FOCAL (an acronym for ''Fast Outgoing Cyclopean Astronomical Lens'') is a proposed

space telescope A space telescope or space observatory is a telescope in outer space used to observe astronomical objects. Suggested by Lyman Spitzer in 1946, the first operational telescopes were the American Orbiting Astronomical Observatory, OAO-2 launch ...

that would use the Sun as a gravity lens. The gravitational lens effect was first derived by

Albert Einstein Albert Einstein ( ; ; 14 March 1879 – 18 April 1955) was a German-born theoretical physicist, widely acknowledged to be one of the greatest and most influential physicists of all time. Einstein is best known for developing the theor ...

, and the concept of a mission to the solar gravitational lens was first suggested by professor Von Eshleman, and analyzed further by Italian astronomer Claudio Maccone and others. In order to use the Sun as a gravity lens, it would be necessary to send the telescope to a minimum distance of 550

astronomical unit The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to or 8.3 light-minutes. The actual distance from Earth to the Sun varies by about 3% as Earth orbi ...

s away from the Sun, enabling very high signal amplifications: for example, at the 203 GHz wavelength, amplification of 1.3·10¹⁵. Maccone suggests that this should be enough to obtain detailed images of the surfaces of

extrasolar planet An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, init ...

Other uses of the mission

Even without using the Sun as the lens, FOCAL could perform various, otherwise impossible measurements: a separate telescope could be used to measure stellar distances by

parallax Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight and is measured by the angle or semi-angle of inclination between those two lines. Due to foreshortening, nearby object ...

, which would, using the baseline of 550 AU, measure the precise position of every star in the

Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked ey ...

, enabling various further scientific discoveries. It could also study the

interstellar medium In astronomy, the interstellar medium is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstella ...

, the

heliosphere The heliosphere is the magnetosphere, astrosphere and outermost atmospheric layer of the Sun. It takes the shape of a vast, bubble-like region of space. In plasma physics terms, it is the cavity formed by the Sun in the surrounding interst ...

, observe

gravitational wave Gravitational waves are waves of the intensity of gravity generated by the accelerated masses of an orbital binary system that propagate as waves outward from their source at the speed of light. They were first proposed by Oliver Heaviside in ...

s, check for the possible variation of the gravitational constant, observe the cosmic infrared background, characterise interplanetary dust within the

Solar System The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...

, more precisely measure the mass of the Solar System and similar.

Limitations

FOCAL does not require any non-existing technology; however, it has various limitations. A space mission of this duration and distance has never been attempted; for comparison, the

Voyager 1 ''Voyager 1'' is a space probe launched by NASA on September 5, 1977, as part of the Voyager program to study the outer Solar System and interstellar space beyond the Sun's heliosphere. Launched 16 days after its twin '' Voyager 2'', ''Voy ...

and

Voyager 2 ''Voyager 2'' is a space probe launched by NASA on August 20, 1977, to study the outer planets and interstellar space beyond the Sun's heliosphere. As a part of the Voyager program, it was launched 16 days before its twin, ''Voyager 1'', on ...

probes are at distances of 147 AU and 122 AU in 2019. A gravity lens will bend objects behind it, so that images from the telescope would be difficult to interpret. FOCAL would be able to observe only objects that are right behind the Sun from its point of view, which means that for every observed object a new telescope would have to be made. A critique of the technology of the gravity lens telescope was given by Landis.Landis, Geoffrey A., “Mission to the Gravitational Focus of the Sun: A Critical Analysis,” paper AIAA-2017-1679, AIAA Science and Technology Forum and Exposition 2017, Grapevine TX, January 9–13, 2017
Preprint
at arXiv.org (accessed 24 December 2016). Some of the problems Landis points out include discussion of the interference of the

solar corona A corona ( coronas or coronae) is the outermost layer of a star's atmosphere. It consists of plasma. The Sun's corona lies above the chromosphere and extends millions of kilometres into outer space. It is most easily seen during a total sola ...

, which will make the telescope signal-to-noise ratio poor, the high magnification of the target, which will make the design of the mission focal plane difficult, and an analysis of the inherent

spherical aberration In optics, spherical aberration (SA) is a type of aberration found in optical systems that have elements with spherical surfaces. Lenses and curved mirrors are prime examples, because this shape is easier to manufacture. Light rays that strike a ...

of the lens will limit the resolution possible.

References

External links

* * {{Space observatories Space telescopes Proposed spacecraft Gravitational lensing