# Keynote lectures/Telescopes for radiation astronomy

The Schmidt Telescope at the former Brorfelde Observatory is now used by amateur astronomers. Credit: Mogens Engelund.

A radiation telescope is an instrument designed to collect and focus radiation so as to make distant sources appear nearer.

## Astronomy

Sunset over four telescopes of the Mauna Kea Observatories is pictured, from left to right: the Subaru Telescope, the twin Keck I and II telescopes, and the NASA Infrared Telescope Facility. Credit: Alan L.

Astronomy may be accomplished by observation using personal senses, or augmented with the use of instruments. The telescope itself can be moved on or by vehicles along the ground, on water, in the air, or above the Earth's atmosphere, and throughout the nearby interplanetary medium.

Observational astronomy benefits from electronics, mechanisms, tools, and machinery.

"With its high altitude, dry environment, and stable airflow, Mauna Kea's summit is one of the best sites in the world for astronomical observation [at left], and one of the most controversial. Since the creation of an access road in 1964, thirteen telescopes funded by eleven countries have been constructed at the summit. The Mauna Kea Observatories are used for scientific research across the electromagnetic spectrum from visible light to radio, and comprise one of the world's largest facilities of their type. Their construction on a "sacred landscape",[1] replete with endangered species and ongoing cultural practices, continues to be a topic of debate and protest. Studies are underway to determine their effect on the summit ecology, particularly on the rare Wēkiu bug. It was designated a National Natural Landmark in 1972.[2]

There are nine telescopes working in the visible and infrared spectrum, three in the submillimeter spectrum, and one in the radio spectrum, with mirrors or dishes ranging from 0.9 m (3.0 ft) to 25 m (82 ft).[3]

In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space.

Def. an action or process of throwing or sending out a traveling ray in a line, beam, or stream of small cross section is called radiation.

Def. the "shooting forth of anything from a point or surface, like the diverging rays of light; as, the radiation of heat"[4] is called radiation.

Radiation that a particular telescope or a telescope array observes consists of fast moving entities from which information is gathered using spectroscopy, spatial distributions, or temporal distributions. A galaxy cluster that is moving is radiation and an astronomical object to be observed. Entities moving faster than the galaxy such as protons or photons are observables.

## Astrodesy

On Earth, telescopes are positioned using geodesy, such fields as surveying, structural geology of the underlying ground, and architecture. The availability of manpower is usually missing for extraterrestrial observatories on the Moon, Mars, or Venus. On the International Space Station, manpower is often available for instrument control and use.

## Instruments

This is an optical telescope that may be used for optical and visual astronomy. Credit: .

Def. any instrument used in astronomy for observing distant objects is called a telescope.

Def. any instrument used in astronomy for observing distant objects (such as a radio telescope) is called a telescope, or an astronomical telescope.

## Aerial telescopes

An engraving of Huygens 210-foot aerial telescope showing the eyepiece and objective mounts and connecting string. Credit: .

An aerial telescope is a type of very-long-focal-length refracting telescope built in the second half of the 17th century that did not use a tube.[5] Instead, the objective was mounted on a pole, tree, tower, building or other structure on a swivel ball-joint. The observer stood on the ground and held the eyepiece, which was connected to the objective by a string or connecting rod. By holding the string tight and maneuvering the eyepiece, the observer could aim the telescope at objects in the sky.

“After about 1675, therefore, astronomers did away with the telescope tube. The objective was mounted on a building or pole by means of a ball-joint and aimed by means of a string.”[5]

## Early telescopes

This image is a photo of the Nimrud lens in the British museum. Credit: Geni.

"There are indeed ancient tablets that mention astronomers' lenses supported by a golden tube to enlarge the pupil, and in Nineveh a rock crystal lens was found (Pettinato 1998). Maybe one day a new archaeological excavation will find a Babylonian telescope for the first time."[6]

## Optics

Optics involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it.[7] Optics usually describes the behavior of visible, ultraviolet, and infrared light. Because light is an electromagnetic wave, other forms of electromagnetic radiation such as X-rays, microwaves, and radio waves exhibit similar properties.[7]

## Colors

"[B]roadband optical photometry of Centaurs and Kuiper Belt objects from the Keck 10 m, the University of Hawaii 2.2 m, and the Cerro Tololo InterAmerican (CTIO) 1.5 m telescopes [shows] a wide dispersion in the optical colors of the objects, indicating nonuniform surface properties. The color dispersion [may] be understood in the context of the expected steady reddening due to bombardment by the ubiquitous flux of cosmic rays."[8]

## Minerals

This shows a colorless and very clean quartz that is transparent. Credit: Zimbres.

Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. Pure quartz, traditionally called rock crystal (sometimes called clear quartz), is colorless and transparent or translucent.

"Bombardment by protostellar cosmic rays may make the rock precursors of [Calcium-aluminum-rich inclusions] CAIs and chondrules radioactive, producing radionuclides found in meteorites that are difficult to obtain with other mechanisms."[9]

"Ice cores contain thin nitrate-rich layers that can be analyzed to reconstruct a history of past events before reliable observations; [this includes] data from Greenland ice cores[10] and others. These show evidence that events of [the magnitude of the solar storm of 1859—as measured by high-energy proton radiation, not geomagnetic effect—occur approximately once per 500 years, with events at least one-fifth as large occurring several times per century.[11] Less severe storms have occurred in 1921 and 1960, when widespread radio disruption was reported.

Ordinary glass is partially transparent to UVA but is opaque to shorter wavelengths, whereas silica or quartz glass, depending on quality, can be transparent even to vacuum UV wavelengths. Ordinary window glass passes about 90% of the light above 350 nm, but blocks over 90% of the light below 300 nm.[12][13][14]

Def. the manufacture and use of telescopes is called telescopy.

## Sources

This image shows a beam of accelerated ions (perhaps protons or deuterons) escaping the accelerator and ionizing the surrounding air causing a blue glow. Credit: Lawrence Berkely National Laboratory.
The image shows the blue glow given off by the synchrotron beam from the National Synchrotron Light Source. Credit: NSLS, Brookhaven National Laboratory.

The image above shows a blue glow in the surrounding air from emitted cyclotron particulate radiation.

At right is an image that shows the blue glow resulting from a beam of relativistic electrons as they slow down. This deceleration produces synchrotron light out of the beam line of the National Synchrotron Light Source.

## Bands

This is a 3D model of the Rosetta Spacecraft. The individual scientific payloads are highlighted in different colours. Credit: IanShazell.

For elongated dust particles in cometary comas an investigation is performed at 535.0 nm (green) and 627.4 nm (red) peak transmission wavelengths of the Rosetta spacecraft's OSIRIS Wide Angle Camera broadband green and red filters, respectively.[15]

## Backgrounds

The frame demonstrates an example of visual snow-like noise. Credit: .

In astronomical CCD technology, background is usually referred to the overall optical "noise" of the system, that is, the incoming light on the CCD sensor in absence of light sources. This background can originate from electronic noise in the CCD, from not-well-masked lights nearby the telescope, and so on. An exposure on an empty patch of the sky is also called a background, and is the sum of the system background level plus the sky's one.

A background frame is often the first exposure in an astronomical observation with a CCD: the frame will then be subtracted from the actual observation result, leaving in theory only the incoming light from the astronomical object being observed.

## Meteor telescopes

The image shows a standard rain gauge. Credit: .

Meteor telescopes per se are often other types of telescopes, such as optical telescopes, that happen or are slewed to observe meteors.

At right is a collection device for rain on Earth as part of meteorology.

