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Notes from the TPF Final Architecture Review, 11-13 December 2001

A Preliminary Architecture Review was held in December 2000, where three out of four contractors favored optical or infrared coronagraph designs for TPF. An interesting result of the Preliminary Review was that optical biomarkers appeared to be very promising. JPL then requested that two coronagraph designs and two interferometer designs be studied in greater detail. TRW and Ball were to study coronagraph designs and Boeing and Lockheed Martin were to study interferometer designs. [Boeing also independently studied a coronagraph design.] The Final Architecture Review was held 11-13 December 2001 in San Diego at the end of the contract.

The results of this work will provide recommendations for technology that should be developed to support the TPF program. The following summarizes my notes from the meeting as well as my own review of the viewgraph packages. The following appears very likely:

• Optical and infrared biomarkers appear to be very promising, and have roughly equal interest and support amongst those involved in the contract work. TPF sponsorted a Biomarkers Working Group study, which concluded in October 2001 with special interest in investigating optical biomarkers.
• No decision will be made concerning the final design of TPF until probably 2006. The most promising candidate designs are variations on coronagraph and interferometer designs, either infrared or optical. The technology for these will be studied in the next few years.
• TPF will be technologically challenging and very expensive. Cost estimates provided by the contractors ranged between 1.3 and 2.8 billion dollars (total life-cycle cost). The importance of precusor missions were emphasized by the contractors and will undoubtedly be studied further, most notably through the NASA Extra-Solar Planets Advanced Mission Concepts Research Announcement. Winners of those contracts should be announced sometime in the spring of 2002.

• An agreement in principle between NASA and ESA is expected to link the TPF and Darwin missions, possibly with Japanese participation, over the next few years.
• From 2002 to 2006 work on TPF will most likely focus on technology development for coronagraphs and interferometers, with a final architecture choice taking place in 2006.
• The current launch date for TPF is estimated to be 2014.

The viewgraph packages distributed at the Preliminary and Final Architecture Review meetings is available on a single CDROM by request to Chris Lindensmith at chrisl@squid.jpl.nasa.gov.

Ball Aerospace

Spergel Pupil
D.N. Spergel, A new optical coronagraph for terrestrial planet detection, Appl. Opt. submitted (2001).
M.J. Kuchner and W.A. Traub, A coronagraph with a band-limited mask for finding terrestrial planets, Astrophys. J. accepted (2001).

• Coronagraph, visible wavelengths
• 10x4-m off-axis primary mirror
• Spergle & Kasdin's binary pupil with Kaiser or prolate-spheroid shaped edges
• 40-60 mas "inner working distance"
• Up to ~1000 mas field of view
• Active wavefront control: 256 x 100 actuators
• 3-130 cycles across mirror: 4.8 nm rms initially, corrected to 0.07 nm rms.
• Monolithic primary constructed from segments with AMSD technology.
• launch vehicle with 5-m fairing, eg. Ariane 5 (10-m long) or Delta IV Heavy
• Arrested-drift earth-trailing orbit, at a distance of 0.2 AU
• Cost estimates: 1.28, 1.33, or 1.9 G$

TRW

R. Simon and Vogt AAS 197 #49.05 7,058 stars withing 50 pc

Infrared Coronagraph

• Coronagraph, infrared wavelengths, 7-17 microns
• 28-m longest diagonal, 36 mirror segments - adequate for dimmest stars on target list
• 80 mas occulting spot diameter
• Diffraction limited at 7 microns
• Coronagraph field of view, 10 x 10 arcseconds
• Primary and secondary mirror passively cooled to 70 K
• Instrument passively cooled to ~30 K
• Surface roughness < 10 nm rms
• Mid spatial frequencies < 4 nm rms
• launch vehicle with 5-m fairing, eg. Delta IV Heavy
• L2 orbit
• Cost estimate: 1.896 G$

Boeing - SVS

Non-redundant Linear Array (Hyper-telescope)

Hypertelescope references:

• A. Labeyrie, Resolved imaging of extra-solar planets with future 10-100km optical interferometric arrays, Astron. Astrophys. Supp. Ser. 118, 517 (1996).
• A. Boccaletti et al, Icarus 145, 628.
• The nulling stellar coronagraph: O. Guyon et al., PASP 111, 1321-1330 (1999).

• Roddier and Roddier PASP 109, 815-820 (1997).
• P. Riaud et al., The Four-Quadrant Phase Mask Coronagraph. II. Simulations. Pub. Astron. Soc. Pac. 113, 1145 (2001)

The hypertelescope is a hybrid interferometer/coronagraph with image-plane beam combination. The interferometer is used to obtain high angular resolution and the coronagraphic stop/mask is used to reject the starlight. At a first image plane, a coronagraphic stop (phase mask) scatters the star-light out of the field of a secondary image plane where planets are detected. Pupil densification is required for the coronagraph to be efficient. For the linear array, outlined here, the input apertures are re-arranged in a densified circular pupil, and are then de-densified afterwards.

• Interferometer, infrared wavelengths, 7-17 microns.
• 100-m structure
• Seven 3-m diameter telescopes in a non-redundant linear array
• Hypertelescope beam combiner, with sub-pupils arranged along the circumference of a circle, about 20 reflections per arm in total beam-train.
• Passively cooled to 70 K
• Three launches required: 2 shuttle and 1 Delta IV
• Human assisted assembly in Low Earth Orbit during the two shuttle missions.
• Boosted from LEO to L2 orbit
• Cost estimate: 2.83 G$

• Coronagraph-type design, visible wavelengths
• 8-m square aperture
• 62 mas minimum detectable planet-star separation
• Jacquinot apodization (30% more throughput than Sonine) in re-imaged pupil
• Rms surface error < lambda/1800; Figure control WFE < lambda/200
• 3 or 4 mirror segmented design
• Cooled by not cryo - Possibly temperature controlled at 220 K
• Delta IV or shuttle launch
• L2 orbit
• Simulated for 3-30 cycles/mirror and lambda/1000 waves.
• No cost estimate provided.

Lockheed Martin

Although a brief bibliography of nulling interferometery exists, the array design that was described by Nick Woolf et al. has yet to be published. However, the design of the nulling combiner is two-staged design based on the combiners described by Serabyn and Colavita:

E. Serabyn, M.M. Colavita, Fully symmetric nulling beam combiners, Appl. Opt. 40, 1668 (2001).

Four-element Double-Bracewell Nulling Arrays
9-m, 21-m, 40-m fixed-structure linear arrays and a Free Flyer design discussed. Only the 40-m array and Free Flyer designs would meet the mission requirements of > 150 stars surveyed for planets, and the 40-m structure will be outlined here.

Biomarkers Study Reference

• Interferometer, infrared wavelengths, 7-17 microns.
• Passively cooled to 40-45 K.
• 40-m double-Bracewell, fixed-structure, linear array
• 4 telescopes, 3.5-m diameter
• Theta-squared null
• Serabyn-Colavita two-stage nuller design with added phase plate.
• Launch vehicle, Atlas 5
• Orbit: L2 or Earth trailing (not specified)
• Cost estimate: 1.741 G$