Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Hierarchical fringe tracking   The limiting magnitude is a key issue for optical interferometry. Pairwise
fringe trackers based on the integrated optics concepts used for example in
GRAVITY seem limited to about K=10.5 with the 8m Unit Telescopes of the VLTI,
and there is a general "common sense" statement that the efficiency of fringe
tracking, and hence the sensitivity of optical interferometry, must decrease as
the number of apertures increases, at least in the near infrared where we are
still limited by detector readout noise. Here we present a Hierarchical Fringe
Tracking (HFT) concept with sensitivity at least equal to this of a two
apertures fringe trackers. HFT is based of the combination of the apertures in
pairs, then in pairs of pairs then in pairs of groups. The key HFT module is a
device that behaves like a spatial filter for two telescopes (2TSF) and
transmits all or most of the flux of a cophased pair in a single mode beam. We
give an example of such an achromatic 2TSF, based on very broadband dispersed
fringes analyzed by grids, and show that it allows piston measures from very
broadband fringes with only 3 to 5 pixels per fringe tracker. We show the
results of numerical simulation indicating that our device is a good achromatic
spatial filter and allowing a first evaluation of its coupling efficiency,
which is similar to this of a single mode fiber on a single aperture. Our very
preliminary results indicate that HFT has a good chance to be a serious
candidate for the most sensitive fringe tracking with the VLTI and also
interferometers with much larger number of apertures. On the VLTI the first
rough estimate of the magnitude gain with regard to the GRAVITY internal FT is
between 2.5 and 3.5 magnitudes in K, with a decisive impact on the VLTI science
program for AGNs, Young stars and planet forming disks.

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The categories of this paper are: physics
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The categories of this paper are: bayes-an
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The categories of this paper are: supr-con
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The categories of this paper are: hep-lat
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The categories of this paper are: cmp-lg
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The categories of this paper are: solv-int
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The categories of this paper are: stat
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The categories of this paper are: ao-sci
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The categories of this paper are: acc-phys
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The categories of this paper are: gr-qc
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The categories of this paper are: math-ph
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The categories of this paper are: funct-an
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The categories of this paper are: plasm-ph
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The categories of this paper are: hep-th
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The categories of this paper are: cond-mat
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The categories of this paper are: nlin