|FGS Instrument Handbook|
As a science instrument the FGS offers unique capabilities not presently available by other means in space or on the ground. Its unique design and ability to sample large areas of the sky with milliarcescond (mas) accuracy or better gives the FGS advantages over all current or planned interferometers.The FGS has two observing modes, Position and Transfer mode. In Position mode the FGS measures the relative positions of luminous objects within its Field of View (FOV) with a per-observation accuracy of ~ 1 mas for targets with 3.0 < V < 16.8. Multi-epoch programs can achieve relative astrometric measurements with accuracies approaching to 0.2 mas.In Transfer mode the FGS is used as a high angular resolution observer, able to detect structure on scales as small as 8 mas. It can measure the separation (with ~1 mas accuracy), position angle, and the relative brightness of the components of a binary system down to ~10 mas for cases where ΔV < 1.5. For systems with a component magnitude difference of 2.0 < Δm < 4.0, the resolution is limited to 20 mas.By using a “combined mode” observing strategy, employing both Position mode (for parallax, proper motion, and reflex motion) and Transfer mode (for determination of a binary’s visual orbit and relative brightnesses of the components), it is possible to derive the total and fractional masses of a binary system, and thus the mass-luminosity relationship for the components.Alternatively, if a double lined spectroscopic system is resolvable by the FGS, then the combination of radial velocity data with Transfer mode observations can yield the system’s parallax and therefore the physical size of the orbit, along with the absolute mass and luminosity of each component.