Gaia in-orbit realignment: overview and data analysis

Mora, A.; Vosteen, Amir

doi:10.1117/12.926313

Cited by 8 publications

(8 citation statements)

References 4 publications

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“…These microlenses focus the light of bright stars transiting the focal plane on a CCD. Comparison of the stellar spot pattern with the pattern of a built-in calibration source (used during initial tests after launch) and with the pattern of stars acquired after achieving best focus (used afterwards) allows reconstruction of the wave front in the form of a series of two-dimensional Legendre polynomials (Zernike polynomials are less appropriate for a rectangular pupil; Mora & Vosteen 2012). The location of the microlenses within the telescope pupils is inferred from the flux collected by the surrounding, partially-illuminated lenslets.…”

Section: Wave-front Sensormentioning

confidence: 99%

TheGaiamission

Prusti¹,

Bruijne²,

Brown³

et al. 2016

A&A

5,534

1,139

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Gaia is a cornerstone mission in the science programme of the European Space Agency (ESA). The spacecraft construction was approved in 2006, following a study in which the original interferometric concept was changed to a direct-imaging approach. Both the spacecraft and the payload were built by European industry. The involvement of the scientific community focusses on data processing for which the international Gaia Data Processing and Analysis Consortium (DPAC) was selected in 2007. Gaia was launched on 19 December 2013 and arrived at its operating point, the second Lagrange point of the Sun-Earth-Moon system, a few weeks later. The commissioning of the spacecraft and payload was completed on 19 July 2014. The nominal five-year mission started with four weeks of special, ecliptic-pole scanning and subsequently transferred into full-sky scanning mode. We recall the scientific goals of Gaia and give a description of the as-built spacecraft that is currently (mid-2016) being operated to achieve these goals. We pay special attention to the payload module, the performance of which is closely related to the scientific performance of the mission. We provide a summary of the commissioning activities and findings, followed by a description of the routine operational mode. We summarise scientific performance estimates on the basis of in-orbit operations. Several intermediate Gaia data releases are planned and the data can be retrieved from the Gaia Archive, which is available through the Gaia home page.

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Section: Wave-front Sensormentioning

confidence: 99%

TheGaiamission

Prusti¹,

Bruijne²,

Brown³

et al. 2016

A&A

5,534

1,139

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show abstract

“…The Gaia focal-plane assembly (e.g., Kohley et al 2012), with 106 CCD detectors, has five dedicated functions: 4 CCDs for metrology, i.e., basic-angle monitoring and wave-front sensing (Gielesen et al 2012;Mora & Vosteen 2012), 14 Sky Mapper (SM) CCDs for object detection and rejection of prompt-particle events, 62 Astrometric Field (AF) CCDs, 14 Blue-Photometer/Red-Photometer (BP/RP) CCDs for low-resolution spectro-photometry, and 12 Radial-VelocitySpectrograph (RVS) CCDs for radial velocities and mediumresolution spectra. The AF, BP/RP, and RVS CCDs see the superimposed light coming from the two telescopes, which look at the sky separated by a basic angle of 106.5 deg along the scan direction.…”