There are favorable locations on Earth, Moon and Mars where meteorites are discovered. These meteorite, or micrometeorite, locations include Antarctica and the equatorial deserts.

## Cosmic-ray telescopes

This is an image of HEAO 3. Credit: .
The charged particle instrument (CPI) is used to detect cosmic rays in the solar system. Credit: NASA.
The cosmic-ray telescope collects data on the composition of the cosmic ray particles and their energy ranges. Credit: NASA.

The [HEAO 3, at right, French-Danish] C-2 experiment measured the relative composition of the isotopes of the primary cosmic rays between beryllium and iron (Z from 4 to 26) and the elemental abundances up to tin (Z=50). Cerenkov counters and hodoscopes, together with the Earth's magnetic field, formed a spectrometer. They determined charge and mass of cosmic rays to a precision of 10% for the most abundant elements over the momentum range from 2 to 25 GeV/c (c=speed of light).

"Recent measurements using the Goddard-University of New Hampshire cosmic-ray telescope [at left] on the Pioneer 10 spacecraft have revealed an anomalous spectrum of nitrogen and oxygen nuclei relative to other nuclei such as He and C, in the energy range 3-30 MeV per nucleon."[16]

## Neutron telescopes

The Imaging Compton Telescope (COMPTEL) utilizes the Compton Effect and two layers of gamma-ray detectors. Credit: NASA.

"In addition to observing gamma rays from a solar flare, [ the Imaging Compton Telescope] COMPTEL is also capable of detecting solar neutrons. Neutron interactions within the instrument occur when an incident solar neutron elastically scatters off a hydrogen nucleus in the liquid scintillator of an upper D1 module. The scattered neutron may then interact and deposit all or a portion of its energy in one of the lower D2 modules, providing the internal trigger signal necessary for a double scatter event. The energy of the scattered neutron is deduced from its time of flight from the upper to lower detector, which is summed with the energy measured for the recoil proton in the upper D1 module to obtain the energy of the incident solar neutron. The computed scatter angle of the neutron, as with gamma rays, yields an event circle on the sky, which can be further constrained since the true source of the detected neutrons is assumed to be the Sun."[17]

## Electron telescopes

This is an image of the Energetic Particles Detector (EPD) aboard the Galileo Orbiter. Credit: NASA.

"[The] two bi-directional, solid-state detector telescopes [of the Galileo Orbiter are] mounted on a platform which [is] rotated by a stepper motor into one of eight positions. This rotation of the platform, combined with the spinning of the orbiter in a plane perpendicular to the platform rotation, [permits] a 4-pi [or 4π] steradian coverage of incoming [electrons]. The forward (0 degree) ends of the two telescopes [have] an unobstructed view over the [4π] sphere or [can] be positioned behind a shield which not only [prevents] the entrance of incoming radiation, but [contains] a source, thus allowing background corrections and in-flight calibrations to be made. ... The 0 degree end of the [Low-Energy Magnetospheric Measurements System] LEMMS [uses] magnetic deflection to separate incoming electrons and ions. The 180 degree end [uses] absorbers in combination with the detectors to provide measurements of higher-energy electrons ... The LEMMS [provides] measurements of electrons from 15 keV to greater than 11 MeV ... in 32 rate channels."[18]

## Positron telescopes

Observation of positrons from a terrestrial gamma ray flash is performed by the Fermi gamma ray telescope. Credit: NASA Goddard Space Flight Center.

The image at right contains a picture of the Fermi gamma-ray telescope that performed observations of positrons from their terrestrial gamma-ray flashes.

The positrons are not directly observed by the INTEGRAL space telescope, but "the 511 keV positron annihilation emission is".[19]

## Neutrinos telescopes

An artist illustration of the Antares neutrino detector and the Nautile. Credit: .
This is an architecture diagram of IceCube. Credit: Nasa-verve.

ANTARES [illustrated at right] is the name of a neutrino detector residing 2.5 km under the Mediterranean Sea off the coast of Toulon, France. It is designed to be used as a directional Neutrino Telescope to locate and observe neutrino flux from cosmic origins in the direction of the Southern Hemisphere of the Earth, a complement to the southern hemisphere neutrino detector IceCube that detects neutrinos from the North.

The IceCube Neutrino Observatory (or simply IceCube) [at left] is a neutrino telescope constructed at the Amundsen-Scott South Pole Station in Antarctica.[1] Similar to its predecessor, the Antarctic Muon And Neutrino Detector Array (AMANDA), IceCube contains thousands of spherical optical sensors called Digital Optical Modules (DOMs), each with a photomultiplier tube (PMT)[20] and a single board data acquisition computer which sends digital data to the counting house on the surface above the array.[21] IceCube was completed on 18 December, 2010, New Zealand time.[22]

## Gamma-ray telescopes

The Imaging Compton Telescope (COMPTEL) utilizes the Compton Effect and two layers of gamma-ray detectors. Credit: NASA.
The Fermi Gamma-ray Space Telescope sits on its payload attachment fitting. Credit: NASA/Kim Shiflett.

The Imaging Compton Telescope, (COMPTEL) by the Max Planck Institute for Extraterrestrial Physics, the University of New Hampshire, Netherlands Institute for Space Research, and ESA's Astrophysics Division was tuned to the 0.75-30 MeV energy range and determined the angle of arrival of photons to within a degree and the energy to within five percent at higher energies. The instrument had a field of view of one steradian. For cosmic gamma-ray events, the experiment required two nearly simultaneous interactions, in a set of front and rear scintillators. Gamma rays would Compton scatter in a forward detector module, where the interaction energy E1, given to the recoil electron was measured, while the Compton scattered photon would then be caught in one of a second layer of scintillators to the rear, where its total energy, E2, would be measured. From these two energies, E1 and E2, the Compton scattering angle, angle θ, can be determined, along with the total energy, E1 + E2, of the incident photon. The positions of the interactions, in both the front and rear scintillators, was also measured. The vector, V, connecting the two interaction points determined a direction to the sky, and the angle θ about this direction, defined a cone about V on which the source of the photon must lie, and a corresponding "event circle" on the sky.

"COMPTEL's upper layer of detectors are filled with a liquid scintillator which scatters an incoming gamma-ray photon according to the Compton Effect. This photon is then absorbed by NaI crystals in the lower detectors. The instrument records the time, location, and energy of the events in each layer of detectors which makes it possible to determine the direction and energy of the original gamma-ray photon and reconstruct an image and energy spectrum of the source."[23]

The Large Area Telescope (LAT) [of the Fermi Gamma-ray Space Telescope ] detects individual gamma rays using technology similar to that used in terrestrial particle accelerators. Photons hit thin metal sheets, converting to electron-positron pairs, via a process known as pair production. These charged particles pass through interleaved layers of silicon microstrip detectors, causing ionization which produce detectable tiny pulses of electric charge. Researchers can combine information from several layers of this tracker to determine the path of the particles. After passing through the tracker, the particles enter the calorimeter, which consists of a stack of caesium iodide scintillator crystals to measure the total energy of the particles. The LAT's field of view is large, about 20% of the sky. The resolution of its images is modest by astronomical standards, a few arc minutes for the highest-energy photons and about 3 degrees at 100 MeV. The LAT is a bigger and better successor to the EGRET instrument on NASA's Compton Gamma Ray Observatory satellite in the 1990s.

"For X-rays, the index of refraction is defined by Rayleigh scattering,"[24] especially in the use of Wolter telescopes.

