical simulations of interferometers with spatial filters and
large apertures are presented. These simulations are particularly
aimed at measuring the fringe (or OPD) motion caused by
atmospherically induced wavefront corrugations (high order Zernike
modes) across the telescope apertures. This component of the fringe
motion results from the coupling in a spatial filter of high-order
Zernike modes to the fringe motion.
The Piston Mode component is found to be an inadequate
approximation to the OPD in an interferometer with large apertures,
spatial filters and tip-tilt correction. The fringe motion at high
temporal frequencies is dominated by the effects of wavefront
corrugations across the telescope apertures.
A search for extrasolar planets using the ESO VLTI PRIMA facility
will become feasible in 2007. An astrometric accuracy of 10 micro-arcseconds will allow us to detect sub-Uranus mass planets around the
most nearby stars, as well as to conduct a planet search around stars of different ages. Most of the PRIMA hardware subsystems are currently being developed by industry. At the same time a scientific Consortium has formed that will deliver the differential delay lines and astrometric software for PRIMA to ESO.
In this paper we describe the planned efforts by the Consortium
related to the "PRIMA astrometry operations and software". These
activities include an overall "PRIMA astrometry error budget", a
"PRIMA astrometry calibration and observation strategy", the "PRIMA astrometry observation preparation tools" and the "PRIMA astrometry data reduction tools". We describe how all these components fit together in an overall approach to the flow of knowledge within the project. First by quantifying the fundamental limits of the VLTI infrastructure and the astronomical sources under study. Followed by elimination or suppression of the errors through either a hardware change to the system, software control of the system, or a proper calibration and observation strategy.
The ultimate goal is being able to calibrate all PRIMA astrometric data acquired over the full lifetime of PRIMA (5 to 10 years) to a uniform accuracy of 10 micro-arcseconds. This will allow identification of long-term trends in the astrometric parameters due to planetary companions around nearby stars and to determine the distances and proper motions for the selected sources.
With ESO's Phase Referenced Imaging and Micro-arcsecond Astrometry (PRIMA) facility well into its procurement phase expectations are made about the astrometric performance. It appears that in almost all respects the instrumentally induced errors are expected to have Power Spectral densities well below those due to the atmosphere.
However for the target performance to be achieved, some effects must reduce by averaging by some 4 orders of magnitude. The most serious worry foreseen is lack of thermal stability in the air-filled Delay Line tunnel, and it is recommended that outside wind influence be impeded.
We present a summary of the activity of the Cambridge Optical Aperture
Synthesis Telescope (COAST) team and review progress on the
astronomical and technical projects we have been working on in the
period 2002--2004. Our current focus has now moved from operating
COAST as an astronomical instrument towards its use as a test-bed for
strategic technical development for future facility arrays. We have
continued to develop a collaboration with the Magdalena Ridge
Observatory Interferometer, and we summarise the programmes we expect
to be working on over the next few years for that ambitious
project. In parallel, we are investigating a number of areas for the
European Very Large Telescope Interferometer and these are outlined
The first-generation COAST array is now primarily operated as a tool
for astrophysics, with any development work aimed at improving
observing efficiency and at prototyping hardware for future arrays. In this paper we summarize the full range of astrophysical results
obtained with COAST in the previous two years. Results of a
program to investigate hotspots on red supergiant stars are
presented in detail.
We present a summary of the status of the Cambridge Optical Aperture
Synthesis Telescope, and review developments at the array through the
period 2000-2002. Summaries of the astronomical and technical
programmes completed, together with an outline of those that are
currently in progress are presented. Since our last report two years
ago in 2000, there have been significant changes in the context for
astronomical interferometry in the UK. We review these developments,
and describe our plans for the near and intermediate term at COAST,
and with colleagues in Europe at the VLTI and in the USA at the
Magdalena Ridge Observatory in New Mexico.
The next generation of optical interferometer arrays will require a large number of unit telescopes in the same manner as the VLA if meaningful scientific objectives are to be achieved. Studies based on the five element COAST array show that something like ten to fifteen telescopes are necessary. For such a project to be viable the unit telescopes must be designed from the outset for this task. The basic criteria are as follows: The wavefront quality and stability should be excellent, high optical throughput, autonomous automatic operation, couple efficiently into the beam transport and combination system, plus maintain acceptable unit cost. To achieve these goals a number of novel designs were considered and are described in this paper. Two of the most suitable designs and which had the least technological risk were studied in more detail by Telescope Technology Ltd. and are described in a separate paper.
The use of faint reference stars for the selection of good short exposure images has recently been demonstrated as a technique which can provide essentially diffraction-limited I band imaging from well-figured ground-based telescopes as large as 2.5 m diameter. The faint limiting magnitude and enhanced isoplanatic patch size for the selected exposures technique means that 20% of the night sky is within range of a suitable reference star for I-band imaging. Typically the 1%-10% of exposures with the highest Strehl ratios are selected. When these exposures are shifted and added together, field stars in the resulting images have Strehl ratios as high as 0.26 and FWHM as small as 90 milliarcseconds. Within the selected exposures the isoplanatic patch is found to be up to 50 arcseconds in diameter at 810 nm wavelength. Images within globular clusters and of multiple stars from the Nordic Optical Telescope using reference stars as faint as I~16 are presented. The technique relies on a new generation of CCDs which provide sub-electron readout noise at very fast readout rates. The performance of the selection technique for various astronomical programs is discussed in comparison with natural guide star Adaptive Optics (AO).
This paper is intended to discuss the impact of noiseless CCD detectors in three significant areas of the development of large telescopes and the instruments that go on them. These are (1) CCDs that have all the characteristics that we are used to seeing in CCDs can now be made with negligible readout noise even at higher pixel rates (> 10 MHz) and will allow rather different approaches to the design of instrument is generally, (2) the technique of achieving diffraction limited imaging in ground-based telescopes known as Lucky Astronomy in which images are taken at high speed has been demonstrated to work under a variety of different conditions and (3) some suggestions as to how these methods may be applied directly to much larger diameter telescopes in order to achieve high resolution imaging and spectroscopy without the expense of laser guide stars or multi-conjugate adaptive optics.
A radically new CCD development by Marconi Applied Technology has enabled substantial internal gain within the CCD before the signal reaches the output amplifier. With reasonably high gain, sub-electron readout noise levels are achieved even at MHz pixel rates. This paper reports a detailed assessment of these devices, including novel methods of measuring their properties when operated at peak mean signal levels well below one electron per pixel. The devices are shown to be photon shot noise limited at essentially all light levels below saturation. Even at the lowest signal levels the charge transfer efficiency is good. The conclusion is that these new deices have radically changed the balance in the perpetual trade-off between read out noise and the speed of readout. They will force a re- evaluation of camera technologies and imaging strategies to enable the maximum benefit to be gained form these high- speed, essentially noiseless readout devices. This new LLLCCD technology, in conjunction with thinning should provide detectors which will be very close indeed to being theoretically perfect.