question: What are lasers used for at observatories?
comment: One use is to detect atmospheric distortion of the light and adjust the telescope's mirrors to compensate. Another is to measure the Earth-Moon distance using retroreflectors left on the surface by Apollo astronauts.
passages: At the correlator station, the actual interferometer is synthesized by processing the digital signals using correlator hardware or software. Common correlator types are the FX and XF correlators. The current trend is towards software correlators running on consumer PCs or similar enterprise hardware. There also exists some amateur radio astronomy digital interferometers, such as the ALLBIN of the European Radio Astronomy Club.
   BULLET::::- Laser based lidar (LIght raDAR) technology has application in geology, seismology, remote sensing and atmospheric physics.
  
  BULLET::::- Lasers have been used aboard spacecraft such as in the Cassini-Huygens mission.
  
  BULLET::::- In astronomy, lasers have been used to create artificial "laser guide stars", used as reference objects for adaptive optics telescopes.
  
  Lasers may also be indirectly used in spectroscopy as a micro-sampling system, a technique termed Laser ablation (LA), which is typically applied to ICP-MS apparatus resulting in the powerful LA-ICP-MS.
  
  The principles of laser spectroscopy are discussed by Demtröder.
  
  Section::::Scientific.:Spectroscopy.
   In late 2010, in Italy, the seeded-FEL source FERMI@Elettra started commissioning, at the Sincrotrone Trieste Laboratory. FERMI@Elettra is a single-pass FEL user-facility covering the wavelength range from 100 nm (12 eV) to 10 nm (124 eV), located next to the third-generation synchrotron radiation facility ELETTRA in Trieste, Italy. The advent of femtosecond lasers has revolutionized many areas of science from solid-state physics to biology.
   Section::::Scientific.:Microscopy.
  
  Confocal laser scanning microscopy and Two-photon excitation microscopy make use of lasers to obtain blur-free images of thick specimens at various depths. Laser capture microdissection use lasers to procure specific cell populations from a tissue section under microscopic visualization.
  
  Additional laser microscopy techniques include harmonic microscopy, four-wave mixing microscopy and interferometric microscopy.
  
  Section::::Military.
  
  Military uses of lasers include applications such as target designation and ranging, defensive countermeasures, communications and directed energy weapons.
  
  Section::::Military.:Directly as an energy weapon.
   The most important factors in the selection of the pulsed light source for are the duration and intensity of the light. Pulsed lasers for the 300 to 500 nm range have now replaced spark caps in fluorescence spectroscopy. There are four general types of pulsing lasers used: lasers with pulsed excitation, lasers with G-switching, mode locked lasers and cavity dumped lasers. Pulsed nitrogen lasers (337 nm) have often been used as an excitation source in time resolved fluorometry.
   List of laser applications
  
  Many scientific, military, medical and commercial laser applications have been developed since the invention of the laser in 1958. The coherency, high monochromaticity, and ability to reach extremely high powers are all properties which allow for these specialized applications.
  
  Section::::Scientific.
  
  In science, lasers are used in many ways, including:
  
  BULLET::::- A wide variety of interferometric techniques"
  
  BULLET::::- Raman spectroscopy
  
  BULLET::::- Laser induced breakdown spectroscopy
  
  BULLET::::- Atmospheric "remote sensing"
  
  BULLET::::- Investigating nonlinear optics phenomena
  
  BULLET::::- Holographic techniques employing lasers also contribute to a number of measurement techniques.
   Section::::Colors and wavelengths.:Yellow.
  
  New 589 nm yellow laser pointers have been introduced using a more robust and of harmonic generation from a DPSS laser system. This "sodium" wavelength, although only 4.5 nm away from the older 593.5 nm, appears more gold in colour compared to the more amber appearance of the 593.5 nm wavelength. Astronomical observatories use a specially tuned dye laser at 589.2 nm (yellow) to create a laser guide star for use with astronomical adaptive optics.
  
  Section::::Colors and wavelengths.:Green.
   On December 8, 2014, it was announced that the lasers had been switched from ion lasers to solid-state lasers. This saves approximately 64 kilowatts of electric power per show. It also means that the laser will no longer need a water cooling system, which also reduced water consumption by thousands of gallons.
  
  Section::::Technical infrastructure.:Moving lights.
   Before the Gemini Planet Imager (GPI) was sent to Gemini South, it was necessary to calibrate its coronagraph. For this matter, a nearly achromatic and collimated source that could cover 0.95-2.4 µm was needed. Photon etc.’s efficient tunable laser source was chosen to test the coronagraph. The tunable source was able to provide an output across the whole GPI wavelength domain.
   Section::::Synergies.
  
  Aside from inherent mission capabilities (particularly sensitivities, which cannot be replicated by ground observatories), the Great Observatories program allows missions to interact for greater science return. Different objects shine in different wavelengths, but training two or more observatories on an object allows a deeper understanding.
   BULLET::::- Each year, they run a Practical Astronomy Course called "SASPAC".
  
  To assist with these activities, the society is authorized by the police to use high-powered lasers for astronomical purposes.
  
  Section::::History.
  
