# Technology

· **TPF General Development**

Studies have identified the key technology developments needed for TPF.
For most of these developments, advanced ground and space based
telescopes and interferometers are currently in operation or under
development and they will provide this technology heritage for TPF to
build on.

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<td><h1>Null-ing</h1></td>
<td><p>Format-ion Flying</p></td>
<td><p>Large Cryo-Optics</p></td>
<td><p>Active Optics</p></td>
<td><p>Metr-ology</p></td>
<td><p>Pointing, Stabilization &amp; Vibration Control</p></td>
<td><p>Passive Cooling</p></td>
<td><p>IR Detectors</p></td>
<td><p>Interfero-</p>
<p>meter System Technology</p></td>
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<td><h1>PTI</h1></td>
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<td><p>√</p></td>
<td><p>√</p></td>
<td><p>√</p></td>
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<td><p>Keck-I</p></td>
<td><p>√</p></td>
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<td><p>√</p></td>
<td><p>√</p></td>
<td><p>√</p></td>
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<td><p>√</p></td>
<td><p>LBT</p></td>
<td><p>√</p></td>
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<td><p>√</p></td>
<td><p>√</p></td>
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<td><p>√</p></td>
<td><p>SIRTF</p></td>
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<td><p>√</p></td>
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<td><p>√</p></td>
<td><p>√</p></td>
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<td><p>SIM</p></td>
<td><p>√</p></td>
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<td><p>√</p></td>
<td><p>√</p></td>
<td><p>√</p></td>
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<td><p>√</p></td>
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<td><p>DS-3</p></td>
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<td><p>√</p></td>
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<td><p>√</p></td>
<td><p>√</p></td>
<td><p>√</p></td>
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These devices and missions may each be accessed on their own web pages.

Palomar Testbed Interferometer, Keck Interferometer, Large Binocular
Telescope,

Space Infrared Telescope Facility, Space Interferometry Mission, DS-3
and

the Next Generation Space Telescope.

Two main items require special additional development, interferometric
nulling in space, and control of separate spacecraft and formation
flying. Both of these items have development planned for precursor
missions and ground observations. Both will also require some special
development for TPF. NASA is already investing in TPF technology, and
plans a \$350M investment in TPF technology development prior to the
mission.

· **Nulling **

Nulling is the process needed to reduce the light recorded from the star
in an observed planetary system, while leaving the planet's light
undimmed, so that the planet observations are not limited by the star's
glare. The process used is interference of starlight.

The light waves from the star have crests and troughs like water waves,
and the amount of light seen is a measure of the depth of the troughs
below the crests. If the starlight from two separated telescopes comes
together so that the troughs from one telescope coincide with the crests
from the other, and vice versa, the recorded starlight will be reduced.
To bring the recorded starlight to zero, the height of the waves must
all be matched exactly from one telescope to the other, for waves of
every length. For light waves, the vibrations of different
crest-to-crest length correspond to different colors.

Because light waves will vibrate differently for up-down and
side-to-side vibrations, we must be careful that we do not partly mix
these two "polarizations".

Development of nulling technology is a process of learning how to
precisely balance wave height or "amplitude", how to make that balance
independent of the length of the waves, and how to keep the
"polarizations" unmixed.

The quality of nulling needed for TPF varies between one part in
1000,000 to 1 part in 2000 across the 7-20µm region of the spectrum
where an earthlike planet shows its atmospheric composition.

The TPF nulling development program has components of observing with
visible light, and observing in the infrared, and each has laboratory
and telescope components. Further, in the lab development there is a
planned progress which starts with light of a single wavelength from a
laser, and progresses to the broad range of wavelengths needed for TPF.

As a part of the TPF program, nulling was first used astronomically at
the MMT to observe dust envelopes around red giant and supergiant stars
by Hinz et al. (1998). The figure shows images from these observations.
The envelopes and the nulled part of the star radiation are shown. The
starlight reduction achieved was about 24:1. Ground observations with
appreciably greater nulling depths will require the use of the new
technique "adaptive optics".

Figure Nulling observations at the MMT. (a) The first two images are of
the dust envelopes. (b)The second two images show the part of the image
that was nulled out in producing figure (a).

Early experiments in preparation for SIM are being conducted at visible
wavelengths. The figure below shows an existing testbed interferometer
at JPL that has enabled the investigation of some of these areas, and
has already reached a null depth of a part in 20,000 on a laser source.

### •Precision Formation Flying

TPF could, if necessary, be flown as a single boom with attached
telescopes. However,

astrophysical studies will need baselines of 1km, and a connected device
of that size is

not practicable. Future astronomical studies are likely to need even
longer baselines, and precision formation flying is being developed to
serve these needs. A first study of separated spacecraft with
interferometer connections is being made with the DS-3 mission.

Much of the technology required for TPF precision formation flying will
be developed and demonstrated on this mission. Precision formation
flying uses a GPS-like system to measure relative 3-dimensional
positions of the spacecraft. ** **Advanced data processing and real-time
estimation technology will be a TPF-unique development based on well
proven estimation theory and advanced sensor fusion techniques under
development in government and industry avionics and robotic systems
technology centers.