280 research outputs found
Astrometric signal profile fitting for Gaia
A tool for representation of the one-dimensional astrometric signal of Gaia
is described and investigated in terms of fit discrepancy and astrometric
performance with respect to number of parameters required. The proposed basis
function is based on the aberration free response of the ideal telescope and
its derivatives, weighted by the source spectral distribution. The influence of
relative position of the detector pixel array with respect to the optical image
is analysed, as well as the variation induced by the source spectral emission.
The number of parameters required for micro-arcsec level consistency of the
reconstructed function with the detected signal is found to be 11. Some
considerations are devoted to the issue of calibration of the instrument
response representation, taking into account the relevant aspects of source
spectrum and focal plane sampling. Additional investigations and other
applications are also suggested.Comment: 13 pages, 21 figures, Accepted by MNRAS 2010 January 29. Received
2010 January 28; in original form 2009 September 3
Chromaticity in all-reflective telescopes for astrometry
Chromatic effects are usually associated with refractive optics, so
reflective telescopes are assumed to be free from them. We show that
all-reflective optics still bears significant levels of such perturbations,
which is especially critical to modern micro-arcsecond astrometric experiments.
We analyze the image formation and measurement process to derive a precise
definition of the chromatic variation of the image position, and we evaluate
the key aspects of optical design with respect to chromaticity. The fundamental
requirement related to chromaticity is the symmetry of the optical design and
of the wavefront errors. Finally, we address some optical engineering issues,
such as manufacturing and alignment, providing recommendations to minimize the
degradation that chromaticity introduces into astrometry.Comment: 10 pages, 8 figure
Performance of an Algorithm for Estimation of Flux, Background, and Location on One-dimensional Signals
Optimal estimation of signal amplitude, background level, and photocentre
location is crucial to the combined extraction of astrometric and photometric
information from focal plane images, and in particular from the one-dimensional
measurements performed by Gaia on intermediate to faint magnitude stars. Our
goal is to define a convenient maximum likelihood framework, suited to
efficient iterative implementation and to assessment of noise level, bias, and
correlation among variables. The analytical model is investigated numerically
and verified by simulation over a range of magnitude and background values. The
estimates are unbiased, with a well-understood correlation between amplitude
and background, and with a much lower correlation of either of them with
location, further alleviated in case of signal symmetry. Two versions of the
algorithm are implemented and tested against each other, respectively, for
independent and combined parameter estimation. Both are effective and provide
consistent results, but the latter is more efficient because it takes into
account the flux-background estimate correlation.Comment: 13 pages; 13 figures; to be published on PAS
Gaia on-board metrology: basic angle and best focus
The Gaia payload ensures maximum passive stability using a single material,
SiC, for most of its elements. Dedicated metrology instruments are, however,
required to carry out two functions: monitoring the basic angle and refocusing
the telescope. Two interferometers fed by the same laser are used to measure
the basic angle changes at the level of as (prad, micropixel), which is
the highest level ever achieved in space. Two Shack-Hartmann wavefront sensors,
combined with an ad-hoc analysis of the scientific data are used to define and
reach the overall best-focus. In this contribution, the systems, data analysis,
procedures and performance achieved during commissioning are presentedComment: 18 pages, 14 figures. To appear in SPIE proceedings 9143-30. Space
Telescopes and Instrumentation 2014: Optical, Infrared, and Millimeter Wav
Gaia Early Data Release 3:Modelling and calibration of Gaia's point and line spread functions
Context: The unprecedented astrometric precision of the Gaia mission relies
on accurate estimates of the locations of sources in the Gaia data stream. This
is ultimately performed by point spread function (PSF) fitting, which in turn
requires an accurate reconstruction of the PSF. Gaia Early Data Release 3
(EDR3) will, for the first time, use a PSF calibration that models several of
the strongest dependences, leading to signficantly reduced systematic errors.
Aims: We describe the PSF model and calibration pipeline implemented for Gaia
EDR3, including an analysis of the calibration results over the 34 months of
data. We include a discussion of the limitations of the current pipeline and
directions for future releases. This will be of use both to users of Gaia data
and as a reference for other precision astrometry missions. Methods: We develop
models of the 1D line spread function (LSF) and 2D PSF profiles based on a
linear combination of basis components. We fit the models to selected primary
sources in independent time ranges, using simple parameterisations for the
colour and other dependences. Variation in time is smoothed by merging the
independent calibrations in a square root information filter, with resets at
certain mission events that induce a discontinuous change in the PSF. Results:
The PSF calibration shows strong time and colour dependences that accurately
reproduce the varying state of the Gaia astrometric instrument. Analysis of the
residuals reveals both the performance and the limitations of the current
models and calibration pipeline, and indicates the directions for future
development. Conclusions: The PSF modelling and calibration carried out for
Gaia EDR3 represents a major step forwards in the data processing and will lead
to reduced systematic errors in the core mission data products. Further
significant improvements are expected in the future data releases.Comment: Accepted by A&A for inclusion in Gaia EDR3 special issu
<i>Gaia</i> Data Release 1. Summary of the astrometric, photometric, and survey properties
Context. At about 1000 days after the launch of Gaia we present the first Gaia data release, Gaia DR1, consisting of astrometry and photometry for over 1 billion sources brighter than magnitude 20.7.
Aims. A summary of Gaia DR1 is presented along with illustrations of the scientific quality of the data, followed by a discussion of the limitations due to the preliminary nature of this release.
Methods. The raw data collected by Gaia during the first 14 months of the mission have been processed by the Gaia Data Processing and Analysis Consortium (DPAC) and turned into an astrometric and photometric catalogue.
Results. Gaia DR1 consists of three components: a primary astrometric data set which contains the positions, parallaxes, and mean proper motions for about 2 million of the brightest stars in common with the HIPPARCOS and Tycho-2 catalogues – a realisation of the Tycho-Gaia Astrometric Solution (TGAS) – and a secondary astrometric data set containing the positions for an additional 1.1 billion sources. The second component is the photometric data set, consisting of mean G-band magnitudes for all sources. The G-band light curves and the characteristics of ∼3000 Cepheid and RR-Lyrae stars, observed at high cadence around the south ecliptic pole, form the third component. For the primary astrometric data set the typical uncertainty is about 0.3 mas for the positions and parallaxes, and about 1 mas yr−1 for the proper motions. A systematic component of ∼0.3 mas should be added to the parallax uncertainties. For the subset of ∼94 000 HIPPARCOS stars in the primary data set, the proper motions are much more precise at about 0.06 mas yr−1. For the secondary astrometric data set, the typical uncertainty of the positions is ∼10 mas. The median uncertainties on the mean G-band magnitudes range from the mmag level to ∼0.03 mag over the magnitude range 5 to 20.7.
Conclusions. Gaia DR1 is an important milestone ahead of the next Gaia data release, which will feature five-parameter astrometry for all sources. Extensive validation shows that Gaia DR1 represents a major advance in the mapping of the heavens and the availability of basic stellar data that underpin observational astrophysics. Nevertheless, the very preliminary nature of this first Gaia data release does lead to a number of important limitations to the data quality which should be carefully considered before drawing conclusions from the data
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