ATSA- A Next Generation Active Telescope for Space Astronomy

We present a design for an active telescope for space astronomy (ATSA). The telescope is capable of both exoplanet work and general astronomyin spectral regions from  100 nm up to 5  m, thus extending the range beyond concepts such as LUVOIR and HabEx up to the shorter wavelengthend of Spitzer or the Origins Space Telescope, as examples.The primary mirror is 6m in diameter, formed by sixteen mirror segments that are precisely phased and supported on rigid body actuators andwith segment optical surface figures fine-tuned using surface figure actuators. The active primary forms a large deformable mirror with wavefronterror correction at the entrance pupil. Thus, the largest source of wavefront error can be removed at the source and is corrected over the entire fieldof view. This enables diffraction-limited performance at 400 nm and a more efficient optical system over a broader wavelength range than could beachieved by a small deformable mirror at a downstream relayed pupil. The telescope is passively cooled to below 100K at Sun-Earth L2, enablingastronomical-background-limited observations out to 5  m. Launched on a SpaceX Starship or alternatively NASA’s SLS, the telescope requiresminimal deployments.A 72m diameter starshade provides a contrast ratio better than 10􀀀10 for exoplanet science. Near the visible region, with a 108% workingbandwidth from 300 to 1000 nm, a working distance of 120Mm provides a 51 mas inner working angle. This band can be moved to shorter orlonger wavelengths by adjusting the starshade range from the telescope. Our first-ever thermal analysis of such a starshade shows that a temperaturebelow 100K can be achieved over a broad range of observing directions, permitting the possibility of working into the infrared. We model the yieldin exoplanets that can be observed.A starshade and associated spectrograph offer significant advantages for exoplanet characterization. They enable a much broader instantaneousspectral bandwidth (here 108%) than current coronagraphs ( 10-20% bandwidth), allow both polarizations to be observed simultaneously and havehigher throughput. The inner working angle is twice as small as can be achieved with a coronagraph and there is no outer working angle. Thesedifferences are particularly pronounced in the UV, where coronagraph performance would be strongly affected by throughput losses, wavefrontaberrations, Fresnel polarization effects at surfaces and thermal instability.