synonymCassegrain telescope(Cassegrain telescope) Generally refers to Cassegrain telescope
Cassegrain telescope: composed of tworeflectorConstituentReflecting telescopeIt was invented by Cassegrain in 1672.The large one in the mirror is calledPrimary mirror, small ones are calledSecondary mirror。Usually in the center of the main mirrorOpening, imaged behind the primary mirror.Its focus is called Cassegrain focus.Sometimes, according to the figureDotted lineAdd one more slantPlane mirror, imaging on the side, this Cassegrain telescope, also known as Nemesis telescope.
The Cassegrain telescope is designed toBernhard SchmidtBased on the Schmidt star cameraSpherical mirrorMake the primary mirror and correct it with the correction plateSpherical aberration；Following the design of Cassegrainconvex mirrordoSecondary mirror, reflect light throughPrimary mirrorThe central hole converges behind the primary mirrorfocal planeOn.Some designs add other optical elements near the focal plane, such as a flat field mirror.
The U.S. Celestron Terran C9.25 Cassegrain type telescope has many deformations (doubleSpherical mirror, DoubleAspherical mirror, or spherical mirror and aspherical mirror), which can be divided into two main design forms: compact and non compact.
In a tight design, the correction plate is close to or just in thePrimary mirrorFocus on;The non compact correction plate is close to or on the main mirrorcurvatureOn the center (twice the focal length).
A typical example of tight design is the Celestron and Meade products, which combine a solid primary mirror with a smallcurvatureLargeSecondary mirror。In this way, although the breadth of vision is sacrificed, the lens tube can be shrunk to a very short length.Most tightly designed Celestron and Meade primary mirrorsCoke ratioIs f/2, while the secondary mirror is negative f/5, resulting in a system focal ratio of f/10.The exception to be mentioned is Celestron's C-9.25. The focal ratio of the primary mirror is f/2.3, and the focal ratio of the secondary mirror is f/4.3. As a result, the lens barrel is longer than the general compact model, and the field of vision is relatively flat.
The non compact design makes the correction plate close to or on the curvature center of the primary mirror. A very good example of Schmidt Cassegrain design is concentric, which means that the curvature center of all mirrors is at one point: the curvature center of the primary mirror.Optically, the non compact design can produce betterFlat fieldAnd modification, but the length of the lens barrel has increased.
The "traditional" Cassegrain telescope hasParabolic mirrorOfPrimary mirror, andHyperboloidOfSecondary mirrorThe light is reflected and passed through the hole in the center of the main mirror, and the design of folding optics makes the length of the mirror tube shrink.In the lens of small telescopes and cameras, the secondary mirror is usually installed in the transparent part of the closed telescope tubeoptical glassOn boardOptical platform。Such device can eliminate the "star shape" caused by spider shaped support frameScattering effect。Although closing the mirror tube will causeAmount of lightBut the barrel can be kept clean,Primary mirrorIt can also be protected.
It uses hyperboloid andparaboloidSome characteristics of reflection,concaveOfParabolic reflectorYou can paralleloptic axisAll incident rays converge at a single point - focus;Convex surfaceOfHyperboloidreflectorThere are two focal points, which reflect all rays passing through one focal point to the other.The lens of this type of telescope is designedHuianIt is placed at a position where the light can share a focus, so that the light canFace mirrorIs imaged at the other focus for observation, usually externaleyepieceIt will also be at this point.The primary mirror of the paraboloid will enter theDirectional lightReflect and converge on the focus. This point is alsohyperbolaA focal point of the mirror.Then the hyperboloid mirror reflects these rays to another focus, where you can observe the image
Ritchey- chretien
(R-C system, Richie Kleikion):
Structural characteristics of optical telescope
Light parallel to the optical axis, satisfaction, etcOptical pathCassegrain telescope with sine condition.It was proposed by H. ChretienR-C telescope。Its focus is called R-C focus.The mainSecondary mirrorShape is very close to rotationHyperboloidIn practice, this system can be approximately regarded as eliminating three levelsSpherical aberrationandComaA system of rotating hyperboloids.Due to the elimination of comafieldIt is larger than other forms of Cassegrain telescope, and the image spot is symmetric ellipse.If a curved negative is used, the field of view will be significantly increased, and the image spot will be circular.One primary mirrorrelative aperture 1/3, the relative aperture of the system is 1/8, and the image is not far behind the primary mirror. The eccentricity of the primary mirror is close to the hyperboloid of 1.06, and the secondary mirrorEccentricityHyperboloid close to 2.56.On the ideal image plane (image plane of paraxial light), if the image spot dispersion is required to be no more than 1, it can be usedField diameterAbout 19 ';If bending negative film is used, the image spot dispersion is still required to be no more than 1, and the field diameter can reach 37 '.To get a larger field of view, you need to addImage field correction lens。joinImage field correctionOfR-C telescopeIt is also better than Cassegrain telescope with parabolic primary mirror.But when the main focus is used in the R-C telescope, the image isSpherical aberrationOf.Therefore, at least one correction lens orreflector。
Typical Cassegrain systemPrimary mirrorIs a paraboloid,Secondary mirrorIt is hyperboloid, so only spherical aberration can be corrected. If the primary mirror is also changed to hyperboloid, two kinds of correction can be madeaberration, spherical aberration sumComa ，fieldIt can also be increased appropriately, but it needs to be corrected in order to further increase the field of viewField music、AstigmatismAnd distortion, which requires adding a set of correction lenses composed of at least two lenses in the image side, which can be calledField mirror。
The Dar Chickham Kaseglin telescope was designed by Hollersdale in 1928 and edited by the then scientific American in 1930Amateur astronomerAlan Chickham and Albert G. Ingel wrote a paper and published it in the magazine.This design uses concave elliptical mirror as the primary mirror, convexSpherical mirrorDo secondreflector。This system is easier to grind than Cassegrain's or Richie Klein's systems, but there is no correction for off-axisComa aberrationandfieldDistortion, so the image quality will quickly deteriorate when it leaves the axis.But for the longCoke ratioSince the influence of F/15 is small, the mirror with focal ratio above F/15 will still adopt this form of design.
