UMBRAS: a matched occulter and telescope for imaging extrasolar planets

Abstract

We describe a 1-meter space telescope plus free-flying occulter craft mission that would provide direct imaging and spectroscopic observations of Jovian and Uranus-sized planets about nearby stars not detectable by Doppler techniques. The Doppler technique is most sensitive for the detection of massive, close-in extrasolar planets while the use of a free-flying occulter would make it possible to image and study stellar systems with planets comparable to our own Solar System. Such a mission with a larger telescope has the potential to detect earth-like planets.Previous studies of free-flying occulters reported advantages in having the occulting spot outside the telescope compared to a classical coronagraph onboard a space telescope. Using an external occulter means light scatter within the telescope is reduced due to fewer internal obstructions and less light entering the telescope and the polishing tolerances of the primary mirror and the supporting optics can be less stringent, thereby providing higher contrast and fainter detection limits. In this concept, the occulting spot is positioned over the star by translating the occulter craft, at distances of 1,000 to 15,000 km from the telescope. Any source within the telescope field-of-view can be occulted without moving the telescope.In this paper, we present our current concept for a 1-m space telescope matched to a free-flying occulter, the Umbral Missions Blocking Radiating Astronomical Sources (UMBRAS) space mission. An UMBRAS space mission consists of a Solar Powered Ion Driven Eclipsing Rover (SPIDER) occulter craft and a matched (apodized) telescope. The occulter spacecraft would be semi-autonomous, with its own propulsion systems, internal power (solar cells), communications, and navigation capability. Spacecraft rendezvous and formation flying would be achieved with the aid of telescope imaging, RF or laser ranging, celestial navigation inputs, and formation control algorithms.Al Schultz is an Instrument Scientist at the Space Telescope Science Institute (STScI). He has worked at STScI for ∼12 years. Since launch, Dr. Schultz has supported HST operations in PODPS, which is now part of OPUS, the GHRS, STIS, NICMOS, and WFPC2 instruments. (Send correspondence to schultz@stsci.edu; Telephone: 410-338-5044)High-Contrast Imaging for Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/17/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx Proc. of SPIE Vol. 4860 57 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/17/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx

Limits on achievable intensity reduction with an optical occulter: comment

“…Transmitting screens have been well studied for both internal coronagraphs and external occulters including studies of partially transmitting screens by the Big Occulting Steerable Satellite and UMBRAS groups [18,19]. The primary issue with partially transmissive screens is that scattering is nearly impossible to control to the necessary level.…”

Limits on achievable intensity reduction with an optical occulter: reply to comment

“…However, they only achieved 4e − 5 orders of magnitude intensity reduction from their optimization scheme at 1 μm. Reference [7] uses an occultation screen based on Sonine functions and obtains 1e − 8 orders of magnitude relative intensity reduction at 1 μm. In WS2011, the authors derive the optimum transmission function represented by a polynomial as described in Appendix B of WS2011.…”

Progress on the occulter experiment at Princeton

“…1 Since then there have been a number of occulter-based mission concepts incorporating different apodization approaches. [2][3][4][5] All of these approaches share the same basic architecture: a large occulting screen on a separate spacecraft flying far (tens to hundreds of thousands of kilometers) from its associated telescope. This distance gives the occulter a small angular size, theoretically allowing objects within 0.1 arcseconds or less of a star to be imaged.…”