Exoplanet detection with simultaneous spectral differential imaging: effects of out-of-pupil-plane optical aberrations

Imaging faint companions (exoplanets and brown dwarfs) around nearby stars is currently limited by speckle noise. To efficiently attenuate this noise, a technique called simultaneous spectral differential imaging (SSDI) can be used. This technique consists of acquiring simultaneously images of the field of view in several adjacent narrow bands and in combining these images to suppress speckles. Simulations predict that SSDI can achieve, with the acquisition of three wavelengths, speckle noise attenuation of several thousands. These simulations are usually performed using the Fraunhofer approximation, i.e. considering that all aberrations are located in the pupil plane. We have performed wavefront propagation simulations to evaluate how out-of-pupil-plane aberrations affect SSDI speckle noise attenuation performance. The Talbot formalism is used to give a physical insight of the problem; results are confirmed using a proper wavefront propagation algorithm. We will show that near-focal-plane aberrations can significantly reduce SSDI speckle noise attenuation performance at several lambda/D separation. It is also shown that the Talbot effect correctly predicts the PSF chromaticity. Both differential atmospheric refraction effects and the use of a coronagraph will be discussed.Comment: 11 pages, 7 figures. To be published in Proc. SPIE Vol. 6269, p. 1147-1157, Ground-based and Airborne Instrumentation for Astronomy; Ian S. McLean, Masanori Iye; Ed

The Space-based Telescopes for Actionable Refinement of Ephemeris (STARE) mission

Recent events, such as the February 2009 Iridium 33-Cosmos 2251 collision, have brought attention to the changing nature of the Low Earth Orbit (LEO) environment. The population of objects recorded by the US Space Catalog has doubled since 1992, resulting in an increased risk of on-orbit collisions. USSTRATCOM’s Space Surveillance Network (SSN) tracks resident space objects (RSO) and publicly releases a subset of these data to support conjunction (collision probability) analyses. However, these early warning systems did not prevent the Iridium – Cosmos collision. Conversely, there have been a number of high profile ISS false alarms where the crew has unnecessarily interrupted operations to take shelter. These examples highlight the need for better Space Situational Awareness (SSA) in LEO. The Space-based Telescopes for Actionable Refinement of Ephemeris (STARE) mission will improve SSA using a low-cost small satellite constellation. An operational STARE constellation of 18 nanosatellites will be able to assess greater than 99% of all conjunctions involving objects larger than 10 cm and has the capability to reduce the current collision false alarm rate by two orders of magnitude up to 24 hours ahead of closest approach, in effect reducing the number of actionable alerts to one per satellite lifetime. This is a significant improvement over today’s capability, which provides so many false alarms (estimated at one per month per satellite for a LEO sun-synchronous orbit) that alerts are regularly ignored due to the inability of the space assets to move frequently