(H-C system, Holden Cassegrain): two spheresreflector
Structural characteristics of optical telescope
Hougton'sCorrectorBy a pieceBiconvex lensAnd a pairConcave mirrorComposition, can be well correctedSpherical aberration，Coma, distortion, availablefieldVery large, the color difference is also very small and can be ignoredaberrationMainly off-axis astigmatism, all surfaces are spherical,Radius of curvatureLarger (unlike the small curvature radius of Maksutov's corrector), it is easy to process and has lower requirements for materialsIn terms of installation,CorrectorThe distance between the two lenses and the distance between them and the main mirror has a large tolerance, mainly for aligning the optical axis
Hougton performs well both visually and photographicallyPersonally, Hougton is bigCoke ratio(Fast) It can better reflect its advantages when used for photographyIf the focal ratio is small, there is no obvious advantage compared with the parabolic Niu inverted telescope. Some foreign DIYers have made Hougton Niu telescopesIt can be said that this type of mirror is the only one DIYER can makeIn addition, in theosloIt has been tested. When the aperture is small (such as 100mm, 120mm)Biconvex lensChange to convex flat mirror, doubleConcave mirrorChange to concave flat mirror, although some will be introducedaberration, but very small (according to the photographic requirements). As long as the requirements are not quite high, they are completely within the acceptable range.Schmidt Cassegrain type
schmit-cassegrain
Structural characteristics of optical telescope
Schmidt Cassegrain telescope is a kind ofCatadioptric telescopeThe folded light path is combined with the correction plate to form a compact astronomical instrument.Schmidt-Cassegrain's design is based onBernhard SchmidtBased on Schmidt star camera, just like Schmidt star cameraSpherical mirrorAs the primary mirrorSchmidt corrector plate To correctSpherical aberration；Following the design of Cassegrainconvex mirrordoSecondary mirror, reflect the light through the hole in the center of the main mirror, and convergePrimary mirrorRearfocal planeOn.Some designs add other optical elements near the focal plane, such as a flat field mirror.
Schmidt Cassegrain type
It has many deformations (double spherical mirror, double aspherical mirror, or spherical mirror and aspherical mirror each), and can be divided into two main design forms: compact and non compact.In a compact design, the correction plate is close to or at the focus of the primary mirror;The non compact correction plate is close to or just on the curvature center of the primary mirror (twice the distance of the focal length).A typical example of tight design is the Celestron and Meade products, which combine a solid primary mirror with a smallcurvatureLargeSecondary mirror。In this way, although the breadth of vision is sacrificed, the lens tube can be shrunk to a very short length.Most tightly designed Celestron and Meade primary mirrorsCoke ratioIs f/2, while the secondary mirror is negative f/5, resulting in a system focal ratio of f/10.The exception to be mentioned is Celestron's C-9.25. The focal ratio of the primary mirror is f/2.3, and the focal ratio of the secondary mirror is f/4.3. As a result, the lens barrel is longer than the general compact model, and the field of vision is relatively flat.The non compact design makes the correction plate close to or on the curvature center of the primary mirror. A very good example of Schmidt Cassegrain design is concentric, which means that the curvature center of all mirrors is at one point: the curvature center of the primary mirror.Optically, the non compact design can produce better flat field and deformation correction than the compact design, but the length of the barrel is increased.