"[T]he strength of the effect drops off as the inverse square of the X-ray energy. This means that at high X-ray energies – and on into low gamma-ray energies – the radiation is not bent enough for a lens to work effectively."[24]

"[T]he index of refraction starts to make a comeback at energies greater than about 700 keV. What is more, while the index of refraction is negative for X-rays, it becomes positive for gamma rays."[24]

"What is new now is that with gamma rays we can really address the extremely high electric field of the nucleus," with Delbrück scattering.[25]

"The measurements indicate that there exists an index of refraction for gamma-ray energies that is substantially larger than people believed before".[26]

"Materials with nuclei that have a large positive charge – such as gold – should be ideal for making gamma-ray lenses".[24]

## X-ray telescopes

The XRT uses a grazing incidence Wolter 1 telescope to focus X-rays onto a state-of-the-art CCD. Credit: .

X-ray telescopes can use a variety of different designs to image X-rays. The most common methods used in X-ray telescopes are grazing incidence mirrors and coded apertures. The limitations of X-ray optics result in much narrower fields of view than visible or UV telescopes.

An extreme example of a reflecting telescope is demonstrated by the grazing incidence X-ray telescope (XRT) of the Swift satellite that focuses X-rays onto a state-of-the-art charge-coupled device (CCD), in red at the focal point of the grazing incidence mirrors (in black at the right).

A Wolter telescope is a telescope for X-rays using only grazing incidence optics. X-rays mirrors can be built, but only if the angle from the plane of reflection is very low (typically 10 arc-minutes to 2 degrees)[27]. These are called glancing (or grazing) incidence mirrors. In 1952, Hans Wolter outlined three ways a telescope could be built using only this kind of mirror.[28][29]. Not surprisingly, these are called Wolter telescopes of type I, II, and III. Each has different advantages and disadvantages.[30]

## Optical telescopes

The Hubble Space Telescope is seen from the departing Space Shuttle Atlantis, flying Servicing Mission 4 (STS-125), the fifth and final human spaceflight to visit the observatory. Credit: Ruffnax (Crew of STS-125).
Mt.Palomar's 200-inch Telescope, pointing to the zenith, is seen from the east side. Note the person standing below the telescope (center-right at the bottom of the image). Credit: NASA.

Def. a monocular optical instrument possessing magnification for observing distant objects is called a telescope.

The Hubble Space Telescope (HST) is an excellent example of a radiation astronomy satellite designed for more than one purpose: the various astronomies of optical astronomy.

The HST is an optical astronomy telescope that “incorporated a set of 48 filters isolating spectral lines of particular astrophysical interest.

Most radiation telescopes, especially optical telescopes, combine a variety of lenses, mirrors, active and adaptive optics, filters, detectors, mounts, image processing, and observatories, in many locations.

## Active optics

Actuators are part of the active optics of the Gran Telescopio Canarias. Credit: .

Active optics is a technology used with reflecting telescopes developed in the 1980s[31], which actively shapes a telescope's mirrors to prevent deformation due to external influences such as wind, temperature, mechanical stress. Without active optics, the construction of 8 metre class telescopes is not possible, nor would telescopes with segmented mirrors be feasible.

Telescopes built since the 1980s use very thin mirrors which are too thin to keep themselves rigidly in the correct shape. Instead, an array of actuators behind the mirror keeps it in an optimal shape. The telescope may also be segmented into many small mirrors, preventing most of the gravitational distortion that occurs in large, thick mirrors. The combination of actuators, a quality-of-image detector, and a real-time computer program to move the actuators to obtain the best possible image is termed active optics. The name active optics means that the system keeps a mirror (usually the primary) in its optimal shape against all environmental factors such as gravity (at different telescope inclinations), wind, temperature changes, telescope axis deformation, et cetera. Active optics correct all factors that may affect image quality at timescales of one second or more. The telescope is therefore actively still, in its optimal shape.

This image shows some of the GRAAL instrument team inspecting GRAAL’s mechanical assembly. Credit: ESO.

Def. an optical system in telescopes that reduces atmospheric distortion by dynamically measuring and correcting wavefront aberrations in real time, often by using a deformable mirror is called adaptive optics.

"Already it has allowed ground-based telescopes to produce images with sharpness rivalling those from the Hubble Space Telescope. The technique is expected to revolutionize the future of ground-based optical astronomy."[32]

At right is an image of the adaptive optics of the GRAAL. GRAAL stands for GRound layer Adaptive optics Assisted by Lasers. It will use the technique of adaptive optics to improve the quality of images by compensating for turbulence in the lower layers of the atmosphere, up to an altitude of 1 kilometre. GRAAL, which will be installed on ESO’s Very Large Telescope (VLT) on Cerro Paranal in Chile, is designed to improve the vision of the VLT’s already excellent HAWK-I camera even further. Currently, HAWK-I operates without adaptive optics. Installing GRAAL will improve the sharpness of HAWK-I’s images, and reduce the exposure times needed by up to a factor of two.

## Refracting telescopes

This is a schematic of a Keplerian refracting telescope which uses two different sizes of planoconvex lenses. Credit: .

The Keplerian Telescope, invented by Johannes Kepler in 1611, is an improvement on Galileo's design.[33] It uses a [plano]convex lens as the eyepiece instead of Galileo's double concave one. The advantage of this arrangement is [that] the rays of light emerging from the eyepiece are converging. This allows for a much wider field of view and greater eye relief but the image for the viewer is inverted. Considerably higher magnifications can be reached with this design but to overcome aberrations the simple objective lens needs to have a very high f-ratio.

All refracting telescopes use the same principles. The combination of an objective lens 1 and some type of eyepiece 2 is used to gather more light than the human eye could collect on its own, focus it 5, and present the viewer with a brighter, clearer, and magnified virtual image 6.

## Reflecting telescopes

The NASA logo on Bldg. 703 at the Dryden Aircraft Operations Facility in Palmdale, California, is reflected in the 2.5 m primary mirror of the SOFIA observatory's telescope. Credit: .
24 inch convertible Newtonian/Cassegrain reflecting telescopeis shown on display at the Franklin Institute. Credit: .

A reflecting telescope (also called a reflector) is an optical telescope which uses a single or combination of curved mirrors that reflect light and form an image.

Def. optical systems that employ both refractive (dioptric) and reflective (catoptric) elements are called catadioptric optical systems.

Def. the construction and use of catadioptric lenses and systems is called catadioptrics.

## Dobsonian telescopes

This is a red Dobsonian telescope on display at Stellafane in the early 1980s. Credit: .

A Dobsonian telescope is an alt-azimuth mounted newtonian telescope design popularized by the [amateur astronomy] John Dobson starting in the 1960s. Dobson's telescopes featured a simplified mechanical design that was easy to manufacture from readily available components to create a large, portable, low-cost telescope. The design is optimized for visually observing faint deep sky objects such as nebulae. This type of observation requires a large objective diameter (i.e. light-gathering power) of relatively short focal length and portability for travel to relatively less light polluted locations.[34][35]

## Schmidt telescopes

The diagram illustrates the optical ray paths inside a Schmidt telescope. Credit: .
The 2 meter diameter (Alfred-Jensch-Telescope at the Karl Schwarzschild Observatory in Tautenburg, Thuringia, Germany, is the largest Schmidt camera in the world. Credit: .

At the top of this lecture/article is the Schmidt Telescope at the former Brorfelde Observatory. It is now used by amateur astronomers. The telescope from 1966 is still located in the same building in Brorfelde as originally. Today the telescope has a 77 cm mirror and a digital 2048x2048 pixel CCD-camera. Originally photographic film was used, and in the lower right part an engineer is showing the former film-box, which was then placed behind the locker at the center of the telescope (at the prime focus).

