Newtonian telescope

TheNewtonian telescope,also called theNewtonian reflectoror just aNewtonian,is a type ofreflecting telescopeinvented by the English scientistSir Isaac Newton,using aconcave primary mirrorand a flat diagonalsecondary mirror.Newton'sfirst reflecting telescopewas completed in 1668 and is the earliest known functional reflecting telescope.^[1]The Newtonian telescope's simple design has made it very popular withamateur telescope makers.^[2]

Description

Newtonian telescope design

A Newtonian telescope is composed of aprimary mirrororobjective,usuallyparabolicin shape, and a smaller flatsecondary mirror.The primary mirror makes it possible tocollect lightfrom the pointed region of the sky, while the secondary mirror redirects the light out of theoptical axisat aright angleso it can be viewed with aneyepiece.

Advantages of the Newtonian design

They are free ofchromatic aberrationfound in refracting telescopes.
Newtonian telescopes are usually less expensive for any given objective diameter (oraperture) than comparable quality telescopes of other types.
Since there is only one surface that needs to be ground and polished into a complex shape, overall fabrication is much simpler than other telescope designs (Gregorians,cassegrains,and early refractors had two surfaces that needfiguring.Laterachromaticrefractor objectives had four surfaces that have to be figured).
A shortfocal ratiocan be more easily obtained, leading to a widerfield of view.
The eyepiece is located at the top end of the telescope. Combined with short f-ratiosthis can allow for a much more compact mounting system, reducing cost and adding to portability.

Disadvantages of the Newtonian design

A large Newtonian with ladder to reach the eyepiece

Newtonians, like other reflecting telescope designs usingparabolic mirrors,suffer fromcoma,an off-axis aberration which causes imagery to flare inward and towards theoptical axis(stars towards edge of thefield of viewtake on acomet-like shape). This flare is zero on-axis, and islinearwith increasingfield angleand inversely proportional to the square of the mirrorfocal ratio(the mirrorfocal lengthdivided by the mirror diameter). The formula for third ordertangential comais 3θ / 16F², where θ is the angle off axis to the image inradiansand F is the focal ratio. Newtonians with afocal ratioof f/6 or lower (f/5 for example) are considered to have increasingly serious coma for visual or photographic use.^[3]Low focal ratio primary mirrors can be combined with lenses that correct for coma to increase image sharpness over the field.^[4]
Newtonians have a central obstruction due to the secondary mirror in the light path. This obstruction and also thediffraction spikescaused by the support structure (called the "spider" ) of the secondary mirror reduce contrast. Visually, these effects can be reduced by using a two or three-legged curved spider. This reduces the diffractionsidelobeintensities by a factor of about four and helps to improve image contrast, with the potential penalty that circular spiders are more prone to wind-induced vibration.
For portable Newtonianscollimationcan be a problem. The primary and secondary can get out of alignment from the shocks associated with transport and handling. This means the telescope may need to be re-aligned (collimated) every time it is set up. Other designs such as refractors and catadioptrics (specificallyMaksutov cassegrains) have fixed collimation.
The focal plane is at an asymmetrical point and at the top of the optical tube assembly. For visual observing, most notably onequatorial mounts,^[5]tube orientation can put theeyepiecein a very poor viewing position, and larger telescopes requireladdersor support structures to access it.^[6]Some designs provide mechanisms for rotating the eyepiece mount or the entire tube assembly to a better position. For research telescopes, counterbalancing very heavy instruments mounted at this focus has to be taken into consideration.

Variations

There are several variations on the Newtonian design that add a lens to the system creating acatadioptric telescope.This is done to correctspherical aberrationor reduce cost.

Schmidt–Newtonian

Schmidt–Newtonian telescope.

A Schmidt–Newtonian telescope combines the Newtonian optical design with a full-apertureSchmidt corrector platein front of the primary mirror that not only correctsspherical aberrationbut can also support the secondary mirror. The resulting system has lesscomaand secondary mirror support induced diffraction effects.^[7]

Maksutov–Newtonian

Similar to a Schmidt–Newtonian, aMaksutovtelescope'smeniscus shaped correctorcan be added to the Newtonian configuration, which gives it minimal aberration over a widefield of view,with one-fourth the coma of a similar standard Newtonian and one-half the coma of a Schmidt-Newtonian.^[8]Diffraction can also be minimized by using a highfocal ratiowith a proportionally small diagonal mirror mounted on the corrector.^[9]

Jones–Bird

A Jones–Bird Newtonian (sometimes called a Bird–Jones) uses a spherical primary mirror in place of a parabolic one, with spherical aberrations corrected bysub-aperture correctorlens^[10]usually mounted inside the focusser tube or in front of the secondary mirror. This design reduces the size and cost of the telescope with a shorter overall telescope tube length (with the corrector extending the focal length in a "telephoto"type layout) combined with a less costly spherical mirror. Commercially produced versions of this design have been noted to be optically compromised, due to the difficulty of producing a correctly shaped sub-aperture corrector, and are targeted at the inexpensive end of the telescope market.^[11]

