Optical interferometry is a powerful technique to achieve high angular resolution.
However, its main issue is its lack of sensitivity, compared to other observation
techniques. Efforts have been made in the previous decade to improve the sensitivity
of optical interferometry, with instruments such as PIONIER and GRAVITY at VLTI, or
MIRC-X and MYSTIC at CHARA. While those instruments pushed on sensitivity, their design
focus was not the sensitivity but relative astrometric accuracy, imaging capability,
or spectral resolution. Our goal is to build an instrument specifically designed to
optimize for sensitivity. This meant focusing our design efforts on different parts
of the instrument and investigating new technologies and techniques. First, we make
use of the low noise C-RED One camera using e-APD technology and provided by First
Light Imaging, already used in the improvement of sensitivity in recent new instruments.
We forego the use of single-mode fibers but still favor an image plane design that
offers more sensitivity than a pupil plane layout. We also use a minimum number of
optical elements to maximize the throughput of the design, using a long focal length
cylindrical mirror. We chose to limit our design to 3 beams, to have the capability
to obtain closure phases, but not dilute the incoming flux in more beam combinations.
We also use in our design an edge filter to have the capability to observe H- and
K-band at the same time. We use a low spectral resolution, allowing for group delay
fringe tracking but maximizing the SNR of the fringes for each spectral channel. All
these elements will lead to a typical limiting magnitude between 10 and 11 in both
H- and K-bands.
