Twinkle twinkle little star, all you like, though near or far

The ability of telescopes to finely resolve images from space is compromised by the presence of the atmosphere, which produces the twinkling effect that we see when we stare up at a star. That is why astronomical observatories tend to be situated on islands and/or at high altitudes. Less atmosphere means less atmospheric distortion. The amount of distortion is quantifiable and the term astronomical seeing has emerged to describe the effect of atmosphere on image. A star is said to twinkle, or scintillate as its brightness fluctuates.

It can be seen then that when imaging a point source of light such as a star the exposure time should be as short as possible in order to produce the most coherent image. As the distortion effects change over time, the resulting image forms a speckle pattern in a short exposure; a blurred image of the star during a long exposure. Space-based telescopes such as Hubble have no seeing problems.

Computerised speckle image processing can offer very high resolution of point-source observations of space and in fact have superior resolution of bright sources when seen from Earth than does Hubble, because of that telescope's smaller diameter mirror. However, Hubble allowed us for the first time to view faint images with great clarity of objects which are impossible to see from Earth.

The highest resolution astronomical images possible are made using astronomical interferometers. An astronomical interferometer is an array of telescopes or mirror segments working together. U.S. Route 60 passes through The Very Large Array located about 80 km west of Socorro, New Mexico. The VLA stands at over 2 km above sea level and is comprised of 27 large radio antennae each with a 25 m diameter. The antennae are situated on rails formed into a Y pattern and can be moved around to preset baseline configurations. A baseline is the effective separation between any two telescopes as seen from the radio source. The VLA's 27 radio telescopes give 351 independent baselines at once. In addition, the rotation of the Earth moves the telescopes to new baselines. Aperture Synthesis is the term given to combining all the telescope data into a single image. Very Long Baseline Interferometry transforms data obtained from telescopes thousands of km apart. Advanced computational algorithms can now be used to transform data from irregularly spaced baselines using whatever data is available in a process called synthesis imaging.

The largest optical telescope arrays consist of 6 telescopes with 15 baselines. These offer poorer image resolution than large array radio telescopes. The challenges of creating an optical array are formidable, since light waves are a million times shorter than radio waves. However, it is possible to achieve better than Hubble resolution at a thousandth the cost. The Cambridge Optical Aperture Synthesis Telescope threatens Hubble's dominance in the field of astronomical imaging.

NASA's planned James Webb Space Telescope is currently undergoing cryogenic testing at the Marshall Space Flight Center. 18 individual mirror segments form a 6.5 m mirror assembly protected by a tennis-court sized sunshield. It is a large infrared-optimized space telescope designed to detect the heat of cosmic bodies billions of light years away. It will also be used to observe stars at visible wavelengths. Although its mass is about half that of Hubble, it will be almost 6 times larger in diameter.

A very large array of James Webb Space telescopes would provide the ultimate in astronomical observatories allowing us to clearly see Earth-like planets orbiting stars hundreds of light years away. I predict that a star that twinkles when viewed from space is a star whose system includes a water-bound planet like Earth, and that the scintillation will be caused by the star's light reflecting from and refracting through that world's oceans.