Point-spread function stability of the SNAP telescope

摘要

SNAP is a proposed space-based experiment designed to study dark energy and alternate explanations of the acceleration of the universe's expansion by performing a series of complementary systematic-controlled astrophysical measurements. The principal mission activities are the construction of an accurate Type Ia supernova Hubble diagram (the supernova program), and conducting a wide-area weak gravitational lensing (WL) survey. WL measurements benefit from a highly constant point spread function (PSF). The goal of this study is to quantify the anticipated variations in PSF arising from on-orbit thermal variations and shrinkage associated with dryout of the composite telescope metering structure. A combined thermo-mechanical-optical analysis tool was developed, and WL metrics whisker, ellipticity and effective anisotropy quantified for thermal and composite structure dryout effects. Stability limits necessary for WL are defined, and compared to stability tolerances defined for the supernova program. The mission is designed for operations at at the Earth-Sun L2 Lagrange point, where thermal disturbances from Earth are minimal. In this study, the effects of seasonal variations in solar flux, transients introduced when pointing the body-fixed Ka-band antenna toward Earth and 90° roll maneuvers (planned every three months of operations) are quantified, and introduced into the optical system. Vector whisker and effective anisotropy change rates were computed, and found to be well below the WL requirement for stability. The effects of composite structure shrinkage due to on-orbit H_2O desorption are discussed, and estimated to be significant, but symmetric and predictable during the supernova/deep WL phase of the mission (3-21 months after launch). When the dedicated WL phase of the mission begins (21 months after launch), structural dryout will be largely complete, with predicted daily changes well within WL limts.