archive: SETI FW: [ASTRO] Spectra question

SETI FW: [ASTRO] Spectra question

Larry Klaes ( )
Mon, 14 Sep 1998 10:56:49 -0400

From: Ka Chun Yu
Sent: Sunday, September 13, 1998 7:35 PM
Subject: Re: [ASTRO] Spectra question

> > SIM will be placed into an Earth-trailing orbit around the
> > Sun. Its multiple telescopes will be used in pairs; the light
> > they gather will be collected and processed to pinpoint the
> > position of stars. The system will synthesize images that could
> > normally only be obtained with a much larger telescope. It also
> > will demonstrate the ability to "null" or cancel out the light
> > from a star, which will help enable future missions to obtain a
> > direct view of planets around other stars. Interferometry will
> This sounds a little like Prof Cohens post where the idea is put
> that say oxygen bearing atmospheres around planets orbiting
> distant stars might be detected by their absorption spectra
> when the "main" star light has been "canceled out". Is that
> right? How is it done?

The concept is a "nulling interferometer." Light reaching the
multiple dishes can be re-combined and will produce fringing
(interference) because the pathlengths from the star to each of the
mirrors is different. You can then build the interferometer in such
a way that a nulling fringe lies right on top of the star, thereby
canceling out the star light.

If the planet signal is still weak, you can try to detect a
modulating signal by spinning the interferometer. The nulling fringe
spins around an axis centered on the star. Twice per revolution, the
planet will fall underneath the nulling fringe, and the planet signal
will disappear. The star signal is always nulled, as long as the
star stays centered in the fringe. Periodic variation of the
photometric signal will thus give you a detection of a companion
around the star.

The nulling interferometer concept was first laid out in a paper by
Bracewell and MacPhie in 1979 (!), and is, I believe, the basis for
the future Terrestrial Planet Finder.

There are plenty of other problems to deal with before a TPF can be
launched. Ensuring that your telescope remains pointed at the star
and doesn't start to wonder will be one technical hurdle. Scattered
light from mirror imperfections can also clobber your planet signal.
This latter problem might be reduced by apodizing the mirror: you can
reduce the effects of the Airy rings by reducing the reflectivity of
the mirror as you move outwards from the center. The Airy pattern is
caused by diffraction from the edge of the mirror; changing the
reflectivity will change the Airy pattern, and if you do it right,
you can make the nulls deeper (and darker).

> Richard

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