A Schmidt camera, also referred to as the Schmidt telescope, is a catadioptric astrophotographic optical telescope designed to provide wide fields of view with limited aberrations.

## Maksutov telescopes

A 150mm aperture Maksutov–Cassegrain telescope is shown. Credit: .
Light path in a typical "Gregory" or "spot" Maksutov–Cassegrain is diagrammed. Credit: .

"The Maksutov is a catadioptric telescope design that combines a spherical mirror with a weakly negative meniscus lens in a design that takes advantage of all the surfaces being nearly "spherically symmetrical".[36] The negative lens is usually full diameter and placed at the entrance pupil of the telescope (commonly called a "corrector plate" or "meniscus corrector shell"). The design corrects the problems of off-axis aberrations such as coma found in reflecting telescopes while also correcting chromatic aberration.

## Ultraviolet telescopes

The Extreme ultraviolet Imaging Telescope (EIT) is an instrument on the SOHO spacecraft used to obtain high-resolution images of the solar corona in the ultraviolet range. The EIT instrument is sensitive to light of four different wavelengths: 17.1, 19.5, 28.4, and 30.4 nm, corresponding to light produced by highly ionized iron (XI)/(X), (XII), (XV), and helium (II), respectively. EIT is built as a single telescope with a quadrant structure to the entrance mirrors: each quadrant reflects a different colour of EUV light, and the wavelength to be observed is selected by a shutter that blocks light from all but the desired quadrant of the main telescope.

EIT is the first long-duration instrument to use normal incidence multilayer coated optics to image the Sun in extreme ultraviolet. This portion of the spectrum is extremely difficult to reflect, as most matter absorbs the light very strongly. Conventionally these wavelengths have been reflected either using grazing incidence (as in a Wolter telescope for imaging X-rays) or a diffraction grating (as flown on Skylab in the mid 1970s).

Vacuum deposition technology allows mirrors to be coated with extremely thin layers of nearly any material. The multilayer mirrors in an EUV telescope are coated with alternate layers of a light "spacer" element (such as silicon) that absorbs EUV light only weakly, and a heavy "scatterer" element (such as molybdenum) that absorbs EUV light very strongly. Perhaps 100 layers of each type might be placed on the mirror, with a thickness of around 10 nm each. The layer thickness is tightly controlled, so that at the desired wavelength, reflected photons from each layer interfere constructively. In this way, reflectivities of up to ~50% can be attained.

The multilayer technology allows conventional telescope forms (such as the Cassegrain or Ritchey-Chretien designs) to be used in a novel part of the spectrum.

## Visual telescopes

This image shows the 26-inch Warner & Swasey refracting telescope at the United States Naval Observatory. Credit: Waldon Fawcett.

“I think everyone can conjure up a mental image of astronomers at every level and place in history, gazing through the eyepieces of their telescopes at sights far away - true visual astronomy.”[37]

## Astronomical filters

Ultraviolet filters are used in astronomy for blocking this part of the spectrum, which causes the camera to heat up when photographing without affecting the image. Credit: .

An astronomical filter is sometimes a telescope accessory used to simply enhance the details of celestial objects [or as part of the photometric system of] filters to understand the astrophysics (such as stellar classification and placement of a celestial body on its Wein Curve), occurring for the object in a given bandpass via photometry.

Most astronomical filters work by blocking a specific part of the color spectrum above and below a bandpass, significantly increasing the signal to noise of the interesting wavelengths, and so making the object more visible, 'contrasty', or defined. While color filters transmit certain colors from the spectrum and are usually used for observation of the planets and the Moon, polarizing filters work by adjusting the brightness, and are usually used for the Moon. The broadband and narrowband filters transmit the wavelengths that are emitted by ... hydrogen and oxygen atoms, and are frequently used for reducing light pollution.[1]

## Infrared telescopes

The image shows the Spitzer Space Telescope prior to launch. Credit: NASA/JPL/Caltech.
The diagram is of a Ritchey-Chrétien reflector telescope. Credit: .
This is an early Ritchey-Chrétien reflector telescope. Credit: P. Shankland.

The Spitzer telescope is a Ritchey–Chrétien telescope a specialized Cassegrain telescope that has a hyperbolic primary mirror and a hyperbolic secondary mirror designed to eliminate optical errors (coma). They have [a] large field of view free of optical errors compared to a more conventional reflecting telescope configuration.

The radii of curvature of the primary and secondary mirrors, respectively, in a two-mirror Cassegrain configuration are

${\displaystyle R_{1}=-{\frac {2DF}{F-B}}}$

and

${\displaystyle R_{2}=-{\frac {2DB}{F-B-D}}}$

where

• ${\displaystyle F}$ is the effective focal length of the system,
• ${\displaystyle B}$ is the back focal length (the distance from the secondary to the focus), and
• ${\displaystyle D}$ is the distance between the two mirrors.

If, instead of ${\displaystyle B}$ and ${\displaystyle D}$, the known quantities are the focal length of the primary mirror, ${\displaystyle f_{1}}$, and the distance to the focus behind the primary mirror, ${\displaystyle b}$, then ${\displaystyle D=f_{1}(F-b)/(F+f_{1})}$ and ${\displaystyle B=D+b}$.

For a Ritchey–Chrétien system, the conic constants ${\displaystyle K_{1}}$ and ${\displaystyle K_{2}}$ of the two mirrors are chosen so as to eliminate third-order spherical aberration and coma; the solution is

${\displaystyle K_{1}=-1-{\frac {2}{M^{3}}}\cdot {\frac {B}{D}}}$

and

${\displaystyle K_{2}=-1-{\frac {2}{(M-1)^{3}}}\left[M(2M-1)+{\frac {B}{D}}\right]}$

where ${\displaystyle M=F/f_{1}=(F-B)/D}$ is the secondary magnification.[38] Note that ${\displaystyle K_{1}}$ and ${\displaystyle K_{2}}$ are less than ${\displaystyle -1}$ (since ${\displaystyle M>1}$), so both mirrors are hyperbolic. (The primary mirror is typically quite close to being parabolic, however.)

The hyperbolic curvatures are difficult to test, especially with equipment typically available to amateur telescope makers or laboratory-scale fabricators; thus, older telescope layouts predominate in these applications. However, professional optics fabricators and large research groups test their mirrors with interferometers. A Ritchey–Chrétien then requires minimal additional equipment, typically a small optical device called a null corrector that makes the hyperbolic primary look spherical for the interferometric test.

The telescope at left is the early Ritchey–Chrétien 1.0 meter telescope at NOFS at the United States Naval Observatory Flagstaff Station.

## Submillimeter telescopes

This photograph shows the 10.4-metre diameter submillimeter wavelength telescope of the Caltech Submillimeter Observatory (CSO). Credit: Samuel Bouchard from Quebec City, Canada; modified by Huntster.
Four antennas of the Atacama Large Millimeter/submillimeter Array (ALMA) gaze up at the star-filled night sky. Credit: ESO/José Francisco Salgado (josefrancisco.org).
The Heinrich Hertz Submillimeter Telescope is shown at night. Credit: Geremia.

The Atacama Large Millimeter/submillimeter Array (ALMA) is being constructed at an altitude of 5000 m on the Chajnantor plateau in the Atacama Desert of Chile. This is one of the driest places on Earth and this dryness, combined with the thin atmosphere at high altitude, offers superb conditions for observing the Universe at millimetre and submillimetre wavelengths. At these long wavelengths, astronomers can probe, for example, molecular clouds, which are dense regions of gas and dust where new stars are born when a cloud collapses under its own gravity. Currently, the Universe remains relatively unexplored at submillimetre wavelengths, so astronomers expect to uncover many new secrets about star formation, as well as the origins of galaxies and planets.

