Cassegrain reflector

Light path in a Cassegrain reflecting telescope
Light path in a Cassegrain reflector telescope
A Cassegrain radio antenna at GDSCC

Combination of a primary concave mirror and a secondary convex mirror, often used in optical telescopes and radio antennas, the main characteristic being that the optical path folds back onto itself, relative to the optical system's primary mirror entrance aperture.

- Cassegrain reflector

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Ritchey–Chrétien telescope

A George Ritchey's 24-inch (0.6 m) reflecting telescope, the first RCT to be built, on display at the Chabot Space and Science Center in 2004.
The 40-inch (1.0 m) Ritchey at United States Naval Observatory Flagstaff Station.
Diagram of a Ritchey–Chrétien reflector telescope
A 41 cm RC Optical Systems truss telescope, part of the PROMPT Telescopes array.

A Ritchey–Chrétien telescope (RCT or simply RC) is a specialized variant of the Cassegrain telescope that has a hyperbolic primary mirror and a hyperbolic secondary mirror designed to eliminate off-axis optical errors (coma).

Focus (optics)

Image point, is a point where light rays originating from a point on the object converge.

Eye focusing ideally collects all light rays from a point on an object into a corresponding point on the retina.
A demonstration of camera focus on different distances, showing a bamboo rooftop
Text on a page that is partially in focus, but mostly not in varying degrees

Conversely, it can focus rays directed at the focal point that is behind the mirror towards the focal point that is in front of the mirror as in a Cassegrain telescope.

Very Large Telescope

Telescope facility operated by the European Southern Observatory on Cerro Paranal in the Atacama Desert of northern Chile.

VLT's four Unit Telescopes
The SPHERE instrument attached to the VLT Unit Telescope 3.
The interior of Antu (UT1), meaning "sun" in the Mapuche language.
Auxiliary Telescope, the Residencia and the heart of the Milky Way.
The soft glow of the Milky Way can be seen behind the VLT Survey Telescope (VST) at ESO's Paranal Observatory.
All four 8.2-metre Unit Telescopes and 1.8-metre Auxiliary Telescopes were connected for the first time on March 17, 2011, becoming the VLT Interferometer (VLTI) with six baselines.
First light for MATISSE interferometric instrument.
Size comparison of primary mirrors. The dotted line shows the theoretical size of the VLT's combined mirrors (dark green).

Each Unit Telescope is a Ritchey-Chretien Cassegrain telescope with a 22-tonne 8.2 metre Zerodur primary mirror of 14.4 m focal length, and a 1.1 metre lightweight beryllium secondary mirror.

Catadioptric system

One where refraction and reflection are combined in an optical system, usually via lenses and curved mirrors (catoptrics).

A 150 mm aperture catadioptric Maksutov telescope
Light path in a Schmidt–Cassegrain
Light path in a meniscus telescope (Maksutov–Cassegrain)
Houghton doublet corrector design equations – special case symmetric design.
Light path in an Argunov Cassegrain telescope
Example of a catadioptric lens using rear surfaced "mangin mirrors" (Minolta RF Rokkor-X 250mm f/5.6)
An example of 'iris blur' or bokeh produced by a catadioptric lens, behind an in-focus light.
500 mm catadioptric lens mounted on a Yashica FX-3
Minolta AF 500 mm F/8 catadioptric lens mounted on a Sony Alpha 55 camera
Samyang 500mm f/8

Some of these designs have been adapted to create compact, long-focal-length catadioptric cassegrains.

Optical telescope

Telescope that gathers and focuses light mainly from the visible part of the electromagnetic spectrum, to create a magnified image for direct visual inspection, to make a photograph, or to collect data through electronic image sensors.

The Large Binocular Telescope uses two curved mirrors to gather light
Schematic of a Keplerian refracting telescope. The arrow at (4) is a (notional) representation of the original image; the arrow at (5) is the inverted image at the focal plane; the arrow at (6) is the virtual image that forms in the viewer's visual sphere. The red rays produce the midpoint of the arrow; two other sets of rays (each black) produce its head and tail.
Eight-inch refracting telescope at Chabot Space and Science Center
The Keck II telescope gathers light by using 36 segmented hexagonal mirrors to create a 10 m (33 ft) aperture primary mirror
These eyes represent a scaled figure of the human eye where 15 px = 1 mm, they have a pupil diameter of 7 mm. Figure A has an exit pupil diameter of 14 mm, which for astronomy purposes results in a 75% loss of light. Figure B has an exit pupil of 6.4 mm, which allows the full 100% of observable light to be perceived by the observer.
Two of the four Unit Telescopes that make up the ESO's VLT, on a remote mountaintop, 2600 metres above sea level in the Chilean Atacama Desert.
Comparison of nominal sizes of primary mirrors of some notable optical telescopes
Harlan J. Smith Telescope reflecting telescope at McDonald Observatory, Texas

These may be integral part of the optical design (Newtonian telescope, Cassegrain reflector or similar types), or may simply be used to place the eyepiece or detector at a more convenient position.

