Constanza Araujo-Hauck

GRAVITY: a four-telescope beam combiner instrument for the VLTI

GRAVITY is an adaptive optics assisted Beam Combiner for the second generation VLTI instrumentation. The instrument will provide high-precision narrow-angle astrometry and phase-referenced interferometric imaging in the astronomical K-band for faint objects. We describe the wide range of science that will be tackled with this instrument, highlighting the unique capabilities of the VLTI in combination with GRAVITY. The most prominent goal is to observe highly relativistic motions of matter close to the event horizon of Sgr A*, the massive black hole at center of the Milky Way. We present the preliminary design that fulfils the requirements that follow from the key science drivers: It includes an integrated optics, 4-telescope, dual feed beam combiner operated in a cryogenic vessel; near-infrared wavefrontsensing adaptive optics; fringe-tracking on secondary sources within the field of view of the VLTI and a novel metrology concept. Simulations show that 10 μas astrometry within few minutes is feasible for a source with a magnitude of mK = 15 like Sgr A*, given the availability of suitable phase reference sources (mK = 10). Using the same setup, imaging of mK = 18 stellar sources in the interferometric field of view is possible, assuming a full night of observations and the corresponding UV coverage of the VLTI.

GRAVITY Spectrometer Metrology laser blocking strategy at OD=12

A two stage blocking system is implemented in the GRAVITY science and the fringe tracking spectrometer optical design. The blocking system consists of a dichroic mirror and a long wave band-pass filter with the top level requirements of high transmission of the science light in the K-Band (1.95 - 2.5 μm) region and high blocking power optical density (OD) ≥ 8 for the metrology laser wavelength at 1.908 μm. The laser metrology blocking filters have been identified as one critical optical component in the GRAVITY science and fringe tracker spectrometer design. During the Phase-B study of GRAVITY we procured 3 blocking filter test samples for demonstration and qualification tests. We present the measurements results of an effective blocking of the metrology laser wavelength with a long wave band-pass filter at OD=12.

Optimizing the transmission of the GRAVITY/VLTI near-infrared wavefront sensor

The GRAVITY instrument’s adaptive optics system consists of a novel cryogenic near-infrared wavefront sensor to be installed at each of the four unit telescopes of the VLT. Feeding the GRAVITY wavefront sensor with light in the 1.4 - 2.4 micrometer band, while suppressing laser light originating from the GRAVITY metrology system, custom-built optical components are required. Here we report on optical and near-infrared testing of the silicon entrance windows of the wavefront sensor cryostat and other reflective optics used in the warm feeding optics.

The GRAVITY spectrometers: optical design and principle of operation

Operating on 6 interferometric baselines, i.e. using all 4 unit telescopes (UTs) of the Very Large Telescope Interferometer (VLTI) simultaneously, the 2nd generation VLTI instrument GRAVITY will deliver narrow-angle astrometry with 10μas accuracy at the infrared K-band. At this angular resolution, GRAVITY will be able to detect the positional shift of the photo-center of a flare at the Galactic Center within its orbital timescale of about 20 minutes, using the observed motion of the flares as dynamical probes of the gravitational field around the supermassive black hole Sgr A*. Within the international GRAVITY consortium, the 1. Physikalische Institut of the University of Cologne is responsible for the development and construction of the two spectrometers of the camera system: one for the science object, and one for the fringe tracking object. In this paper we present the phase-B optical design of the two spectrometers as it got derived from the scientific and technical requirements and as it passed the preliminary design review (PDR) at the European Southern Observatory (ESO) successfully in late 2009.

Signatures of strong gravity with GRAVITY

The dynamics of stars and gas undoubtedly shows the existence of a 4 million solar mass black hole at the center of the Milky Way: Sagittarius A* (SgrA*). Violent flare emission allows us to probe the immediate environment of the central mass. Near-infrared polarimetry now shows signatures of strong gravity that are statistically significant against randomly polarized red noise. Using these signatures we can derive spin and inclination information of SgrA*. A combined synchrotron self Compton (SSC) and adiabatic expansion model with source components peaking in the sub-mm domain can fully account for the observed flare flux densities and the time delays towards the (sub-)mm flares that have been reported in some cases. We discuss the expected centroid paths of the NIR images and summarize how the geometrical structure of the emitting region (i.e. spot shape, presence of a torus or spiral-arm pattern etc.) affects this centroid tracks. While most of the mentioned geometries are able to fit the observed fluxes, future NIR interferometry with GRAVITY at the VLT will break some of the degeneracies between different emission models. In this contribution we summarize several GRAVITY science cases for SgrA*. Our simulations propose that focusing GRAVITY observations on the polarimetry mode could reveal a clear centroid track of the spot(s). A non-detection of centroid shifts cannot rule out the multi-component model or spiral arms scenarios. However, a clear wander between alternating centroid positions during the flares will prove the idea of bright long-lived spots occasionally orbiting the central black hole.