ALMA began scientific observations in the second half of 2011 and the first images were released to the press on 3 October 2011. The array has been fully operational since March 2013.[39]

The Submillimeter Telescope (SMT), formerly known as the Heinrich Hertz Submillimeter Telescope [at lower right], is a submillimeter wavelength radio telescope located on Mount Graham, Arizona. It is a 10-meter-wide parabolic dish inside a building to protect it from bad weather. The building front doors and roof are opened when the telescope is in use. The dryness of the air around and above Mt. Graham is particulatly vital for EHF (extremely low wavelength radio) and far-infrared observations - a region of the spectrum where the electromagnetic waves are strongly attenuated by any water vapor or clouds in the air.

This 64 meter radio telescope is at Parkes Observatory Credit: John Sarkissian (CSIRO Parkes Observatory).

Def. “a device for studying astronomical sources of radio waves”[40] is called a radio telescope.

A radio telescope is a form of directional radio antenna, as used in tracking and collecting data from satellites and space probes that [operates] in the radio frequency portion of the electromagnetic spectrum. Radio telescopes are typically large parabolic ("dish") antennas used singly or in an array. Radio observatories are preferentially located far from major centers of population to avoid electromagnetic interference (EMI) from radio, TV, radar, and other EMI emitting devices.

## Microwave telescopes

The Planck telescope was launched in 2009 to observe the Cosmic Microwave Background Radiation. Credit: ESA.

"The basic scientific goal of the Planck mission is to measure [cosmic microwave background] CMB anisotropies at all angular scales larger than 10 arcminutes over the entire sky with a precision of ~2 parts per million. The model payload consists of a 1.5 meter off-axis telescope with two focal plane arrays of detectors sharing the focal plane. Low frequencies will be covered by 56 tuned radio receivers sensitive to 30-100 GHz, while high frequencies will be covered by 56 bolometers sensitive to 100-850 GHz."[41]

This image shows the early planetary radar at Pluton, USSR, 1960. Credit: Rumlin.
The Arecibo Radio Telescope, Arecibo, Puerto Rico, at 1000 feet (305 m) across, is the largest dish antenna in the world. Credit: H. Schweiker/WIYN and NOAO/AURA/NSF, NOAA.
The image is of the Evpatoria RT-70 radar telescope in the Ukraine. Credit: Bebo.
In this artist's impression TerraSAR-X and TanDEM-X are in orbit. Credit: Astrium GmbH.

The image at right shows planetary radar telescopes at Pluton, USSR, in 1960.

The "Arecibo Observatory in Puerto Rico [is] the world's largest, and most sensitive, single-dish radio telescope."[42]

"The 1,000-foot-diameter (305 meters) Arecibo telescope [... provides] access to state-of-the-art observing for scientists in radio astronomy, solar system radar and atmospheric studies, and the observatory has the unique capability for solar system and ionosphere (the atmosphere's ionized upper layers) radar remote sensing."[42]

The telescope has three radar transmitters, with effective isotropic radiated powers of 20 TW at 2380 MHz, 2.5 TW (pulse peak) at 430 MHz, and 300 MW at 47 MHz. The telescope is a spherical reflector, not a parabolic reflector. To aim the telescope, the receiver is moved to intercept signals reflected from different directions by the spherical dish surface. A parabolic mirror would induce a varying astigmatism when the receiver is in different positions off the focal point, but the error of a spherical mirror is the same in every direction.

At second lower left is the Evpatoria RT-70 radar telescope in the Ukraine.

At lower right is an artist's impression of the two radar satellites TerraSAR-X and TanDEM-X.

The diagram shows a possible layout for an astronomical interferometer, with the mirrors laid out in a parabolic arrangement (similar to the shape of a conventional telescope mirror). Credit: .

An astronomical interferometer is an array of telescopes or mirror segments acting together to probe structures with higher resolution by means of interferometry. The benefit of the interferometer is that the angular resolution of the instrument is nearly that of a telescope with the same aperture as a single large instrument encompassing all of the individual photon-collecting sub-components. The drawback is that it does not collect as many photons as a large instrument of that size. Thus it is mainly useful for fine resolution of the more luminous astronomical objects, such as close binary stars.

Very Long Baseline Interferometry uses a technique related to the closure phase to combine telescopes separated by thousands of kilometers to form a radio interferometer with the resolution which would be given by a single dish which was thousands of kilometers in diameter. Astronomical interferometers can produce higher resolution astronomical images than any other type of telescope. At radio wavelengths image resolutions of a few micro-arcseconds have been obtained.

## Superluminal telescopes

The Cherenkov telescopes do not actually detect the gamma rays directly but instead detect the flashes of visible light [Cherenkov radiation] produced when gamma rays are absorbed by the Earth's atmosphere.[43]

## Plasma-object telescopes

2 kW Hall thruster is in operation as part of the Hall Thruster Experiment at the Princeton Plasma Physics Laboratory. Credit: Dstaack.
This is a xenon 6 kW Hall thruster in operation at the NASA Jet Propulsion Laboratory. Credit: NASA/JPL-Caltech.

In spacecraft propulsion, a Hall thruster is a type of ion thruster in which the propellant is accelerated by an electric field. Hall thrusters trap electrons in a magnetic field and then use the electrons to ionize propellant, efficiently accelerate the ions to produce thrust, and neutralize the ions in the plume. Hall thrusters are sometimes referred to as Hall effect thrusters or Hall current thrusters. Hall thrusters are often regarded as a moderate specific impulse (1,600 s) space propulsion technology. ... Hall thrusters operate on a variety of propellants, the most common being xenon. Other propellants of interest include krypton, argon, bismuth, iodine, magnesium, and zinc.

While these thrusters are not plasma-object telescopes, they may serve to maneuver or slew a space telescope. As sources of blue light they mat serve as calibrated light sources.

## Gaseous-object telescopes

This is an image of a Sun Gun Telescope. Credit: .

The Sun Gun Telescope is designed so that large groups of people can view the sun safely - in particular it was created as a way to encourage children to become interested in astronomy. With this safe and portable device, both amateur science enthusiasts and professionals alike can observe sun spots.

The Sun Gun has a 60mm dia. 900mm fl. optical tube which is mounted inside a 3" PVC which is in turn connected to a 20" plastic flower planter. A rear projection screen is mounted on the top of the flower planter. The entire Sun Gun can be made from items easily found at most local hardware stores. The scope itself is an inexpensive 60mm refractor available from many sources.

## Liquid-object telescopes

This is a liquid mirror telescope. Credit: .

Liquid mirror telescopes are telescopes with mirrors made with a reflective liquid. The most common liquid used is mercury, but other liquids will work as well (for example, low melting alloys of gallium). The container for the liquid is rotating so that the liquid assumes a paraboloidal shape. A paraboloidal shape is precisely the shape needed for the primary mirror of a telescope. The rotating liquid assumes the paraboloidal shape regardless of the container's shape. To reduce the amount of liquid metal needed, and thus weight, a rotating mercury mirror uses a container that is as close to the necessary parabolic shape as possible. Liquid mirrors can be a low cost alternative to conventional large telescopes. Compared to a solid glass mirror that must be cast, ground, and polished, a rotating liquid metal mirror is much less expensive to manufacture.

A telescope with a liquid metal mirror can only be used [as a] zenith telescope that looks straight up.

