OBSERVATION - 2

THARINDRA KORALEGEDARA

TELESCOPES

Telescope history In 1608 inthe Netherlands, whena patent was submittedby Hans Lippershey,an eyeglass maker.

Galileo improved on thisdesign the following year andapplied it to astronomy. Isaac Newton is credited withbuilding the first reflector in1668 with a design thatincorporated a small flatdiagonal mirror to reflect thelight to an eyepiece mountedon the side of the telescope.

Classification oftelescopes Refracting telescopes (Dioptrics) Achromatic telescope dialytic refractor Apochromatic Binoculars Opera glasses Copyscope Galileoscope Monocular Non-achromatic Galilean telescope Keplerian Telescope Aerial telescope Superachromat Baden-Powell's unilens

Reflecting telescopes (Catoptrics) Cassegrain telescope Dall–Kirkham telescope Nasmyth telescope Ritchey–Chrétien telescope Crossed Dragone Gregorian telescope Herschelian telescope Large liquid mirror telescope Newtonian telescope Dobsonian telescope Pfund telescope Schiefspiegler telescope Stevick–Paul telescope Three-mirror anastigmat Toroidal reflector / Yolo telescope Wolter telescope

Catadioptric telescopes (Catadioptrics) Argunov–Cassegrain telescope Catadioptric dialytes Klevtsov–Cassegrain telescope Lurie–Houghton telescope Maksutov telescope Maksutov camera Maksutov–Cassegrain telescope Gregory (Spot) Maksutov-Cassegrain telescope Rutten Maksutov-Cassegrain telescope Sub-aperture corrector Maksutov-Cassegraintelescope Maksutov Newtonian telescope Modified Dall–Kirkham telescope Schmidt camera Baker-Nunn camera Baker-Schmidt camera Lensless Schmidt telescope Mersenne-Schmidt camera Schmidt–Cassegrain telescope ACF Schmidt–Cassegrain telescope (MeadeInstruments) Schmidt–Newton telescope Schmidt-Väisälä camera

Telescope Mounts A telescope mount is a mechanical structure which supports a telescope.Telescope mounts are designed to support the mass of the telescope andallow for accurate pointing of the instrument. Many sorts of mounts havebeen developed over the years, with the majority of effort being put intosystems that can track the motion of the fixed stars as the Earth rotates. Fixed mountsFixed altitude mountsTransit mountsAltitude – Altitude mountsHexapod mountAltazimuth mountEquatorial mount

Altazimuth mount An altazimuth or alt-azimuthmount is a simple two-axismount for supporting androtating an instrument abouttwo perpendicular axes –one vertical and the otherhorizontal. Rotation aboutthe vertical axis varies theazimuth (compass bearing)of the pointing direction ofthe instrument. Rotationabout the horizontal axisvaries the altitude (angle ofelevation) of the pointingdirection.

When used as an astronomicaltelescope mount, the biggestadvantage of an alt-azimuth mount isthe simplicity of its mechanicaldesign. The primary disadvantage isits inability to follow astronomicalobjects in the night sky as the Earthspins on its axis. On the other hand,an equatorial mount only needs to berotated about a single axis, at aconstant rate, to follow the rotationof the night sky (diurnal motion).Altazimuth mounts need to berotated about both axes at variablerates, achieved via microprocessorbased two-axis drive systems, totrack equatorial motion.

Equatorial Mount An equatorial mount is a mount forinstruments that compensates forEarth's rotation by having onerotational axis parallel to the Earth'saxis of rotation. This type of mount isused for astronomical telescopes andcameras. The advantage of anequatorial mount lies in its ability toallow the instrument attached to it tostay fixed on any celestial object withdiurnal motion by driving one axis ata constant speed. Such anarrangement is called a sidereal orclock drive.

In astronomical telescope mounts, theequatorial axis (the right ascension) ispaired with a second perpendicular axis ofmotion (known as the declination). Theequatorial axis of the mount is oftenequipped with a motorized "clock drive",that rotates that axis one revolution every23 hours and 56 minutes in exact sync withthe apparent diurnal motion of the sky.They may also be equipped with settingcircles to allow for the location of objects bytheir celestial coordinates. Equatorialmounts differ from mechanically simpleraltazimuth mounts, which require variablespeed motion around both axes to track afixed object in the sky. Also, forastrophotography, the image does notrotate in the focal plane, as occurs withaltazimuth mounts when they are guided totrack the target's motion, unless a rotatingerector prism or other field-derotator isinstalled.