Maksutov-cassegrain
Maksutov Kaseglin style:
Structural characteristics of optical telescope
Maksutov is a refracting reflecting (mirror lens) telescope designed to reduce off-axisaberration, for exampleComa aberration。In 1944,Soviet UnionPhotologist Demetri Maksutov invented this type of telescopeSpherical mirrorAs the primary mirror and combined withincident lightHoleyLunateTo correctSpherical aberration, this isReflecting telescopeAnd other types ofMajor issues。The biggest disadvantage of the Maksutov type is that it can not produce large caliber (>250mm/10in), because the weight and production cost will rise due to the inhibition of the correction plate.
Maksutovobjective lenseNot able to correct the whole beamSpherical aberration, only the edge spherical aberration can be corrected, so there is residual spherical aberrationaberrationFor example, only correctionComa , cannot be correctedAstigmatism。At the time of his invention, Marksutov himself suggested that it was possible to replace Cassegrain's "folding" optical structure.Perkin ElmerThe designer John Gregory developed the Maksutov Cassegrain telescope from the idea of Maksutov.Later, GregorySky and telescopeThe magazine published the landmark design of the f/15 and f/23 Maksutov Cassegrain telescopes, which clearly foresaw the commercial use of this design for Perkin Elmer.
Optical telescope
Many of the Mark Sutov styles that have been made use of the design of "Kaseglin" (sometimes called "Dot Mark Sutov"), the originalSecondary mirrorIt is replaced by a small piece of aluminum spot on the inside of the correction plate.The advantage is that it has been fixed without alignment and calibration, and it also eliminates the problems caused by spider shaped support frameDiffraction fringe。The disadvantage is that a certain degree of freedom (the radius of curvature of the secondary mirror) is lost, because the radius of curvature of the secondary mirror must be consistent with the inside of the meniscus correction plate.Gregory himself, for the second time,RedesignIf the speed is fast (f/15), the front of the correction plate or the main mirror is aspheric to reduce the aberration.
Schmidt-meniscus Cassegrain
Schmidt lunate Cassegrain
This type of telescope can be said to combine the advantages of Schmidt and Marksutov, so it is equivalent to calling two kinds of correctors, Schmidt is used for correctionSpherical aberrationThe crescent moon is used to correct the coma, but this type of Cassegrain is too long for large caliber.
Mangin-Cassegrain
Agunov Kaseglin
Structure of Cassegrain Telescope
The Agunov Cassegrain telescope was designed by PP. Agunov was first introduced to the world.All his optics areSpherical mirrorAnd the traditional Cassegrain styleSecondary mirrorReplace with three lens elements with air gaps.The lens furthest from the main mirror is the Manjing mirror, which functions as the surface of the second mirror, and has a reflective coating on the side facing the sky.Agunov's system only uses spherical surfaces to avoidaspheric surfaceManufacturing and testing.However, it seems that the benefits obtained are very few, because this system is actually very difficult to make, and it requires an accurate radius of curvature of the free zone sphere to replace the equivalent aspherical mirror.
Pressmann-Camichel
Presman Carmichael Cassegrain
Optical telescope
Compared with the above types of Cassegrain, the Pressmann Camichel Type is the easiest to manufacture, but its quality is poor. It can only be used with Schmidt corrector.
Schiefspiegler
Off Axis or Oblique ReflectionreflectorCassegrain:
Optical telescope
The Schiefspiegler ("off-axis" or "oblique reflection") mirror is a very strangeCassegrain reflector, he willprimary mirrorTilt to avoid the second reflectorPrimary mirrorCreates a shadow on the.Although the diffraction pattern is eliminated, it leads to other differentaberrationIt must be corrected.
Three-mirror Cassegrain
Cassegrain
Cassegrain telescope
threereflectorThe system is composed of three mirrors, with two spacing, three radii and three cone coefficients, totally eight variables. In addition to meeting the system focusSpherical aberration、ComaAstigmatism, field curvature, etcsystem performance In addition to image quality requirements, there are enough variables for system layout and structureoptimal design 。Three mirror system is better than two mirror systemfieldLarge and easy to controloptical system Stray radiation of the off axis field of viewLuminous fluxTo make the illumination of the image surface more uniform.along withspace technologyThe development of total reflection optical system, especially the threeReflective optical systemIt is gradually becoming the main form of space optical system.
This design is very common on telescopes provided by manufacturers to consumers, because sphericalOptical surfaceNot only is it better than long focal lengthRefracting telescopeEasy to make.Although this kind of telescope is better thanSame caliberOfReflecting telescopeThe price is more expensive, but because of the compact optical design, it is easy to carry within the designed aperture, making itAstronomerIt has become a mainstream amateur high-end astronomical observation instrument.HighCoke ratioIt means it is different from the formerSchmidtThe astrograph is not a wide-angle telescope, but its narrow field of view is very suitable for observing planets anddeep space 。[1]