LSST mirror system status: from design to fabrication and integration

In the construction phase since 2014, the Large Synoptic Survey Telescope (LSST) is an 8.4 meter diameter wide-field (3.5 degrees) survey telescope located on the summit of Cerro Pachón in Chile. The reflective telescope uses an 8.4 m f/1.06 concave primary, an annular 3.4 m meniscus convex aspheric secondary and a 5.2 m concave tertiary. The primary and tertiary mirrors are aspheric surfaces figured from a monolithic substrate and referred to as the M1M3 mirror. This unique design offers significant advantages in the reduction of degrees of freedom, improved structural stiffness for the otherwise annular surfaces, and enables a very compact design. The three-mirror system feeds a threeelement refractive corrector to produce a 3.5 degree diameter field of view on a 64 cm diameter flat focal surface. This paper describes the current status of the mirror system components and provides an overview of the upcoming milestones including the mirror coating and the mirror system integrated tests prior to summit integration.

Beating the confusion limit: the necessity of high angular resolution for probing the physics of Sagittarius A* and its environment: opportunities for LINC-NIRVANA (LBT), GRAVITY (VLTI) and and METIS (E-ELT)

The super-massive 4 million solar mass black hole (SMBH) SgrA* shows variable emission from the millimeter to the X-ray domain. A detailed analysis of the infrared light curves allows us to address the accretion phenomenon in a statistical way. The analysis shows that the near-infrared flux density excursions are dominated by a single state power law, with the low states of SgrA* are limited by confusion through the unresolved stellar background. We show that for 8-10m class telescopes blending effects along the line of sight will result in artificial compact star-like objects of 0.5-1 mJy that last for about 3-4 years. We discuss how the imaging capabilities of GRAVITY at the VLTI, LINC-NIRVANA at the LBT and METIS at the E-ELT will contribute to the investigation of the low variability states of SgrA*.

The GRAVITY spectrometers: optical design

Operating on 6 interferometric baselines, i.e. using all 4 unit telescopes (UTs) of the Very Large Telescope Interferometer (VLTI) simultaneously, the 2nd generation VLTI instrument GRAVITY will deliver narrow-angle astrometry with 10μas accuracy at the infrared K-band. At this angular resolution, GRAVITY will be able to detect the positional shift of the photo-center of a flare at the Galactic Center within its orbital timescale of about 20 minutes, using the observed motion of the flares as dynamical probes of the gravitational field around the supermassive black hole Sgr A*. Within the international GRAVITY consortium, the 1. Physikalische Institut of the University of Cologne is responsible for the development and construction of the two spectrometers of the camera system: one for the science object, and one for the fringe tracking object, both being operated at cryo-vacuum. In this paper we present the phase-C final optical design of the two spectrometers as it got derived from the scientific and technical requirements and as it was presented and reviewed successfully at the Final Design Review (FDR) at the European Southern Observatory (ESO) in October 2011.

The GRAVITY spectrometers: system design

Operating on 6 interferometric baselines, i.e. using all 4 UTs, the 2nd generation VLTI instrument GRAVITY will deliver narrow angle astrometry with 10μas accuracy at K-band. We present the system design of the science and fringe tracking spectrometers of GRAVITY: The fringe tracking spectrometer is optimised for highest sensitivity, providing a fixed spectral resolution. The science spectrometer provides 3 different low - medium spectral resolutions. Both spectrometers provide detector focus stages and deployable Wollaston prisms. The two spectrometers also feed the beams of the metrology laser system of GRAVITY backwards into the integrated optics beam-combiner, propagating back to the M2 mirrors of the 4 telescopes.

The GRAVITY spectrometers: design report of the optomechanics and active cryogenic mechanisms

The work package of the University of Cologne within the GRAVITY consortium included the development and manufacturing of two spectrometers for the beam combiner instrument. Both spectrometers are optimized for different tasks. The science spectrometer provides 3 different spectral resolutions. In the highest resolution the length of the spectral lines is close to the borders of the imaging area of the detector. Also the integration time of these high resolution images is relative long. Therefor the optical pathes have to be controlled by the feedback of a faster spectrometer. The fringe tracking spectrometer has only one low resolution to allow much shorter integration times. This spectrometer provides a feedback for the control loops which stabilize the optical pathes of the light from the telescope to the instrument. This is a new key feature of the whole GRAVITY instrument. Based on the optical layout my work was the design of the mechanical structure, mountings, passive and active adjustment mechanisms. This paper gives a short review about the active mechanisms and the compliant lens mounts. They are used similarly in both spectrometers. Due to the observation and analysis of near-infrared light the mechanisms have to run at cryogenic temperatures and in a high vacuum. Except the linear stages, the motorized mechanisms will get used for several times per observation.