A high-Strehl low-resolution optical imager (BESSEL): Detection of a 0.7λ/D separation binary from the ground

We have constructed a high-speed image stabilization system, BESSEL, which mounts to the 8-in. refractive telescope coupled to the ray white telescope at steward observatory. The high-speed tip/tilt mirror platform is controlled by an Andor electron multiplication EMCCD enabling wavefront correction at a rate exceeding 1 kHz. BESSEL achieves on-sky Strehl ratios of 98-99% at λ = 800 nm when the telescope aperture is stopped down to half the Fried parameter (typically D = 25.4 mm, where D is the diameter of the effective telescope aperture). Utilizing high Strehls and the technique of roll subtraction enabled BESSEL to resolve the binary, ADS 10418AB (more commonly known as α Her), with separation of only 0.71λ/D and a delta magnitude of ∼3 mag at 800 nm. In addition, Arcturus was observed with BESSEL to explore a relatively untested region of Strehl versus D/r₀ parameter space in the optical, specifically at small (∼0.5D/r₀). We find that in this regime space quality, very high Strehl (∼99%), optical imaging can be obtained from the ground when D/r₀ < 0.5 as one approaches the inner scale of turbulence. Because of BESSEL's demonstrated high Strehls, it is an ideal platform for inexpensive, initial, on-sky characterization of space-based coronagraphs, for which it will be used in the near future.