Section: Introductionmentioning

confidence: 99%

Detecting stars, galaxies, and asteroids withGaia

Bruijne

Allen

Azaz

et al. 2015

A&A

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Context. Gaia is Europe's space astrometry mission, aiming to make a three-dimensional map of 1000 million stars in our Milky Way to unravel its kinematical, dynamical, and chemical structure and evolution.Aims. We present a study of Gaia's detection capability of objects, in particular non-saturated stars, double stars, unresolved external galaxies, and asteroids. Gaia's on-board detection software autonomously discriminates stars from spurious objects like cosmic rays and solar protons. For this, parametrised criteria of the shape of the point spread function are used, which need to be calibrated and tuned. This study aims to provide an optimum set of parameters for these filters. Methods. We developed a validated emulation of the on-board detection software, which has 20 free, so-called rejection parameters which govern the boundaries between stars on the one hand and sharp (high-frequency) or extended (low-frequency) events on the other hand. We evaluate the detection and rejection performance of the algorithm using catalogues of simulated single stars, resolved and unresolved double stars, cosmic rays, solar protons, unresolved external galaxies, and asteroids. Results. We optimised the rejection parameters, improving -with respect to the functional baseline -the detection performance of single stars and of unresolved and resolved double stars, while, at the same time, improving the rejection performance of cosmic rays and of solar protons. The optimised rejection parameters also remove the artefact of the functional-baseline parameters that the reduction of the detection probability of stars as a function of magnitude already sets in before the nominal faint-end threshold at G = 20 mag. We find, as a result of the rectangular pixel size, that the minimum separation to resolve a close, equal-brightness double star is 0.23 arcsec in the along-scan and 0.70 arcsec in the across-scan direction, independent of the brightness of the primary. To resolve double stars with ∆G > 0 mag, larger separations are required. We find that, whereas the optimised rejection parameters have no significant impact on the detectability of pure de Vaucouleurs profiles, they do significantly improve the detection of pure exponential-disk profiles, and hence also the detection of unresolved external galaxies with intermediate profiles. We also find that the optimised rejection parameters provide detection gains for asteroids fainter than 20 mag and for fast-moving near-Earth objects fainter than 18 mag, although this gain comes at the expense of a modest detection-probability loss for bright, fast-moving near-Earth objects. The major side effect of the optimised parameters is that spurious ghosts in the wings of bright stars essentially pass unfiltered.

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“…A five-degrees-of-freedom mechanism is implemented behind the secondary mirrors of the two telescopes for re-aligning the telescopes in orbit to cancel errors due to mirror micro-settings and gravity release. Re-alignment will be carried out during in-orbit commissioning using the feedback from measurements of two Shack-Hartmann-type wave-front sensors (WFS) at different positions in the focal plane [6]. Figure 4 shows the position of the WFS assemblies mounted in front of the dedicated WFS CCDs in the FPA.…”

Section: Payload Modulementioning

confidence: 99%

Gaia's FPA: sampling the sky in silicon

Kohley

Garé

Vetel³

et al. 2012

Space Telescopes and Instrumentation 2012: Optical, Infrared, and Millimeter Wave

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ESA´s astrometry satellite Gaia is scheduled for launch in 2013. In a combination of outstanding hardware performance, autonomous object detection and sophisticated data processing, Gaia will chart more than a billion stars of the entire sky to unprecedented accuracy during its 5 years mission. A key element to its mission success is the focal plane assembly (FPA), the largest ever flown to space, comprising a close-butted almost Giga-pixel mosaic of 106 large area CCDs. Manufacturing and extensive testing of the individual devices and detector system units as well as integration on the single-piece, silicon-carbide support structure has been a challenge. The focal plane is now assembled and has undergone its final tests during 2012.The paper summarizes the expected in-flight performances of Gaia´s FPA and the implemented tools and procedures to monitor its operation in space. Accurate knowledge of the impact of FPA performance parameters on individual measurements and its evolution in time is critical to achieve the high accuracy needed in calibrating the science data. An example is the radiation-induced deterioration of the CCD charge transfer efficiency, which acts on distorting the detected object PSFs while observing the sky in continuous scan mode. Through dedicated calibration procedures and directly through the scientific data processing, Gaia will therefore closely track the radiation environment at L2 from the FPA output itself. Detection of transient effects and analysis of persistent damage on the CCDs mainly caused by solar protons converts Gaia's FPA inherently into the largest ever radiation monitor in space.

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Gaia in-orbit realignment: overview and data analysis

Cited by 8 publications

References 4 publications

TheGaiamission

TheGaiamission

Detecting stars, galaxies, and asteroids withGaia

Gaia's FPA: sampling the sky in silicon

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