Single Conjugate AO
The Single Conjugate Adaptive Optics (SCAO) mode of HARMONI is designed to deliver diffraction-limited performance by correcting atmospheric turbulence using a single bright natural guide star located on-axis or within a small patrol field around the science target. The correction is optimised for the direction of that guide star and therefore provides the highest possible image quality over a limited field of view.
In SCAO mode, visible light from the guide star is separated from the science beam by a dichroic and sent to a dedicated adaptive optics wavefront sensor. HARMONI uses a pyramid wavefront sensor, which offers high sensitivity to small wavefront aberrations when operated with appropriate modulation. The wavefront sensor measures distortions introduced by atmospheric turbulence and telescope aberrations at high temporal frequency. These measurements are processed in real time by the adaptive optics control system, which computes the corrective commands applied to the ELT deformable mirror. The correction is applied continuously, closing the adaptive optics loop and stabilising the wavefront delivered to the science instrument.
In addition to the main high-order correction loop, SCAO includes a low-order loop that compensates for slowly varying aberrations such as non-common path errors between the wavefront sensing and science channels. This ensures that the wavefront quality measured by the sensor is accurately transferred to the science focal plane, maintaining long-term stability of the point spread function.
The expected performance of SCAO is driven by a top-level requirement of achieving high Strehl ratio in the near-infrared. Under median atmospheric conditions and with a sufficiently bright guide star, simulations show that the system can achieve Strehl ratios of around 70% in K band, and in many cases close to or exceeding 80%. Performance degrades gracefully with worsening seeing or fainter guide stars, but remains compliant with requirements up to approximately magnitude 12 under typical conditions. The delivered image quality is dominated by a diffraction-limited core with a stable halo, enabling high spatial resolution spectroscopy at the smallest HARMONI spaxel scales.