HERO will provide new view of x-ray universe: Replication process mass produces high-quality mirrors

July 08, 1999

July 8, 1999: X-ray astronomers in the 21st century may use a mass-produced HERO to give them a view of the sky at higher energies than the best telescopes can do today. The HERO - High-Energy Replicated Optics - telescope is a promising new development at NASA's Marshall Space Flight Center.

The production method coats a form, called a mandrel, with the right material and then pops the two apart. The resulting mirror only needs a little finishing work before going into the telescope, and the mandrel is reused to make another mirror. Tests with the first HERO mirrors are encouraging.

"We have tested our first 6-meter focal-length mirror," said Brian Ramsey, an X-ray astronomer at NASA/Marshall. "We got 33 arc-seconds resolution, a result we were very pleased with. This was our very first one, and I'm confident that we can do even better on subsequent mirrors."

Resolution is a measure of how much detail a telescope will show. For comparison, the apparent diameter of the Moon is about 1,860 arc-seconds, and the human eye has a resolution of about 60 arc-seconds. HERO's first test mirror would be able to see details 33 arc-sec (at what the astronomers half-power diameter), about 1/60th the apparent diameter of the Moon, or twice as fine as the human eye can see.

But that's a bit of an apples and oranges comparison since HERO will observe objects, both in our galaxy and beyond, that emit most of their energy in x-rays, far more energetic than what the human eye or even the Hubble Space Telescope can see. Objects from white dwarves and neutron stars to distant objects like quasars do some of their most interesting work in this part of the spectrum.

Turning up the brightness
X-ray astronomy started in the 1960s and has developed very quickly. The first satellite devoted to X-ray astronomy, named Uhuru, was launched in 1970 and was able to map a few hundred bright sources.

We are now poised to have large orbiting Observatories, such as Chandra, with a hundred thousand times the sensitivity of Uhuru, that can 'see' literally millions of galactic and extra-galactic sources.

"However, their mirrors work at low energies, less than 10 keV (10,000 electron volts) and we want to go beyond that," said Ramsey. By comparison, visible light has an energy of 1 to 2 eV.

"The region above 10 keV is extremely interesting. It's the transition region between thermal processes, where temperatures largely control emissions, and mechanisms like synchrotron radiation, X-rays given off by electrons spiraling along intense magnetic fields, and Compton scattering, when low-energy photons hit high-energy electrons and get boosted to X-rays."

HERO will be able to focus X-rays into images in the hard-X-ray region, 10 to 75 keV. To date, this range has been studied only by large area non-focusing detectors that have much coarser resolution than HERO can achieve, and much poorer sensitivity.

"Focusing is an extremely powerful tool that has revolutionized X-ray astronomy at lower energies," explained Ramsey. "By concentrating the X rays onto a very small area of the detector we prevent the signal from being overwhelmed by the background. This gives us an enormous increase in signal to noise. For example, the Chandra observatory and the detectors on Uhuru had roughly the same collecting area, so they would collect the same number of x rays. Chandra is 100,000 times more powerful because it focuses them to an extremely tiny point and this reduces background almost to zero."

X-rays are challenging to focus because they pass through most materials. However, if they strike a smooth surface at a shallow angle, the surface acts as a mirror. This "grazing incidence reflection" is why light glares off a piece of glass if you hold it almost edge-on.

It's all done with mirrors
In X-ray astronomy, the effect is put to work with mirrors that resemble tubes. The Chandra X-ray Observatory uses four primary mirrors nested within each other, and four nested secondary mirrors lined up precisely behind the primaries.

The mirrors follow mathematical curves - the parabola and hyperbola - derived by slicing through an imaginary cone at different angles. In Chandra, the mirrors are made of large pieces of special optical glass, meticulously polished to a precision shape and coated with iridium for reflectivity.

While Chandra will give us new vistas in the X-ray astronomy universe, with superb images in soft X rays, its vision fades sharply at higher energies - near 10 keV - where the angle of incidence must be ever shallower in order to be effective.

In order to get significant collecting areas at higher energies, many shallow-angle mirrors of successively smaller diameters must be nested. Each mirror has to be light weight and very thin, because there are many of them and they need to be packed tightly. This precludes the use of thick polished glass which would be too bulky and way too costly to fabricate in large numbers.

