Silmaril: final design and on-sky performance

Silmaril is a new 3-telescope beam combiner at the CHARA Array. In this presentation, we present the current design of the instrument, its on-sky measured and theoretical best performance, and its future development. Its design is specifically made to push for sensitivity. It combines 3 beams of the CHARA Array, allowing closure phase measurement but limiting the loss in sensitivity. It limits the number of optical elements to the minimum needed for combining the 3 beams. Using a CRED ONE camera allows for sub-electron readout noise, which means that our sensitivity is limited by the thermal background. Long focal-length cylindrical mirrors limit the thermal background by using an f/20 cold stop in front of the camera. We use a low spectral resolution prism to allow fringe tracking without compromising the instrument's sensitivity. Thanks to an ingenious edge filter design, we can observe both H- and K-band simultaneously, with a low thermal background on the H-band side of the detector. In the future, we intend to extend the instrument with a second set of 3 beams and add a Narcissus mirror to lower the thermal background and improve the sensitivity even more.