Haifa researchers breaking the telescope diffraction limit

The earliest-known telescope was built in 1608 in the Netherlands by eyeglass maker Hans Lippershey. Although he applied for a patent but didn’t receive one, news of the invention soon spread across Europe. 

The design of these early refracting telescopes consisted of a convex objective lens and a concave eyepieceGalileo improved on this design the following year and applied it to astronomy, and a couple of years later, Johannes Kepler described how a far-more-useful telescope could be made with a convex objective lens and a convex eyepiece lens. 

Sixty years after Lippershey’s achievement, Isaac Newton built with building the first reflector in 1668 with a design that included a small flat diagonal mirror to reflect the light to an eyepiece mounted on the side of the telescope. 

Since then, telescopes aimed at space have gone far, and thanks to researchers at the Technion-Israel Institute of Technology in Haifa, new developments could significantly improve the resolution of telescopes. 

The research, which was performed by doctoral student Gal Gumpel and supervised by Dr. Erez Ribak from the Technion’s physics department, was published in the Journal of the Optical Society of America B under the title “Optical amplification for astronomical imaging at higher resolution.” 

 

The resolution of a telescope – how sharp its images are – is the smallest angle between two observed objects, where they can still be visibly separated. The resolution limit is set by diffraction; light rays diffract and scatter around objects in their path – in this case the telescope mirror, as they travel to the focal plane, where the detector (camera) is.  

The original angle of the light is blurred, causing the observed object, in this case a star, to look like a fuzzy spot, so two nearby stars will look like overlapping fuzzy spots, which we cannot distinguish any more. 

 

There are two basic ways to reduce diffraction and improve resolution, as set by the uncertainty principle of quantum mechanics. One is reduction of the wavelength, for example by illuminating the object by blue light, instead of red; the other is to increase the telescope aperture. Since in astronomy, we observe natural light that is out of our control, we cannot reduce the wavelength, but we can increase the telescope aperture. In fact, the giant telescopes being built in recent decades provide very high resolution. In mid-sized telescopes or space telescopes, limited by launcher volume, resolution is still an issue.

These stimulated photons are identical to the original photon, both in direction and in wavelength. These “daughter photons” also obey the initial diffraction limit, but by their mere number they allow for a better measurement of the angle at which the original, astronomical photon has crossed the telescope aperture. This is an improvement on direct detection, which is based only on the original photon, without an amplifier, improves the resolution of the telescope without increasing its size.

Such light amplification has been disfavored because the stimulated emission is also accompanied by constant spontaneous emission by the same atoms. Large numbers of spontaneous photons are emitted in all directions, unlike the stimulated ones, creating a bright background, and reducing the achieved increase in resolution. 

As a result, Gumpel and Ribak had to separately measure also the spontaneous photons. In the lab experiment they blocked the “star” light part of the time, thus measuring only the background, while the rest of the time served to measure both stimulated and spontaneous photons. The image of the object was obtained by subtraction of the background image from the combined image, leaving only the clean image of the source. This is the first time such an experiment is performed with white light, since most light amplifiers, such as those in lasers,operate only at one wavelength. 


“One of the possible drawbacks of the method is the loss of sensitivity in the final images, but this is a worthy price to pay for the increased resolution,” the authors wrote. “Moreover, the loss of sensitivity can be overcome partially by increasing the exposure times, namely the observation period. We show that it is possible to increase the weight of the stimulated photons by considering photon statistics and observe an improvement in resolution. Most importantly, we demonstrate a method that can apply for all imaging purposes.”

 


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