Abstract

Abstract
Context. FU Orionis objects are a class of young stars with powerful bursts in luminosity that show evidence of accretion and ejection activity. It is generally accepted that they are surrounded by a Keplerian circumstellar disk and an infalling envelope. The outburst occurs because of a sudden increase in the accretion rate. Aims. We study the regions closer to the central star in order to observe the signs of the accretion and ejection activity. Methods. We present optical observations of the Ha line using the Integral Field Spectrograph OASIS, at the William Herschel Telescope, combined with adaptive optics. Since this technique gives the spectral information for both spatial directions, we carried out a two-dimensional spectro-astrometric study of the signal. Results. We measured a clear spectro-astrometric signal in the north-south direction. The cross-correlation between the spectra showed a spatial distribution in velocity suggestive of scattering by a disk surrounding the star. This would be one of the few spatial inferences of a disk observed in an FU Orionis object. However, to fully understand the observed structure, higher angular and spectral resolution observations are required. V1515 Cyg now appears to be an important object to be observed with a new generation of instruments to increase our knowledge about the disk and outflow structure in FU Orionis objects.

Abstract
The GRAVITY Acquisition Camera was designed to monitor and evaluate the optical beam properties of the four ESO/VLT telescopes simultaneously. The data is used as part of the GRAVITY beam stabilization strategy. Internally the Acquisition Camera has four channels each with: several relay mirrors, imaging lens, H-band filter, a single custom made silica bulk optics (i.e. Beam Analyzer) and an IR detector (HAWAII2-RG). The camera operates in vacuum with operational temperature of: 240k for the folding optics and enclosure, 100K for the Beam Analyzer optics and 80K for the detector. The beam analysis is carried out by the Beam Analyzer, which is a compact assembly of fused silica prisms and lenses that are glued together into a single optical block. The beam analyzer handles the four telescope beams and splits the light from the field mode into the pupil imager, the aberration sensor and the pupil tracker modes. The complex optical alignment and focusing was carried out first at room temperature with visible light, using an optical theodolite/alignment telescope, cross hairs, beam splitter mirrors and optical path compensator. The alignment was validated at cryogenic temperatures. High Strehl ratios were achieved at the first cooldown. In the paper we present the Acquisition Camera as manufactured, focusing key sub-systems and key technical challenges, the room temperature (with visible light) alignment and first IR images acquired in cryogenic operation.

Abstract
The GRAVITY acquisition camera has four 9x9 Shack-Hartmann sensors operating in the near-infrared. It measures the slow variations of a quasi-distorted wavefront of four telescope beams simultaneously, by imaging the Galactic Center field. The Shack-Hartmann lenslet images of the Galactic Center are generated. Since the lenslet array images are filled with the crowded Galactic Center stellar field, an extended object, the local shifts of the distorted wavefront have to be estimated with a correlation algorithm. In this paper we report on the accuracy of six existing centroid algorithms for the Galactic Center stellar field. We show the VLTI tunnel atmospheric turbulence phases are reconstructed back with a precision of 100 nm at 2 s integration.

Abstract
The acquisition camera for the GRAVITY/VLTI instrument implements four functions: a) field imager: science field imaging, tip-tilt; b) pupil tracker: telescope pupil lateral and longitudinal positions; c) pupil imager: telescope pupil imaging and d) aberration sensor: The VLTI beam higher order aberrations measurement. We present the dedicated algorithms that simulate the GRAVITY acquisition camera detector measurements considering the realistic imaging conditions, complemented by the pipeline used to extract the data. The data reduction procedure was tested with real aberrations at the VLTI lab and reconstructed back accurately. The acquisition camera software undertakes the measurements simultaneously for all four AT/UTs in 1 s. The measured parameters are updated in the instrument online database. The data reduction software uses the ESO Common Library for Image Processing (CLIP), integrated in to the ESO VLT software environment.

Abstract
The so-called "phase delay tracking" attempts to estimate the effects of the turbulence on the phase of the interferograms in order to numerically cophase the measured complex visibilities and to coherently integrate them. This is implemented by the "coherent fringe analysis" of MIDI instrument1 but has only been used for high SNR data. In this paper, we investigate whether the sensitivity of this technique can be pushed to its theoretical limits and thus applied to fainter sources. In the general framework of the maximum likelihood and exploiting the chromatic behavior of the turbulence effects, we propose a global optimization strategy to compute various estimators of the differential pistons between two data frames. The most efficient estimators appear to be the ones based on the phasors, even though they do not yet reach the theoretical limits.