One approach, being pursued by other groups equally eager to explore this exciting energy range, is to develop mirrors from hundreds of curved foils, and to coat these foils to enhance high-energy reflectivity.

"This is a good technique and real progress is being made, but we'd like better resolution than these look like providing" commented Ramsey.

That's where HERO comes to the rescue for future missions. Ramsey and colleagues working in the Science Directorate at NASA/Marshall, together with Darell Engelhaupt of the University of Alabama in Huntsville, have utilized a replication method for manufacturing multiple high-quality copies of a mirror.

"In these mirrors, we're using two straight cones instead of curved surfaces," Ramsey explained. "This means the mirrors will provide images that are just a little out of focus, but it doesn't matter. The effect is very tiny, much smaller than the average resolution we expect to achieve. The departure is insignificant to the imaging properties."

It also simplifies manufacture with the replication method - the R in HERO - that Ramsey and Englehaupt have utilized.

Taking a spin, then taking a bath
They start with an aluminum mandrel - a mold like the centerpiece of an angelfood cake - that is machined into part of a double shallow cone (called frustra since these cones don't reach a point). It is coated with nickel and superpolished to a roughness of less than half a nanometer, about the thickness of a few atoms. Ramsey said that Raytheon engineer Chet Speegle has refined the polishing process from three months to one month for a mandrel.

Next, the mandrel is immersed in a chemical bath to electroplate it with a shell of a proprietary nickel alloy to a thickness of about 0.25 mm (0.010 in).

"It's what's called a glassy nickel," Ramsey explained. "The shell material behaves more like a ceramic than a metal. It's brittle and will not bend. That's ideal for a mirror because it means it does not permanently deform it as pure nickel would. We developed this material to satisfy future needs for large-area, high-resolution X-ray optics."

The electroforming takes about three days but may be shortened to less than a day as the process is improved. Further, several mandrels can be electroplated at a time, so a complete set of nested mirrors could be made at once out of the same bath.

Finally, the mandrel is cooled so it contracts and the electroplated shell pulls away, forming the new mirror, something that conventional electroformed pure-nickel mirrors would not survive without deforming. The HERO mirror is then vacuum coated with iridium, a hard reflective metal, and is ready for assembly into a nested array to make a telescope.

The HERO team recently completed its first mirror and tested it in the 100-meter (328 ft) beam tube at MSFC. Based on the accuracy and finish of the mandrel, the team predicted that the mirror would provide a resolution of 28 arc-seconds. It achieved 33.

"That shows that everything in the process worked well and that we replicated the shape and the surface qualities of the mandrel."

Ramsey now is aiming for the first HERO test flight aboard a balloon-borne observatory jointly operated with Harvard College Observatory. Speegle is completing those mandrels now. At the same time, other teams in the Science Directorate are developing the rest of the HERO flight package. Lead Engineer Jeff Apple and software lead Kurt Dietz are developing the observatory pointing and data handling systems, while Cheryl Alexander is completing a unique star-tracker camera that will give fine pointing information during both day and night observations. In parallel with this, Robert Austin of the Universities Space Research Association is developing special detectors, containing rare gas at high pressures, that will sit at the focus of each mirror module.

Multiple mirrors for a better view
NASA/Marshall's payload on the first flight will comprise two 3-meter (10-ft) focal length telescopes, each with a pair of nested mirrors.

"This flight will allow us to check everything out, particularly the new high-accuracy pointing system that is critical for realizing the full potential of the X-ray mirrors," Ramsey said. In three years, the team plans a flight of 16 6-meter telescopes, each composed of twelve sets of mirrors - a total of 192 mirrors - ranging from 5 cm (2 in) to 7 cm (2.75 in) in diameter.

Beyond that, the HERO team hopes for a 10-day flight on a long-duration balloon, and, eventually to fly the payload on ultra-long-duration flights as NASA is developing the capability to fly payloads on 100-200-day missions with super-pressure balloons. "This offers the exciting possibility of balloon flights competing with satellites at a fraction of the cost," said Ramsey. "We can do this in hard x-rays as the remaining atmosphere is reasonably transparent, so we get good observations, just currently limited in time. A 100 to 200-day flight would offer unprecedented sensitivity, and carry the X-ray optics 'revolution' into this important, yet relatively unexplored, energy range."

NASA/Marshall Space Flight Center--Space Sciences Laboratory

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