archive: SETI FW: [ASTRO] Spectra question

SETI FW: [ASTRO] Spectra question

Larry Klaes ( lklaes@zoomtel.com )
Tue, 15 Sep 1998 10:01:21 -0400

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From: Ka Chun Yu
Sent: Monday, September 14, 1998 9:42 PM
To: astro@lists.mindspring.com
Subject: Re: [ASTRO] Spectra question

> Thanks for your detailed reply Kachun,
> I see what you mean. Just a couple of question if I may??
>
> Will some filtering be applied before nulling so that
> "pollution" from other wavelengths that aren't nulled is removed?

The nulling interferometer can be set up so that it's wavelength
dependent to a degree. The size of the interferometric fringe will
depend on the wavelength of light, but there are some set-ups where
this won't make a difference. In Bracewell & MacPhie's paper, they
suggest a telescope where light from two apertures are sent to the
focal points via two off-axis paraboloid mirrors which switches the
phase of the radiation. Other set-ups are of course possible.

I suspect that when the TPF is built, observations will have to be
made through at least broad-band filters, since the optical path
will vary to a degree on wavelength. You will also want to look at
the region of the planetary spectrum where the star-to-planet contrast
is lowest (or the planet-to-star contrast is highest), which is in
the mid-infrared, somewhere between 10 and 20 microns. It would be
natural to filter out other wavelengths that would just add noise to
your signal.

> Will the detectors have the dynamic range to "see" the periodic
> variation (due to the rotating apparatus) without prior filtering?
> Is this expected soon/never or not relevant?

Not sure what you mean by this. Depending on how much star light is
nulled by the interferometer, you could have a set-up whereby you can
make the detection without having to modulate the signal at all.
(You will still need to spin the interferometer to get total UV-plane
coverage--for those familiar with interferometerese.) If you're
trying to detect a puny terrestrial planet that is close to the star
and whose signal is near the noise, going for a modulated signal
might work best. For a warm Jupiter farther out and less affected by
the star light and by the zodiacal dust of the extra-solar system,
you might see the signal right off the bat.

> Richard.

--kachun +** Center for Astrophysics and Space Astronomy, CB 389 **+
+** University of Colorado, Boulder, CO 80309 **+
+** Email: kachun@casa.colorado.edu **+
+** http://casa.colorado.edu/~kachun **+