Hubble Space Telescope

Space telescope that was launched into low Earth orbit in 1990 and remains in operation.

Astronaut Owen Garriott working next to Skylab's crewed solar space observatory, 1973.
Dr. Nancy Grace Roman with a model of the Large Space Telescope that was eventually developed as the Hubble Space Telescope. While listed as a 1966 photo, this design was not the standard until the mid-1970s.
Grinding of Hubble's primary mirror at Perkin-Elmer, March 1979.
The backup mirror, by Kodak. Its inner support structure can be seen because it is not coated with a reflective surface.
The OTA, metering truss, and secondary baffle are visible in this image of Hubble during early construction.
DF-224 in Hubble, before it was replaced in 1999.
Exploded view of the Hubble Space Telescope
Hubble Control Center at Goddard Space Flight Center, 1999
STS-31 lifting off, carrying Hubble into orbit
Hubble being deployed from Discovery in 1990
An extract from a WF/PC image shows the light from a star spread over a wide area instead of being concentrated on a few pixels.
Optical evolution of Hubble's primary camera system. These images show spiral galaxy M100 as seen with WFPC1 in 1993 before corrective optics (left), with WFPC2 in 1994 after correction (center), and with WFC3 in 2018 (right).
COSTAR being removed in 2009
Astronauts Musgrave and Hoffman install corrective optics during SM1
Hubble as seen from Discovery during its second servicing mission
Nickel–hydrogen battery pack for Hubble
One of Hubble's most famous images, Pillars of Creation, shows stars forming in the Eagle Nebula.
The Frontier Fields program studied MACS0416.1-2403.
Star cluster Pismis 24 with nebula
The HST is sometimes visible from the ground, as in this 39-second exposure when it is in Orion. Maximum brightness about magnitude 1.
Hubble Extreme Deep Field image of space in the constellation Fornax
An illustration of a black hole
Hubble's STIS UV and ACS visible light combined to reveal Saturn's southern aurora
Brown spots mark Comet Shoemaker–Levy 9 impact sites on Jupiter's southern hemisphere. Imaged by Hubble.
Hubble and ALMA image of MACS J1149.5+2223
Evolution of detecting the early Universe
Some of the Carina nebula by WFC3
Hubble precision stellar distance measurement has been extended ten times further into the Milky Way.
Data analysis of a spectrum revealing the chemistry of hidden clouds
In 2001, NASA polled internet users to find out what they would most like Hubble to observe; they overwhelmingly selected the Horsehead Nebula.
One-quarter scale model at the courthouse in Marshfield, Missouri, a hometown of Edwin Hubble
A pillar of gas and dust in the Carina Nebula. This Wide Field Camera 3 image, dubbed Mystic Mountain, was released in 2010 to commemorate Hubble's 20th anniversary in space.
Hubble views the Fomalhaut system. This false-color image was taken in October 2004 and July 2006 with the Advanced Camera for Surveys.
Illustration of the Soft Capture Mechanism (SCM) installed on Hubble
Hubble and JWST mirrors (4.0 m2 and 25 m2 respectively)

Optically, the HST is a Cassegrain reflector of Ritchey–Chrétien design, as are most large professional telescopes.

Maksutov telescope

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".

A 150mm aperture Maksutov–Cassegrain telescope
Dmitry Dmitrievich Maksutov
Light path in a typical "Gregory" or "spot" Maksutov–Cassegrain.
Meade ETX "spot" Maksutov–Cassegrain.
Light path in a typical Rutten Maksutov–Cassegrain.
Light path in a typical sub-aperture Maksutov–Cassegrain.

The design is most commonly seen in a Cassegrain variation, with an integrated secondary, that can use all-spherical elements, thereby simplifying fabrication.

Laurent Cassegrain

Light path in a Cassegrain Reflector

Laurent Cassegrain (c. 1629 – 1 September 1693) was a Catholic priest who is notable as the probable inventor of the Cassegrain reflector, a folded two-mirror reflecting telescope design.

Anton Kutter

German film director and screenwriter.

Kutter's observatory in Biberach

He invented the Schiefspiegler telescope which is a modified Cassegrain reflector featuring superb optical definition due to an off-axis secondary mirror.

Gran Telescopio Canarias

10.4 m reflecting telescope located at the Roque de los Muchachos Observatory on the island of La Palma, in the Canaries, Spain.

Dome of the GTC at sunset.

In the future, when the Cassegrain focus of the telescope is commissioned, it is expected that CanariCam will move to this focus, which will provide superior performance with the instrument.