  The Society was formed in June 1961, and was then known as the "James Cook Astronomers Club" (JCAC), named after Captain James Cook, English explorer who first landed in Australia at Kurnell (a suburb in the Sutherland Shire) after successfully observing a transit of Venus from Tahiti.
   The NIROSETI instrument employs a new generation of near-infrared (900 to 1700 nm) detectors, cooled at -25 °C, that have a high speed response (1 GHz) and gain comparable to photomultiplier tubes, while also producing very low noise, and significantly reducing false positives. Its field-of-view is 2.5"x2.5" each, and focuses
  
  on detecting short (nanosecond) pulsed laser emissions. The NIROSETI instrument is also being used to study variability of very short natural near-infrared transient stars.
  
  Section::::Targets.
   Section::::History.
  
  The Vulcan is the first operational laser at the CLF. By 1997, when a new director was appointed, M. H. R. Hutchinson, formerly of Imperial College London, CLF was also operating a second laser, the Titania, at that time said to be the world brightest krypton fluoride laser.
  
  Section::::Current lasers.
  
  Section::::Current lasers.:Vulcan.
   Tractor/Repulsor Beams have been realized as Laser-based Optical tweezers, and more recently as a pair of Bessel beams. These instruments use the radiation from the laser beam to manipulate microscopic particles in what is called an "optical trap" along the length of the beam as desired.
  
  Fictional Tractor beams have been prominently used in the "Star Wars" universe and in the "Star Trek" universe. In an early scene of "" a large spaceship uses such a beam to seize a small one, in order to capture the protagonists.
   Being above the atmosphere, these space observatories can also avoid the effects of atmospheric turbulence that plague ground based telescopes, although new generations of adaptive optics telescopes have since then dramatically improved the situation on the ground. The space high vacuum environment also frees the detectors from the ancestral diurnal cycle due to the atmospheric blue light background of the sky, thereby increasing significantly the observation time.
   PROMPT (also known as Skynet) is run by the University of North Carolina. The Panchromatic Robotic Optical Monitoring and Polarimetry Telescopes is made up of 4 x 0.41 Ritchey Chretien telescopes is entirely robotic and queue based. They are able to have rapid and simultaneous multi-wavelength observations of gamma ray burst afterglows and other transient objects.
   Relatively cheap, mass-produced ~2 meter telescopes have recently been developed and have made a significant impact on astronomy research. These allow many astronomical targets to be monitored continuously, and for large areas of sky to be surveyed. Many are robotic telescopes, computer controlled over the internet (see "e.g." the Liverpool Telescope and the Faulkes Telescope North and South), allowing automated follow-up of astronomical events.
   List of astronomical interferometers at visible and infrared wavelengths
  
  Here is a list of currently existing astronomical optical interferometers (i.e. operating from visible to mid-infrared wavelengths), and some parameters describing their performance.
  
  Section::::Current performance of ground-based interferometers.
   Because even a low-powered green laser is visible at night through Rayleigh scattering from air molecules, this type of pointer is used by astronomers to easily point out stars and constellations. Green laser pointers can come in a variety of different output powers. The 5 mW green laser pointers (classes II and IIIa) are the safest to use, and anything more powerful is usually not necessary for pointing purposes, since the beam is still visible in dark lighting conditions.
   The LLE was founded on the University of Rochester's campus in 1970, by Dr. Moshe Lubin. Working with outside companies such as Kodak the team built Delta, a four beam laser system in 1972. Construction started on the current LLE site in 1976. The facility opened a six beam laser system in 1978 and followed with a 24 beam system two years later. In 2018, Donna Strickland and Gérard Mourou shared a Nobel prize for work they had undertaken in 1985 while at LLE. They invented a method to amplify laser pulses by "chirping" for which they would share the 2018 Nobel Prize in Physics. This method disperses a short, broadband pulse of laser light into a temporally longer spectrum of wavelengths. The system amplifies the laser at each wavelength and then reconstitutes the beam into one color. Chirp pulsed amplification became instrumental in building the National Ignition Facility and the Omega EP system. In 1995, the omega laser system was increased to 60 beams, and in 2008 the Omega extended performance system was opened.
   Other optical devices, such as spinning mirrors, modulators, filters, and absorbers, may be placed within the optical resonator to produce a variety of effects on the laser output, such as altering the wavelength of operation or the production of pulses of laser light.
   When the Apollo astronauts visited the moon, they planted retroreflector arrays to make possible the Lunar Laser Ranging Experiment. Laser beams are focused through large telescopes on Earth aimed toward the arrays, and the time taken for the beam to be reflected back to Earth measured to determine the distance between the Earth and Moon with high accuracy.
  
  Section::::Scientific.:Photochemistry.
   The three major components are the "Spectrometer Unit", the "Control and Excitation Unit" (includes the power converters), and "Optical head".
  
  Section::::Principle and operation.
   Research shows that scientists may one day be able to induce rain and lightning storms (as well as micro-manipulating some other weather phenomena) using high energy lasers. Such a breakthrough could potentially eradicate droughts, help alleviate weather related catastrophes, and allocate weather resources to areas in need.
  
  Section::::Scientific.:Lunar laser ranging.
   The calibration of measuring instruments (e.g. : photodetector, spectrometer) is essential if researchers want to be able to compare their results with those of different research groups and if we want to maintain high standards. Spectral calibration is often needed and requires a well-known source that can cover a wide part of the electromagnetic spectrum. Tunable laser sources possess all of the above requirements and are hence particularly appropriate for this type of calibration.
  