Ground-based astrometry with wide field imagers

Libralato, Mattia; Bellini, Andrea; Bedin, Luigi R.; Piotto, G.; Platais, I.; Kissler-Patig, M.; Milone, A. P.

doi:10.1051/0004-6361/201322059

Cited by 50 publications

(75 citation statements)

References 33 publications

(35 reference statements)

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“…A successful approach to model GDs has been proposed for the first time by Anderson & King (2003), who found a distortion solution for the HST Wide Field Planetary Camera 2. In recent years, other similar works aimed at measuring the GD of the ACS and the Wide Field Camera 3 Bellini et al 2011Bellini et al , 2014 onboard HST or of ground-based imagers such as LBT/LBC, ESO/WFI, VLT/HAWK-I, VISTA/VIRCAM (Anderson 2006;Yadav et al 2008;Bellini et al , 2010Libralato et al 2014Libralato et al , 2015, have been published.…”

Section: Gems/gsaoi Astrometric Performancementioning

confidence: 99%

GeMS/GSAOI PHOTOMETRIC AND ASTROMETRIC PERFORMANCE IN DENSE STELLAR FIELDS

Dalessandro

Saracino

Origlia

et al. 2016

ApJ

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Ground-based imagers at 8m class telescopes assisted by Multi conjugate Adaptive Optics are primary facilities to obtain accurate photometry and proper motions in dense stellar fields. We observed the central region of the globular clusters Liller 1 and NGC 6624 with the Gemini Multi-conjugate adaptive optics System (GeMS) feeding the Gemini South Adaptive Optics Imager (GSAOI) currently available at the Gemini South telescope, under different observing conditions. We characterized the stellar Point Spread Function (PSF) in terms of Full Width at Half Maximum (FWHM), Strehl Ratio (SR) and Encircled Energy (EE), over the field of view. We found that, for sub-arcsec seeing at the observed airmass, diffraction limit PSF FWHM (≈ 80 mas), SR ∼ 40% and EE ≥ 50% with a dispersion around 10% over the 85 ×85 field of view, can be obtained in the K s band. In the J band the best images provide FWHMs between 60 and 80 mas, SR > 10% and EE > 40%. For seeing at the observed airmass exceeding 1 , the performance worsen but it is still possible to perform PSF fitting photometry with 25% EE in J and 40% in K s . We also computed the geometric distortions of GeMS/GSAOI and we obtained corrected images with an astrometric accuracy of ∼1 mas in a stellar field with high crowding.

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Section: Gems/gsaoi Astrometric Performancementioning

confidence: 99%

GeMS/GSAOI PHOTOMETRIC AND ASTROMETRIC PERFORMANCE IN DENSE STELLAR FIELDS

Dalessandro

Saracino

Origlia

et al. 2016

ApJ

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“…A main asset of our reduction process was the derivation of an ad-hoc GD correction for the LBC-red in the r Sloan -filter, starting from the recipes described in and improving the polynomial solution that follows prescriptions provided in Libralato et al (2014). Here, we briefly outline the key points of the reduction process while referring to the aforementioned papers for more details.…”

Section: Data Reductionmentioning

confidence: 99%

“…Figure 4 shows the σ(Positional residuals), computed as in Libralato et al (2014) after each step of our correction. In the bottom panel, we show the σ(Positional residuals) for a master frame derived using general 6-parameter linear transformations.…”

Section: R Sloan -Filter Geometric Distortion Solution For the Lbc-redmentioning

confidence: 99%

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A bag of tricks: Using proper motions of Galactic stars to identify the Hercules ultra-faint dwarf galaxy members