## Hydrogen telescopes

Here is an example of an amateur solar telescope equipped with a hydrogen-alpha filter system. Credit: .

In the field of amateur astronomy amateurs use hydrogen-alpha filter systems.

## Alloys

The image shows the dome of the NASA Orbital Debris Observatory near Cloudcroft, New Mexico. Credit: NASA.
This image shows the CCD Debris Telescope that is under the NODO dome. Credit:

The NASA-LMT was 3 m (9.8 ft) aperture liquid mirror telescope located in NODO's main dome. It consisted of a 3 m diameter parabolic dish that held 4 U.S. gallons (15 l) of a highly reflective liquid metal, mercury, spinning at a rate of 10 rpm, with sensors mounted above on a fixed structure. Due to the primary mirror's material, the NASA-LMT was configured as a zenith telescope. Using 20 narrowband filters, it cataloged space debris in Earth's orbit.

The 32 cm (13 in) CCD Debris Telescope (CDT) was a portable Schmidt camera equipped with a 512×512 pixel charge-coupled device (CCD) sensor. It operated at NODO from October of 1997 until December of 2001, and was used to characterize debris at or near geosynchronous orbit.

## Meteorites

The image contains a 27.70 g fragment of the Carancas meteorite fall. The scale cube is 1 cm3. Credit: Meteorite Recon.

On September 20, the X-Ray Laboratory at the Faculty of Geological Sciences, Mayor de San Andres University, La Paz, Bolivia, published a report of their analysis of a small sample of material recovered from the impact site. They detected iron, nickel, cobalt, and traces of iridium — elements characteristic of the elemental composition of meteorites. The quantitative proportions of silicon, aluminum, potassium, calcium, magnesium, and phosphorus are incompatible with rocks that are normally found at the surface of the Earth.[44]

## Shelters

Telescope domes have a slit or other opening in the roof that can be opened during observing, and closed when the telescope is not in use. In most cases, the entire upper portion of the telescope dome can be rotated to allow the instrument to observe different sections of the night sky. Radio telescopes usually do not have domes.

## Spectroscopy

Astronomical spectroscopy is the technique of spectroscopy used in astronomy. The object of study is the spectrum of electromagnetic radiation, including visible light, which radiates from stars and other celestial objects. Spectroscopy can be used to derive many properties of distant stars and galaxies, such as their chemical composition, but also their motion by Doppler shift measurements.

## Spectrometers

The Oriented Scintillation Spectrometer Experiment (OSSE) consists of four NaI scintillation detectors, sensitive to energies from 50 keV to 10 MeV. Credit: NASA GSFC.

"The Oriented Scintillation Spectrometer Experiment (OSSE) will conduct a broad range of observations in the 0.05-250 MeV energy range. Major emphasis is placed on scientific objectives in the 0.1-10.0 MeV region with a limited capability above 10 MeV, primarily for observations of solar gamma-rays and neutrons and observations of high-energy emission from pulsars."[17]

## Planetary telescopes

A telescope on an alt-azimuth GoTo mount. Note the keypad, resting on the platform between the tripod's legs, that is the telescope's hand control. Batteries are stored in the circular compartment just above the tripod. In this picture, the compartment is just above the hand control.

In amateur astronomy, "GoTo" refers to a type of telescope mount and related software which can automatically point a telescope to astronomical objects that the user selects. Both axes of a GoTo mount are motor driven and are controlled by either a microprocessor-based integrated controller or a personal computer, as opposed to the single axis semi-automated tracking of a traditional clock drive mount. This allows the user to command the mount to point the telescope to a right ascension and declination that the user inputs or have the mount itself point the telescope to objects in a pre-programmed data base including ones from the Messier catalogue, the New General Catalogue, and even major solar system bodies (the Sun, Moon, and planets).

## Solar telescopes

This view is of the McMath-Pierce Solar Telescope at Kitt Peak National Observatory, near Tucson, Arizona. Credit: ocean yamaha.

A solar telescope is a special purpose telescope used to observe the Sun. Solar telescopes usually detect light with wavelengths in, or not far outside, the visible spectrum.

## Asteroid telescopes

The Lowell astrograph is a dedicated astrophotography telescope. Credit: .

The Lowell astrograph imaged at right is a 13-inch, f/5.3 astrograph at Lowell Observatory, a refractor with a 3 element Cooke triplet lens.[45]) that was used in the discovery of Pluto.

An astrograph (astrographic camera) is a telescope designed for the sole purpose of astrophotography. Astrographs are usually used in wide field surveys of the night sky as well as detection of objects such as asteroids, meteors, and comets.

## Comet-seeker telescopes

A comet seeker is a type of small telescope adapted especially to searching for comets: commonly of short focal length and large aperture, in order to secure the greatest brilliancy of light.

## Stellar telescopes

The Ron Stone/Flagstaff Astrometric Scanning Transit Telescope of the U.S.Naval Observatory, built by Farrand Optical Company, 1981, is imaged. Credit: .

The meridian circle is an instrument for timing of the passage of stars across the local meridian, an event known as a transit, while at the same time measuring their angular distance from the nadir. These are special purpose telescopes mounted so as to allow pointing only in the meridian, the great circle through the north point of the horizon, the zenith, the south point of the horizon, and the nadir. Meridian telescopes rely on the rotation of the Earth to bring objects into their field of view and are mounted on a fixed, horizontal, east-west axis.

A modern day example of this type of telescope is the 8 inch (~0.2m) Flagstaff Astrometric Scanning Transit Telescope (FASTT) at the United States Naval Observatory (USNO) Flagstaff Station Observatory.[46] Modern meridian circles are usually automated. The observer is replaced with a [Charge-coupled device] CCD camera. As the sky drifts across the field of view, the image built up in the CCD is clocked across (and out of) the chip at the same rate. This allows some improvements:[47]

## Galactic telescopes

NGC 891 is selected as first light. Credit: NASA.
This is an image of the Large Binocular Telescope with protective doors open. Credit: NASA.

The Large Binocular Telescope at left is located on Mount Graham (10,700-foot (3,300 m)) in the Pinaleno Mountains of southeastern Arizona, and is a part of the Mount Graham International Observatory.

The first image taken [shown at right] combined ultraviolet and green light, and emphasizes the clumpy regions of newly formed hot stars in the spiral arms.

## Locations on Earth

VERITAS is located at the basecamp of the Smithsonian Astrophysics Observatory's Fred Lawrence Whipple Observatory (FLWO) in southern Arizona. Credit: VERITAS.
The four Unit Telescopes form the VLT together with the Auxiliary Telescopes. Credit: .

"VERITAS (Very Energetic Radiation Imaging Telescope Array System) is a ground-based gamma-ray instrument operating at the Fred Lawrence Whipple Observatory (FLWO) in southern Arizona, USA. It is an array of four 12m optical reflectors for gamma-ray astronomy in the GeV - TeV energy range. These imaging Cherenkov [a bluish light] telescopes are deployed such that they have the highest sensitivity in the VHE energy band (50 GeV - 50 TeV), with maximum sensitivity from 100 GeV to 10 TeV. This VHE observatory effectively complements the NASA Fermi mission."[48]

The Collaboration between Australia and Nippon for a Gamma Ray Observatory in the Outback, (CANGAROO) is for very high energy cosmic gamma ray observation by telescope detecting Cherenkov light. It is located on the Woomera Prohibited Area in South Australia. [49]

The Very Large Telescope (VLT) is a telescope operated by the European Southern Observatory on Cerro Paranal in the Atacama Desert of northern Chile. The UTs are equipped with a large set of instruments permitting observations to be performed [in] the near-ultraviolet. It includes large-field imagers, adaptive optics corrected cameras and spectrographs, as well as high-resolution and multi-object spectrographs and covers a broad spectral region, from [the] deep ultraviolet (300 nm).

