Hierarchical fringe tracker to co-phase and coherence very large optical interferometers

Abstract

The full scientific potential of the VLTI with its second generation instruments MATISSE and GRAVITY require fringe tracking up to magnitudes K>14 with the UTs and K>10 with the ATs. The GRAVITY fringe tracker (FT) will be limited to K~10.5 with UTs and K~7.5 with ATs, for fundamental conceptual reasons: the flux of each telescope is distributed among 3 cophasing pairs and then among 5 spectral channels for coherencing. To overcome this limit we propose a new FT concept, called Hierarchical Fringe Tracker (HFT) that cophase pairs of apertures with all the flux from two apertures and only one spectral channel. When the pair is cophased, most of the flux is transmitted as if it was produced by an unique single mode beam and then used to cophase pairs of pairs and then pairs of groups. At the deeper level, the flux is used in an optimized dispersed fringe device for coherencing. On the VLTI such a system allows a gain of about 3 magnitudes over the GRAVITY FT. On interferometers with more apertures such as CHARA (6 telescopes) or a future Planet Formation Imager (12 to 20 telescopes), the HFT would be even more decisive, as its performance does not decrease with the number of apertures. It would allow building a PFI reaching a coherent magnitude H~10 with 16 apertures with diameters smaller than 2 m. We present the HFT concept, the first steps of its feasibility demonstration from computer simulations and the optical design of a 4 telescopes HFT prototype.

Hierarchical Fringe Tracking, sky coverage and AGNs with the VLTI

“…In [9]we described a very broad band bulk optics TBSF that proved operational but difficult to realize and to maintain accurately aligned. The figure 1 shows how a standard ABCD integrated optics beam combiner can be evolved in TBSF by merging two of its outputs after correcting their fixed phase difference.…”

“…Despite its state-of-the-art design, control system, integrated optics chip and high performance Saphira detector, the GFT is not reaching the sensitivity limit that is achievable on the VLTI with the current technology. Boskri et al 2021 [8] show that the new generation hierarchical fringe tracker (HFT) proposed by [9] using the J, H and K bands should achieve K~15.9 on UTs and K~12.2 on ATs. These magnitudes are necessary to achieve, among other programs, the full potential of MATISSE for Young Stellar Objects studies as well as for the AGN program both with MATISSE and in the J and H bands.…”

“…Hence optimizing the SNR of each individual phase delay estimator, independently from the number of apertures, should be a main specification of any future FT. This paper reminds the key features of the hierarchical fringe tracker architecture with performances independent from the number of apertures proposed in [9] in section 2 and describes its implementation in integrated optics chips. The first set of experimental tests of such integrated optics chips is described in section 3 where we validate the key features of the HFT concepts.…”

A two-band approach to nλ phase error corrections with LBTI's PHASECam

“…In order to obtain sensitive, accurate interferometric observations, the fringes must be stabilized by reducing the differential OPD between apertures to a fraction of the observing wavelength. 2 In practice, this is often achieved using a fringe tracker, a device which measures the fringe phase and uses this to correct the differential OPD in real time. However, when these measurements are carried out using quasi-monochromatic fringes, there exists a 2π ambiguity in the phase measurement, which leads to a λ ambiguity in the OPD correction.…”

“…This ambiguity can be avoided with multi-wavelength phase sensing, which will remove the ambiguity out to the first common multiple of all observed wavelengths. 2 The Large Binocular Telescope Interferometer (LBTI) is a NASA-funded nulling and imaging instrument designed to coherently combine the two primary mirrors of the Large Binocular Telescope (LBT) [3][4][5] for highsensitivity, high-contrast, and high-resolution infrared imaging (1.5 -13 µm). 6,7 The LBTI is equipped with two science cameras: LMIRCam 8-10 (the L and M Infrared Camera, 3-5 µm) and NOMIC 11 (Nulling Optimized Mid-Infrared Camera, 8-14 µm).…”