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X-ray Interferometry


Webster Cash

Center for Astrophysics and Space Astronomy
University of Colorado
Boulder, CO 80309-0389


Before the time of Galileo, all scientists used the unaided eye to observe the heavens. When Galileo trained his tiny telescope on the sky, its magnification gave him the same view as if he had traveled 90% of the way to his target. With his 10 times improvement in angular scale, he saw a new view of the universe, and revolutionized astronomy. The unaided eye can see detail as fine as one arcminute, we shall approximate as 100" (arcseconds).  Galileo was able to resolve 10" features.

Over the ensuing three centuries telescopes improved, but were limited by the twinkling of the Earth's atmosphere. This "Seeing" limited all telescopes, no matter how well made, to about 1".  Until the latter half of the 20th Century, a scientist could use an instrument to show the view from a distance 100 times closer, but no further.

The development of long baseline radio interferometry showed the first images of the sky significantly finer than one arcsecond. The intercontinental baselines reached resolutions as fine as .001", revealing detail as if the observer were 100,000 times closer to the object. This revealed such unusual behavior as superluminal expansion. Unfortunately, radio sources tend to be dim or diffuse, so most classes of object could not be observed.

In the 1990's the Hubble Space Telescope (HST) became operational. After the correction of the spherical aberration, it achieved resolution of 0.1", ten times the resolution of the ground images. As observers it moved us from 99%  to 99.9% of the way to the target. The results have been spectacular, demonstrating how important an improvement in visual clarity, or alternatively, getting closer to the target, can be.

An observatory with sufficient resolution can provide images that make you feel like you are actually visiting another astronomical object. However, a factor of 100 to 1000 is insufficient for most astronomical objects. We to improve our resolution by many orders of magnitude if we are to visit the stars without a warp drive.

Two barriers stand in the way of super high resolution imaging.  First is the twinkling and absorption of the Earth's atmosphere, which we can circumvent by placing our observatories in space.  Second is the diffraction limit of telescopes. The resolution is limited by the number of wavelengths of light across the diameter of the primary optic. HST is diffraction limited in the visible at about 0.1". A factor of a few improvement can be realized by using a bigger mirror, but mirrors much larger than 10 meters across are impractical to build. Instead we resort to interferometry, coupling two or more telescopes together optically, to synthetically build an aperture equal to the separation of the telescopes. The Space Interferometry Mission (SIM) is a NASA project specifically designed to improve imaging of the sky.

We contend that the natural band for the very highest spatial resolution is the x-ray. X-rays have very short wavelengths, typically one thousandth that of visible light. This makes required baselines a thousand times smaller as well. Historically, x-ray optics have been expensive and low quality, effectively blocking progress. However, recent developments now show that x-ray interferometry is within our grasp with today's technology.

Exploring the Universe With X-rays                    Building an X-ray Interferometer