ALGOR FEA SOFTWARE HELPS DESIGN SPACE-BASED TELESCOPES

A computer graphic illustration of the Hubble Space Telescope in orbit. Drawing provided by the Space Telescope Science Institute.

One of the most exciting fields of study in astronomy is the search for planets outside the solar system, especially planets that may be similar to Earth. Locating such planets would not only be important for confirming theories of planet formation, but also would be of tremendous philosophical interest. The recently orbited Hubble Space Telescope is designed to provide information of the type that may lead to the discovery of extrasolar planets.

Pierre Bely, an engineer with the Space Telescope Science Institute, is one of the men working on the next generation of powerful space telescopes. With the help of Algor's Finite Element Analysis (FEA) system, Bely hopes to battle problems that continue to prevent advanced telescopes, like the Hubble, from doing all that they can to locate extrasolar planets.

As Bely explains, "The detection of possible planets around nearby stars is a very challenging problem because of the extremely high intensity of the light from stars compared to that of planets, and the high resolution needed to separate a star from any accompanying planets. The Hubble Telescope will most-likely be unable to detect extrasolar planets for two reasons.

"The first reason is that detection is best achieved in the near infrared, say 10 micron wavelength, as opposed to visible wavelengths, 0.5 microns. It is in the near infrared that the ratio of the flux of the planet to that of the star is the highest, thus making detection easiest. By design, the Hubble is a 'warm telescope'. Most of its components are temperature controlled to about 20 degrees Celsius. As a result, it naturally 'radiates' in the infrared, essentially to the same wavelength as that of a planet. The Hubble's own radiation blocks any attempt to detect the very faint radiation of distant planets.

"The second problem is caused by 'microroughness' in the optics of the Hubble. Although the telescope's mirrors are essentially perfect, they do have residual polishing defects which create a minute scatter of any light hitting them - a similar effect can be noticed while looking at a bright object with dirty eyeglasses. For most astronomical observations, this effect is too small to be observed. However, in the case of detecting extrasolar planets, the scatter is obvious. Judging by our own solar system, the intensity of a star at visible wavelengths is typically 10 billion times higher than that of an earth-like planet. And of course, seen from the distance of the earth, any planet around even the closest stars will appear extremely close to the star. It takes incredibly smooth optics to avoid the light from the parent star as it 'spills over' into the area where the planets may be. Unfortunately, the superb optics of the Hubble are still not good enough to avoid this problem."

Pierre Bely (left) of the Space Telescope Science Institute and Johns Hopkins University student Steve Chong use Algor to study space telescopes of the 21st century.

Bely's approach to the problem is to work in the near infrared with a large diameter telescope. Working in the infrared improves the chances of detection by a factor of 1,000 to 10,000. To be able to work in the infrared, the telescope must be passively cooled to about 70-100 degrees Kelvin. This is done by shielding the telescope from the sun and allowing it to radiate against the sky.

Using a large diameter telescope increases the resolution (the apparent separation between the parent star and the planet), thus reducing the intensity of the spill over. A high resolution necessitates a large diameter, but does not mean that the collecting aperture must be filled with optics. In other words, it is sufficient to have a small number of mirror segments distributed over a 20 meter circle instead of a fully covered 20 meter circle. This "incomplete" telescope is called an "interferometer".

Bely, along with Space Telescope Science Institute intern Steve Chong, designed a proposed interferometer with Algor software. The interferometer has an aperture composed of twelve 1.2 meter mirrors on a 20 meter ring. The subaperatures are distributed to optimize the image when the interferometer is rotated about its axis.

A proposed 20-meter diameter space interferometer modeled with Algor. The backbone structure is composed of a very thin membrane which is folded for launch, deployed by inflation, and chemically rigidified by exposure to the sun. The bottom torus supports the 12 mirrors of the interferometer which are actively controlled to compensate for thermal deformation and background vibration of the structure. The interferometer is shielded from the sun by a screen (not shown) and passively cooled to about 70 degrees Kelvin by radiation against the sky. The high resolution of the instrument combined with its low temperature could permit infrared detection of earth-like planets around nearby stars.

"We are not in the business of designing or building scientific spacecraft," explains Bely. "Instead we need to obtain quick and realistic answers on the feasibility of various concepts which are proposed for future space-based astronomical observatories. We find Algor the ideal tool for such preliminary studies. With Algor we can rapidly model a proposed concept, size its backbone structure and obtain the order of magnitude of its natural frequency, launch stresses, thermal behavior, and its overall mass.

"When designing the interferometer we first built a beam model, which was used to perform a preliminary sizing of the members, and obtain an estimate of the overall mass and inertias and of the modal frequencies," says Bely. "We then built the membrane model. Partial modeling confirmed the validity of the beam model for individual members. In the future, I intend to investigate buckling, modal frequencies, and thermal effects on the interferometer in more detail."

Other space-based concepts Bely has analyzed with Algor include:

• an ultra-lightweight mirror with thermal actuators for a large infrared telescope.
• a high resolution X-ray telescope.
• a 16-meter telescope which may be located on the moon in the next century.

"We work exclusively in the metric system and like the "unit-independent" configuration of Algor processors," Bely continues. "This is important for us since our applications are in gravity-free environment, space, or on the moon where gravity is only 1/6 of what it is on Earth."

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