Telescopes as cameras Most astronomicaltelescopes are cameras –they form images ofobjects both onand off the optical axis inthe focal plane. Although a telescope’soptics can becomplex, we can profitablyrepresent them with asingle ‘‘equivalent thinlens’’ ofmatching aperture and afocal length thatreproduces the imageforming properties of thetelescope.

Image scale and image size The image scale, s, describes the mapping of the sky by any camera. Theimagescale is the angular distance on the sky that corresponds to a unit lineardistance inthe focal plane of the camera. Figure shows the equivalent lens diagram of acamera of focal length f. We draw the paths followed by two rays, onefrom a staron the optical axis, the other from a star separated from the first by asmall angle hon the sky. Rays pass through the vertex of the lens without deviation, soassuming the paraxial approximation, ϴ tan ϴ, it should be clear from thediagram

Focal ratio and image rightness For example, the 20-inch telescope at Vassar College Observatory hasa focal length of 200 inches, so R 10. This is usually expressed as‘‘f /10’’.You can show that the brightness (energy per unit area in the focalplane) of anextended source in the focal plane is proportional to R-2, so that images in an f /5 system, for example, will be four times asbright as images in an f /10 system.

Image quality: telescopic resolution The wave properties of light set afundamental limit on the quality ofa telescopic image. Following figure illustrates theformation of an image by atelescope outside the atmosphere.

The diffraction limit Even though the source is a point, its image – created by aperfect telescope operating in empty space – will have a finite size because ofdiffraction of the wave. This size, the diffraction limit of the telescope, dependson both the wavelength of light and on Diameter of the telescope. The diffraction of a plane wavefrontby a circular aperture is a messy problem in wave theory, and its solution, firstworked out in detail by the English astronomer George Airy in 1831, saysthat the image is a bulls-eye-like pattern, with the majority (84%) of the lightfocused into a spot or ‘‘disk.’’ Concentric bright rings, whose brightnessdecreases with distance from the center, surround the very bright centralspot, the Airy disk. The angular radius of the dark ring that borders the Airydisk is

If two-point sources lie close together, their blended Airypatterns may not be distinguishablefrom that of a single source. If we can say for sure that aparticular pattern is due to two sources, not one, the sources areresolved.

Atmospheric refraction We can approximate the Earth’s atmosphere as a series of plane-parallel plates,and the surface as an infinite plane. for example, we imagine an atmosphere of just two layers that have indices, n2 n1. A ray incident at anglea refracts at each of the two interfaces, and ultimately makes a new angle, a 1Da, with the surface: thus, refraction shifts the apparent position of a sourcetowards the zenith. we imagine that the atmosphere consistsof a very large number of thin layers, so in the limit, the effect of refractionis to curve the path of the incident ray

Chromatic aberration Since n(λ) for optical glasses decreases with wavelength in the visible, thenthefocal length of a convex lens will be longer for red wavelengths than for blue.Different colors in an image will focus at different spots. This inability toobtain perfect focus is called chromatic aberration. Chromatic aberration will not occur in all-reflecting optics. To correct it in alens, the usual strategy is to cement together two lenses made of differentglasses, a positive (convex) lens with low chromatic dispersion, and a negativelens with lower absolute power, but higher chromatic dispersion.

Monochromatic wavefront aberrations An aberration is an imperfection intelescope design that degrades animage. Straight lines in the sky shouldproduce straight lines in the imageplane. This mapping of the pointsand lines on the sky to points andlines in theimage plane is called a collineartransformation. If an optical systemfails toproduce this collineartransformation it is said to exhibitaberrations. Spherical aberrationComaAstigmatismCurvature of fieldDistortion

Sphericalaberration Except for sphericalaberration, all thewavefront errors inTable 5.4 vanishfor sources on axis For visual astronomy,where one typicallyexaminesonly on-axis images, SAis the onlymonochromaticaberration that istroublesome.

Coma Prior to the end of the nineteenthcentury, visual observers wereconcerned onlywith a telescope’s on-axisperformance. The advent ofphotography changedthose concerns forever, andtelescope design has since neededto satisfy morestringent optical criteria. Coma is the wavefront aberrationwhich means,like spherical aberration, it isparticularly a problem of largeapertures. UnlikeSA, coma increases with objectdistance from the axis.

Astigmatism

Spot Diagrams

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light to an eyepiece mounted on the side of the telescope. Classification of telescopes Refracting telescopes (Dioptrics) . Dobsonian telescope Pfund telescope Schiefspiegler telescope . and ultimately makes a new angle, a 1 Da, with the su