## Recent history

This is the 6-inch transit circle of the U.S. Naval Observatory. Credit: .

The recent history period dates from around 1,000 b2k to present.

The 6-inch transit circle [imaged at right] of the U.S. Naval Observatory was built by Warner and Swasey in 1898.

## Coded apertures

Some X-ray telescopes use coded aperture imaging. This technique uses a flat aperture grille in front of the detector, which weighs much less than any kind of focusing X-ray lens, but requires considerably more post-processing to produce an image.

## Mirrors

This is a diagram of Wolter telescopes of Types I, II, and III. Credit: .

The mirrors can be made of ceramic or metal foil.[50] The most commonly used grazing angle incidence materials for X-ray mirrors are gold and iridium. The critical reflection angle is energy dependent. For gold at 1 keV, the critical reflection angle is 3.72 degrees. A limit for this technology in the early 2000s with Chandra and XMM-Newton was about 15 keV light.[51] Using new multi-layered coatings, computer aided manufacturing, and other techniques the [X-ray] mirror for the NuStar telescope pushed this up to 79 keV light.[51] To reflect at this level, glass layers were multi-coated with Tungsten (W)/Silicon (Si) or Platinum(Pt)/Siliconcarbite(SiC).[51].

Although optical telescopes can image the near ultraviolet, the ozone layer in the stratosphere absorbs ultraviolet radiation shorter than 300 nm so most ultra-violet astronomy is conducted with satellites. Ultraviolet telescopes [10 nm - 400 nm] resemble optical telescopes, but conventional aluminium-coated mirrors cannot be used and alternative coatings such as magnesium fluoride or lithium fluoride are used instead. The OSO 1 satellite carried out observations in the ultra-violet as early as 1962. The International Ultraviolet Explorer (1978) systematically surveyed the sky for eighteen years, using a 45 cm (18 in) aperture telescope with two spectroscopes. Extreme-ultraviolet astronomy (10–100 nm) is a discipline in its own right and involves many of the techniques of X-ray astronomy; the Extreme Ultraviolet Explorer (1992) was a satellite operating at these wavelengths.

## Modulation collimators

The diagram shows the principles of operation of the four-grid modulation collimator. Credit: H. Bradt, G. Garmire, M. Oda, G. Spada, and B.V. Sreekantan, P. Gorenstein and H. Gursky.

A modulation collimator consists of “two or more wire grids [diffraction gratings] placed in front of an X-ray sensitive Geiger tube or proportional counter.”[52] Relative to the path of incident X-rays (incoming X-rays) the wire grids are placed one beneath the other with a slight offset that produces a shadow of the upper grid over part of the lower grid.[53]

## Computers

The ALMA correlator is one of the most powerful supercomputers in the world. Credit: ALMA (ESO/NAOJ/NRAO), S. Argandoña.

"The ALMA correlator [shown at right], one of the most powerful supercomputers in the world, has now been fully installed and tested at its remote, high altitude site in the Andes of northern Chile. This view shows lights glowing on some of the racks of the correlator in the ALMA Array Operations Site Techical Building. This photograph shows one of the four quadrants of the correlator. The full system has four identical quadrants, with over 134 million processors, performing up to 17 quadrillion operations per second."[54]

## Telescope mounts

A telescope mount is a mechanical structure which supports a telescope. Telescope mounts are designed to support the mass of the telescope and allow for accurate pointing of the instrument.

Def. an object on which another object is attached for support is called a mount.

## Altazimuth mounts

A heliostat is shown at the THÉMIS experimental station in France. The mirror rotates on an alt-azimuth mount. The pointing direction of the mirror is perpendicular to its surface. Credit: .

An altazimuth or alt-azimuth mount is a simple two-axis mount for supporting and rotating an instrument about two mutually perpendicular axes; one vertical and the other horizontal. Rotation about the vertical axis varies the azimuth (compass bearing) of the pointing direction of the instrument. Rotation about the horizontal axis varies the altitude (angle of elevation) of the pointing direction.

When used as an astronomical telescope mount, the biggest advantage of an alt-azimuth mount is the simplicity of its mechanical design. The primary disadvantage is its inability to follow astronomical objects in the night sky as the Earth spins on its axis the way that an equatorial mount can. Equatorial mounts only need to be rotated about a single axis, at a constant rate, to follow the rotation of the night sky (diurnal motion). Altazimuth mounts need to be rotated about both axes at variable rates, achieved via microprocessor based two-axis drive systems, to track equatorial motion. This imparts an uneven rotation to the field of view that also has to be corrected via a microprocessor based counter rotation system.[55] On smaller telescopes an equatorial platform[56] is sometimes used to add a third "polar axis" to overcome these problems, providing an hour or more of motion in the direction of right ascension to allow for astronomical tracking. The design also does not allow for the use of mechanical setting circles to locate astronomical objects although modern digital setting circles have removed this shortcoming.

## Equatorial mounts

An example of an equatorial mount is photographed. Credit: Peter Rucks.

The equatorial mount has north-south "polar axis" tilted to be parallel to Earth's polar axis that allows the telescope to swing in an east-west arc, with a second axis perpendicular to that to allow the telescope to swing in a north-south arc. Slewing or mechanically driving the mounts polar axis in a counter direction to the Earth's rotation allows the telescope to accurately follow the motion of the night sky.

## Hexapod mounts

This is an image of the top part of the Dutch Open Telescope. Credit: Tim van Werkhoven.

Instead of the classical mounting using two axles, the mirror is supported by six extendable struts (hexapod). This configuration allows moving the telescope in all six spatial degrees of freedom and also provides a strong structural integrity.

## Clock drives

The clock drive mechanism in the pier of the german equatorial mount for the 8-inch refracting telescope at Aldershot Observatory is shown in the image. Credit: .

"[A] Clock drive is a regulatory mechanism used to move an equatorial mounted telescope along one axis to keep the telescope in exact sync with the apparent motion of the celestial sky (diurnal motion)."[57]

Clock drives work by rotating a telescope mount's polar axis, the axis parallel to the Earth's polar axis (also called the right ascension axis) in the opposite direction to the Earth's rotation one revolution every 23 hours and 56 minutes (called sidereal day), thereby canceling that motion.[58] This allows the telescope to stay [fixed] on a certain point in the sky without having to be constantly re-aimed due to the Earth's rotation. The mechanism itself used to be clockwork but nowadays is usually electrically driven. Clock drives can be light and portable for smaller telescopes[59] or can be exceedingly heavy and complex for larger ones such as the 60 inch telescope at the Mount Wilson Observatory.[60] Clock-driven equatorial platforms are sometimes used in non-tracking type mounts, such as altazimuth mounts.[61]

## Clocks

The FOCS 1 is a continuous cold caesium fountain atomic clock in Switzerland. Credit: .

An atomic clock is a clock device that uses an electronic transition frequency in the microwave, optical, or ultraviolet region[62] of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element. Atomic clocks are the most accurate time and frequency standards known, and are used as primary standards for international time distribution services, to control the wave frequency of television broadcasts, and in global navigation satellite systems such as GPS.

The FOCS 1 continuous cold cesium fountain atomic clock started operating in 2004 at an uncertainty of one second in 30 million years. The clock is in Switzerland.

