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  • A dual injection system for the Automated Alignment System of the MROI

    Introduction

    The Automated Alignment System (AAS) and Back End Active Stabilization of Shear and Tilt (BEASST) together have been proposed to align the beam trains in the MROI and to keep these aligned during the night. A Preliminary Design for the AAS has been developed and some of the subsystems of the AAS have been prototyped. On the other hand, BEASST has been developed to the conceptual design stage only. We propose here a modification to the AAS concept which will allow the existing goals to be met while addressing drifts in alignment during the night more rapidly and robustly. At the same time, some of the constraints on the BEASST system can be relaxed, allowing a higher-performance system to be built more quickly.

    In the following we recall the original goals and concept for the AAS and outline a proposed modification. The advantages and disadvantages of this modification are then discussed and a way forwards proposed.

    Functional requirements

    The functions of the combined AAS and BEASST systems (hereafter collectively referred as “the AAS” since they can be considered to be a single system addressing alignment needs) can be briefly described as:

    1. when a new beam train is installed or configured, the AAS must allow for coarse (and possibly manual) alignment of the beam train to allow an alignment beam to pass between the beam combining area (BCA) and the unit telescope mount (UTM) without vignetting;

    2. at the beginning of each observing night, the AAS must re-align the beam train so that beams of starlight arriving at the beam combiners meet the tilt and shear error budget;

    3. during the night, the AAS must make fine adjustments to the alignment so that the tilt and shear error budget is maintained in the presence of thermal drifts, typically introduced by those optics located in temperature unstable environments;

    4. when a new beam train is installed or configured, the AAS must allow for measurement of the “piston” term due to the optical path between the beam combiners and the telescopes, in order to allow for rapid acquisition of stellar fringes;

    5. at the beginning of the night, the AAS must allow the pathlength differences between the fringe tracker and the science instrument to be measured so that these can be equalized using the switchyard mirrors.

    Outline of the existing scheme

    The existing design of the AAS is shown schematically in Fig. \ref{fig:classic}. A beam of light is injected from the alignment table in the BCA (the BCA beam injection system is also known as the Magical Optical Box or MOB), with one beam going “upstream” towards the telescopes and one going “downstream” towards the beam combiners (Fringe Tracker FT and Science Instrument SI). This pair of injected beams (known as the “primary fiducial”) can consist of either laser light at a visible wavelength or broadband light which has components at visible and infrared wavelengths. The design of the MOB ensures that the upstream and downstream beams are anti-parallel to one another.

    \label{fig:classic} Schematic layout of the existing AAS design. Hardware elements which are built as part of the AAS or BEASST are indicated as gray boxes, but these are combined with other hardware and software components to make up the complete AAS/BEASST subsystem.