Fabrizio

Raimondo

Brocato

et al. 2014

A&A

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Context. Discovered in the last decade as overdensities of resolved stars, the ultra-faint dwarfs (UFDs) are among the least luminous, most dark-matter dominated, and most metal-poor galaxies known today. They appear as sparse, loose objects with high mass-to-light ratios. Hercules is the prototype of the UFD galaxies. To date, there are still no firm constraints on its total luminosity due to the difficulty of disentangling Hercules bona-fide stars from the severe Galactic field contamination. Aims. To better constrain Hercules properties, we aim at removing foreground and background contaminants in the galaxy field using the proper motions of the Milky Way stars and the colour-colour diagram. Methods. We have obtained images of Hercules in the r Sloan , B Bessel and U spec bands with the Large Binocular Telescope (LBT) and LBC-BIN mode capabilities. The r Sloan new dataset combined with data from the LBT archive span a time baseline of about 5 yr, allowing us to measure proper motions of stars in the Hercules direction for the first time. The U spec data along with existing LBT photometry allowed us to use colour-colour diagram to further remove the field contamination.Results. Thanks to a highly-accurate procedure to derive the r Sloan -filter geometric distortion solution for the LBC-red, we were able to measure stellar relative proper motions to a precision of better than 5 mas yr −1 down to r Sloan 22 mag and disentangle a significant fraction (>90%) of Milky Way contaminants. We ended up with a sample of 528 sources distributed over a large portion of the galaxy body (∼0.12 deg 2 ). Of these sources, 171 turned out to be background galaxies and additional foreground stars from the analysis of the U spec − B Bessel vs. B Bessel − r Sloan colour-colour diagram. This leaves us with a sample of 357 likely members of the Hercules UFD. We compared the cleaned colour-magnitude diagram (CMD) with evolutionary models and synthetic CMDs, confirming the presence in Hercules of an old population (t = 12 ± 2 Gyr) with wide spread metallicity (−3.3 < [Fe/H] < −1.8). Conclusions. Our procedure to estimate star proper motions proved to be a very effective way to identify likely members for stellar systems as far as 130 kpc. The present selection, based on both proper motions and colour-colour diagram, provides a robust identification of Hercules members and a new target list for more spectroscopic investigations.

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“…The approach has been employed with HST (Anderson & King 2003;Bellini et al 2011) and ground-based imagers (Libralato et al 2014), with astrometric precisions after correction of the order of 0.1 -1 mas. This is at least an order of magnitude larger than what we seek to achieve for exoplanet detection, but the design and observation strategy of WFIRST should enable higher precision because of the thermally stable environment in L2 orbit and the abundance of calibration products from the microlensing survey.…”

Section: Optical Distortion Correctionmentioning

confidence: 99%

In the Crosshair: Astrometric Exoplanet Detection with WFIRST's Diffraction Spikes

Spergel

Lanz

2018

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WFIRST will conduct a coronagraphic program of characterizing the atmospheres of planets around bright nearby stars. When observed with the WFIRST Wide Field Camera, these stars will saturate the detector and produce very strong diffraction spikes. In this paper, we forecast the astrometric precision that WFIRST can achieve by centering on the diffraction spikes of highly saturated stars. This measurement principle is strongly facilitated by the WFIRST H4RG detectors, which confine excess charges within the potential well of saturated pixels. By adopting a simplified analytical model of the diffraction spike caused by a single support strut obscuring the telescope aperture, integrated over the WFIRST pixel size, we predict the performance of this approach with the Fisher-matrix formalism. We discuss the validity of the model and find that 10 µas astrometric precision is achievable with a single 100 s exposure of a R AB = 6 or a J AB = 5 star. We discuss observational limitations from the optical distortion correction and pixel-level artifacts, which need to be calibrated at the level of 10 -20 µas so as to not dominate the error budget. To suppress those systematics, we suggest a series of short exposures, dithered by at least several hundred pixels, to reach an effective per-visit astrometric precision of better than 10 µas. If this can be achieved, a dedicated WFIRST GO program will be able to detect Earth-mass exoplanets with orbital periods of 1 yr around stars within a few pc as well as Neptunelike planets with shorter periods or around more massive or distant stars. Such a program will also enable mass measurements of many anticipated direct-imaging exoplanet targets of the WFIRST coronagraph and a "starshade" occulter.

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Ground-based astrometry with wide field imagers

Cited by 50 publications

References 33 publications

GeMS/GSAOI PHOTOMETRIC AND ASTROMETRIC PERFORMANCE IN DENSE STELLAR FIELDS

GeMS/GSAOI PHOTOMETRIC AND ASTROMETRIC PERFORMANCE IN DENSE STELLAR FIELDS

A bag of tricks: Using proper motions of Galactic stars to identify the Hercules ultra-faint dwarf galaxy members

In the Crosshair: Astrometric Exoplanet Detection with WFIRST's Diffraction Spikes

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