## Motion calibrators

POA CALFOS is the improved Post Operational Archive version of the Faint Object Spectrograph (FOS) calibration pipeline. The current version corrects for image motion problems that have led to significant wavelength scale uncertainties in the FOS data archive. The improvements in the calibration enhance the scientific value of the data in the FOS archive, making it a more homogeneous and reliable resource.

## Detectors

This is an image of a real X-ray detector. The instrument is called the Proportional Counter Array and it is on the Rossi X-ray Timing Explorer (RXTE) satellite. Credit: .

Radiation detectors provide a signal that is converted to an electric current. The device is designed so that the current provided is proportional to the characteristics of the incident radiation.

Detectors such as the X-ray detector at right collect individual X-rays (photons of X-ray light), count them, discern the energy or wavelength, or how fast they are detected. The detector and telescope system can be designed to yield temporal, spatial, or spectral information.

## Image processors

Def. "any form of information processing for which both the input and output are images"[63] is called image processing.

Def. a representation of anything upon canvas, paper, or other surface” is called a picture.

Def. a "representation of a real object"[64] is called an image.

Def. the set of points that map to a given point (or set of points) under a specified function is called an inverse image.

Under the function given by ${\displaystyle f(x)=x^{2}}$, the inverse image of 4 is ${\displaystyle \{-2,2\}}$, as is the inverse image of ${\displaystyle \{4\}}$.

A telescope's "imaging system's resolution can be limited either by aberration or by diffraction causing blurring of the image. These two phenomena have different origins and are unrelated. Aberrations can be explained by geometrical optics and can in principle be solved by increasing the optical quality — and cost — of the system. On the other hand, diffraction comes from the wave nature of light and is determined by the finite aperture of the optical elements. The lens' circular aperture is analogous to a two-dimensional version of the single-slit experiment. Light passing through the lens interferes with itself creating a ring-shape diffraction pattern, known as the Airy pattern, if the wavefront of the transmitted light is taken to be spherical or plane over the exit aperture.

The interplay between diffraction and aberration can be characterised by the point spread function (PSF). The narrower the aperture of a lens the more likely the PSF is dominated by diffraction.

Two point sources are regarded as just resolved when the principal diffraction maximum of one image coincides with the first minimum of the other.[65]

## Robotic telescopes

“El Enano” is a robotic telescope. Credit: .

A robotic telescope is an astronomical telescope and detector system that makes observations without the intervention of a human. In astronomical disciplines, a telescope qualifies as robotic if it makes those observations without being operated by a human, even if a human has to initiate the observations at the beginning of the night, or end them in the morning. A robotic telescope is distinct from a remote telescope, though an instrument can be both robotic and remote.

## Spotting telescopes

This is a 100 mm spotting scope with a coaxial 30 mm finderscope. Credit: .

A spotting scope is a small portable telescope with added optics to present an erect image, optimized for the observation of terrestrial objects.

The light-gathering power and [angular] resolution of a spotting scope is determined by the diameter of the objective lens, typically between 50 and 80 mm. The larger the objective, the more massive and expensive the telescope.

The optical assembly has a small refracting objective lens, an image erecting system that uses either image erecting relay lenses or prisms (porro prisms or roof prisms), and an eyepiece that is usually removable and interchangeable to give different magnifications. Other telescope designs are used such as Schmidt and Maksutov optical assemblies. They may have a ruggedised design, a mounting for attaching to a tripod, and an ergonomically designed and located knob for focus control.

## Observatories

This equatorial room is at the University of Illinois Observatory. Credit: .

Historically, observatories [are] as simple as [using or placing stably] an astronomical sextant (for measuring the distance between stars) or Stonehenge (which has some alignments on astronomical phenomena). Most optical telescopes are housed within a dome or similar structure, to protect the delicate instruments from the elements. Telescope domes have a slit or other opening in the roof that can be opened during observing, and closed when the telescope is not in use. In most cases, the entire upper portion of the telescope dome can be rotated to allow the instrument to observe different sections of the night sky. Radio telescopes usually do not have domes.

An equatorial room, in astronomical observatories, is the room which contains an equatorial mounted telescope. It is usually referred to in observatory buildings that contain more than one type of instrument: for example buildings with an "equatorial room" containing an equatorial telescope and a "transit room" containing a transit telescope.[66] Equatorial rooms tend to be large circular rooms to accommodate all the range of motion of a long telescope on an equatorial mount and are usually topped with a dome to keep out the weather.

## Lofting technology

Many devices for lofting technology have been developed to improve radiation astronomy.

## Balloons

BLAST is hanging from the launch vehicle in Esrange near Kiruna, Sweden before launch June 2005. Credit: Mtruch.
NASA's balloon-carried BLAST sub-millimeter telescope is hoisted into launch position on Dec. 25, 2012, at McMurdo Station in Antarctica. Credit: NASA/Wallops Flight Facility.

The Balloon-borne Large Aperture Submillimeter Telescope (BLAST) is a submillimeter telescope that hangs from a high altitude balloon. It has a 2 meter primary mirror that directs light into bolometer arrays operating at 250, 350, and 500 µm. BLAST's primary science goals are:[67]

• Measure photometric redshifts, rest-frame FIR luminosities and star formation rates of high-redshift starburst galaxies, thereby constraining the evolutionary history of those galaxies that produce the FIR/submillimeter background.
• Measure cold pre-stellar sources associated with the earliest stages of star and planet formation.
• Make high-resolution maps of diffuse galactic emission over a wide range of galactic latitudes.

High-altitude balloons and aircraft ... can get above [much] of the atmosphere. The BLAST experiment and SOFIA are two examples, respectively, although SOFIA can also handle near infrared observations.

At left above "NASA's balloon-carried BLAST sub-millimeter telescope is hoisted into launch position on Dec. 25, 2012, at McMurdo Station in Antarctica on a mission to peer into the cosmos."[68] The giant helium-filled balloon is slowly drifting about 36 km above Antarctica. It was "[l]aunched on Tuesday (Dec. 25) from the National Science Foundation's Long Duration Balloon (LDB) facility ... This is the fifth and final mission for BLAST, short for the Balloon-borne Large-Aperture Submillimeter Telescope. ... "BLAST found lots of so-called dark cores in our own Milky Way — dense clouds of cold dust that are supposed to be stars-in-the-making. Based on the number of dark cores, you would expect our galaxy to spawn dozens of new stars each year on average. Yet, the galactic star formation rate is only some four solar masses per year." So why is the stellar birth rate in our Milky Way so low? Astronomers can think of two ways in which a dense cloud of dust is prevented from further contracting into a star: turbulence in the dust, or the collapse-impeding effects of magnetic fields. On its new mission, BLAST should find out which process is to blame. ... [The 1800-kilogram] stratospheric telescope will observe selected star-forming regions in the constellations Vela and Lupus."[69]

## Aircraft assisted launches

The Array of Low Energy X-ray Imaging Sensors (ALEXIS) X-ray telescopes feature curved mirrors whose multilayer coatings reflect and focus low-energy X-rays or extreme ultraviolet light the way optical telescopes focus visible light. ... The Launch was provided by the United States Air Force Space Test Program on a Pegasus Booster on April 25, 1993.[70]

## Hypotheses

1. Ancients had and used telescopes.

A control group for a radiation telescope would contain

1. an aperture, or an entry avenue into the instrument,
2. collimators, or lenses, to concentrate radiation,
3. moderators, to systematically reduce the incoming radiation so as to allow determination of incoming direction,
4. detectors, or sensors, to convert the incoming radiation into electrical impulses,
5. amplifiers, or processors, and
6. supports, to provide orientation and stability of all components.

Proof of concept consists of a prototype instrument or device that makes a distant source appear nearer.

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