Why UMBRAS?

A Better Technique?

Direct imaging of extrasolar planetary systems is extremely difficult due to their low brightness and close proximity to the parent stars as viewed from the Earth. In the optical and near-infrared (IR) wavelengths, planets shine by reflected light. The difference in apparent magnitude between a planet and a star can be as high as 20-30 magnitudes, while in the infrared the difference could be in the range of 15-25 magnitudes. Without an occulter or coronagraph or some other clever light suppressing technique to reduce the light from the parent star, a planet would be lost in the glare of the star.

As viewed from a disant solar-type star, Jupiter (which is 5 times as far away from the Sun as the Earth is), would be about a billion times fainter in visible light than the star. It is not merely the difference in brightness which causes difficulty in imaging extrasolar planets, but diffraction of the starlight by the telescope. Much of this light spreads out in the image-plane where planets would be found, preventing detection.

It is the giant Jupiter-like planets that are currently being detected by ground based observers using [Doppler techniques]. However, almost all of these objects are much closer to their stars than Jupiter is, and nearly all orbit their stars closer than the Earth orbits the sun. Several of the Doppler detected planets about nearby stars (Upsilon Andromeda, 47 Ursa Majoris, GL 614, & Epsilon Eridani) could be directly imaged using the UMBRAS observing technique using the large N-class occulting screen. The outer of two planets about the star 55 rho¹ Cancri could possibly be imaged with HST instruments.

UMBRAS Spacecraft

The UMBRAS space mission has several advantages over other types of coronagraphic designs. Since the occulter is not built into the telescope as an add-on instrument, scattered light is reduced due to fewer optical surfaces and apertures. There are no unwanted diffraction spikes resulting from coronagraph supports, and the complexity of internal instruments is reduced. Some of the difficulties with conventional coronagraphs are the result of small scale imperfections in manufacturing and materials, and these are avoided with an external occulter.

UMBRAS can move while the science instrument is fixed on a background target. The target of interest may be placed anywhere in the image plane of any science instrument, unlike in conventional coronagraphs. All science instruments can be used with UMBRAS, imaging arrays as well as spectrographs. The study of extrasolar planet properties, photometrically and spectroscopically, could be achieved.

In simple terms, UMBRAS uses the oldest trick in the book: blocking out glare. We do it with sun-visors in our car, or with our hand when we look up on a clear-sky day near the sun to see birds or planes flying overhead. You can even use the technique to help see Venus near quadrature during the daytime if you know where to look (and are blessed with a clear atmosphere).