|Space Telescope Science Institute|
|FGS Instrument Handbook|
It is possible to use the FGS in both Transfer and Position mode during a given observing session (visit). As was mentioned in Section 3.4, by using Transfer mode observations to determine a binary system’s true relative orbit, and using Position mode observations of nearby reference stars (and perhaps the binary as well) to determine the binary’s parallax, the system’s total mass can be derived. Furthermore, if the data are of sufficient quality, i.e., if the uncertainty in the position of each component with respect to the reference stars is small compared to the semi-major axis of the binary’s orbit, then the motion of each component about the system’s barycenter can be determined. From this information, the mass ratio, and hence the mass to luminosity ratio of each of the two stars, can be calculated.Investigations of an extended source, such as a giant star, can also benefit from a combination of Transfer and Position mode observations. If the FGS in Transfer mode can resolve the disk and measure its angular size, and nearby stars are measured in Position mode to derive the object’s parallax, then the physical size of the disk can be determined.In order to achieve these scientific objectives, the Transfer mode data must be related to the Position mode data. If the binary is observable in Position mode, then it is straightforward to determine the offsets between the Position mode reference frame and Transfer mode positions. If the binary cannot be observed in Position mode, then the task of relating the Transfer data to the Position data is more complex.Linking a Transfer mode observation of a binary system to Position mode observations of reference stars in the same visit requires that the x,y coordinates of each of the binary’s components, derived from analysis of the Transfer function, be mapped onto the same x,y coordinate system as the Position mode observations (the visit level “plate”). This implies that all pertinent corrections and calibrations applied to Position mode data must be applied to the Transfer mode centroids, i.e., visit-level corrections for low frequency oscillations of the spacecraft’s pointing, FOV drift, the OFAD, and the differential velocity aberration. The level of difficulty in accomplishing this task depends on the structure of the binary (i.e., the separation of the components and their relative brightness).