News 1997


Feasibility Study on Synthetic Aperture Optics for Earth Observation (Findings to appear in the December 1997 ICSO 97 Proceedings) submitted by Frédéric Cassaing, Office National d'Études et Recherches Aérospatiales

ONERA has recently completed a feasability study for the application of Synthetic Aperture Optics (SAO) to Earth Observation from Space.

This study showed that the major difficulty of such an application resides in the extremely extended field of view, which has a major impact on the optical design of the instrument and on its active systems.
Key aspects studied in the course of this project included:
  • the aperture configuration optimization;
  • the conditions for interferometric imaging in the whole field;
  • the need for a cophasing system, the optical modes to be corrected, the type of reference source (internal or external);
  • the design of a cophasing sensor and of the associated phase measurement algorithms; and
  • the simulation of the image acquisition (polychromatic imaging, including short-term and long-term aberrations), and the restoration of such images.
Some conclusions drawn from this study will be detailed in our ICSO'97 communication (see below). The most important ones are: the demonstration of the feasability of the instrument by simulations, the conceptual design of the cophasing system, an optical design of the instrument that complies with the specifications, the significant gain in weight, and the full compatibility with the Ariane V launcher.

Related publications:
  • L. M. Mugnier, G. Rousset and F. Cassaing, "Aperture Configuration Optimality Criterion for Phased Arrays of Optical Telescopes", J. Opt. Soc. Am. A 13 (12):2367--2374, dec 1996.
  • F. Cassaing, L. M. Mugnier, G. Rousset and B. Sorrente, "éléments-clés de la conception d'un instrument spatial à synthèse d'ouverture optique", to appear in the proceedings of ICSO'97 (International Conference on Space Optics 1997), Toulouse, France.
  • F. Cassaing, "Analyse d'un instrument à synthèse d'ouverture optique: méthods de cophasage et imagerie à haute résolution angulaire", PhD thesis, Université de Paris XI, France, to be published.

What is the JPL Interferometry Center of Excellence and its Mission?
submitted by Bill Goss
The Interferometry Center of Excellence (ICE) is one out of five Centers of Excellence that JPL has created to ensure the development and maintenance of a leading edge capability in science and technology. ICE's science and technology focus is that of optical and near infrared interferometric astrometry and imaging.
The Center is responsible for:
  • providing leadership and vision and participating in long-range planning for the institutional interferometry capability,
  • ensuring that the quality of technical proposals and work in interferometry meets the highest standards,
  • developing infrastructure necessary for the successful implementation of interferometry projects,
  • helping to coordinate the definition of interferometer organizational roles
  • and serving as a focus for the Laboratory's technical level interfaces with the external scientific and technological interferometry community.
ICE is currently recruiting qualified scientists and engineers for several interferometry projects. See the Jobs Listing section of this newsletter for more information.
The ICE staff are:
The ICE office mail address is Building 306, MS .473. The ICE fax number is (818) 393-4053.

Keck Interferometer Plans Revealed
submitted by Dr. Paul Swanson
Starting in FY '98, the Keck Interferometer will be funded by NASA, JPL and the California Association for Research in Astronomy (CARA) for the complete array including four or five outrigger telescopes. The outriggers will be 1.5 to 2 meters in diameter and be positioned so as to maximize the imaging capability of the complete array with the two 10- meter Keck telescopes. Initial operation with two outrigger telescopes will be in 2000. The two Keck telescopes will be combined in 2001 and the complete imaging array will be operational in 2002. The imaging array will have approximately 5 milliarcsec angular resolution with a sensitivity at 2 microns of 22 mag. In the astrometric mode the angular resolution will be 10 microarcsec at 21 mag.

Palomar Testbed Interferometer Engineering Update
submitted by M. Mark Colavita and J. Kent Wallace
Recent engineering work at PTI has been in two areas: a) improving the system to perform more robust dual star measurements, and reducing sources of systematic errors in this measurement, and b) improving visibility calibration for single-star measurements. Science observations in single-star mode are interspersed with the engineering activities, and include measurements of stellar diameters and binary-star parameters.

Constant-Term Laser Metrology
The constant-term laser metrology system on PTI measures the difference in pathlengths between the primary and secondary interferometer beam trains. This system has been upgraded in several ways. The most significant was moving the common corner cube for the primary and secondary beams at each pier from in front of the telescope to behind the dual-star beamsplitter. In this location, the corner cube can be centered on the optical axis so that the metrology, which is injected off-axis to avoid central obscurations, returns symmetric to the true optical axis, eliminating any coupling of wavefront tilt to pathlength. With this new configuration, a polarizer, which is needed to separate the beams from the primary and secondary metrology beams, can be moved to be in front of the corner cube, so that it is now common mode, eliminating a potential systematic error. Additional changes to the system include modifying the heterodyne frequencies to simply primary from secondary beam separation, and the use of dichroic beamsplitters for injection and extraction of the metrology signal.

Vacuum Pipes
To reduce the effects of turbulence on the laser metrology, it was decided to replace the current sealed 'air tubes' with vacuum tubes. The current tubes are 6 inch diameter insulated aluminum with quartz windows at each end to prevent air flow arising from temperature and pressure differences between the piers and the main building. The new tubes will be arriving this summer.

Single-Mode Fibers
A single-mode optical fiber has been added to the spectrometer feeds of the both primary and secondary beam combiners. The use of the fiber has significantly improved the calibration of our amplitude data. While we use a free-space beam combiner, the use of the fiber in the combined beam still provides considerable benefit, especially when the intensity ratio from the two arms is incorporated. This latter measurement is part of the observation sequence for each star.

Siderostat Accelerometers

To eliminate residual piston noise due to mount motions, we have added accelerometers to our primary siderostat mirrors. These measure accelerations normal to the siderostat mirror, which when integrated twice and combined with a model of the star-mirror geometry, provide a signal which can be fed forward to the delay lines for compensation. This feedback eliminates the effect of residual mount modes which could be seen in some data.

Beam Combiner Optics
The primary beam combiner has been upgraded with new achromats to allow operation at both H and K. The wider band will provide improved group-delay measurements, and enables observations which take advantage of color effects across the two bands.

Third Baseline

Most of the components to add a third baseline (to be combined with one of the other two siderostats) are in place; system integration will take place later in the year.

Visit the Palomar Interferometer Testbed web site at:http://huey.jpl.nasa.gov/palomar