History

Newton's idea for a reflecting telescope was not new.Galileo GalileiandGiovanni Francesco Sagredohad discussed using a mirror as the image formingobjectivesoon after the invention of the refracting telescope,^[12]and others, such asNiccolò Zucchi,claimed to have experimented with the idea as far back as 1616.^[13]Newton may even have readJames Gregory's1663 bookOptica Promotawhich described reflecting telescope designs usingparabolic mirrors^[14](a telescope Gregory had been trying unsuccessfully to build).^[15]

Newton built his reflecting telescope because he suspected it could prove his theory thatwhite lightis composed of aspectrumof colours.^[16]Colour distortion (chromatic aberration) was the primary fault ofrefracting telescopesof Newton's day, and there were many theories as to what caused it. During the mid-1660s with his work onthe theory of colour,Newton concluded this defect was caused by the lens of the refracting telescope behaving the same asprismshe was experimenting with, breaking white light into a rainbow of colours around brightastronomical objects.^[17]^[18]If this were true, then chromatic aberration could be eliminated by building a telescope which did not use a lens – a reflecting telescope.

A replica of a second reflecting telescope Newton presented to theRoyal Societyin 1672 (the first one he made in 1668 was loaned to an instrument maker but there is no further record of what happened to it).^[19]

In late 1668 Isaac Newton built hisfirst reflecting telescope.He chose analloy(speculum metal) oftinandcopperas the most suitable material for hisobjectivemirror. He later devised means for shaping and grinding the mirror and may have been the first to use apitch lap^[20]to polish the optical surface. He chose a spherical shape for his mirror instead of a parabola to simplify construction; even though it would introducespherical aberration,it would still correct chromatic aberration. He added to his reflector what is the hallmark of the design of a Newtonian telescope, a secondary diagonally mounted mirror near the primary mirror's focus to reflect the image at a 90° angle to aneyepiecemounted on the side of the telescope. This unique addition allowed the image to be viewed with minimal obstruction of the objective mirror. He also made the tube,mount,and fittings. Newton's first version had a primary mirror diameter of 1.3 inches (33 mm) and afocal ratioof f/5.^[21]He found that the telescope worked without colour distortion and that he could see the fourGalilean moonsofJupiterand thecrescent phase of the planet Venuswith it. Newton's friendIsaac Barrowshowed a second telescope to a small group from theRoyal Society of Londonat the end of 1671. They were so impressed with it that they demonstrated it toCharles IIin January 1672. Newton was admitted as a fellow of the society in the same year.

Like Gregory before him, Newton found it hard to construct an effective reflector. It was difficult to grind the speculum metal to a regular curvature. The surface alsotarnishedrapidly; the consequent low reflectivity of the mirror and also its small size meant that the view through the telescope was very dim compared to contemporary refractors. Because of these difficulties in construction, the Newtonian reflecting telescope was initially not widely adopted. In 1721John Hadleyshowed a much-improved model to the Royal Society.^[22]Hadley had solved many of the problems of making aparabolicmirror. His Newtonian with a mirror diameter of 6 inches (150 mm) compared favourably with the largeaerial refracting telescopesof the day.^[23]

Notes

^Hall, A. Rupert (1992).Isaac Newton: Adventurer in Thought.Cambridge University Press.p. 67.ISBN 9780521566698.
^Ingalls, Albert G.,ed. (1935).Amateur Telescope Making(4th ed.). Munn and Co., Inc.[1]
^Sacek, Vladimir (2006-07-14)."8.1.1. Newtonian off-axis aberrations".Retrieved2009-09-29.off-axis performance of the paraboloidal mirror drops so quickly with the increase in relative aperture beyond ~ƒ/6
^Knisely, David (2004)."Tele Vue Paracor Coma Corrector for Newtonians"(PDF).Cloudy Nights Telescope Review.Retrieved2010-11-29.
^Hebra, Alex (2010).The Physics of Metrology: All about Instruments: From Trundle Wheels to Atomic Clocks.Springer Science+Business Media.pp. 258–259.ISBN 978-3-211-78381-8.
^Cooke, Antony (2009).Make Time for the Stars: Fitting Astronomy into Your Busy Life.Springer Science+Business Media.p. 14.ISBN 978-0-387-89341-9.
^"Schmidt-Newton telescope".telescopeOptics.net.Retrieved28 August2012.
^Rutten, Harrie G. J.; van Venrooij, Martin A.M. (1988).Telescope Optics: Evaluation and design.ISBN 9780943396187.
^Mollise, Rod (28 February 2009).Choosing and Using a New CAT.p. 101.ISBN 9780387097725.
^10.1.2. Sub-aperture corrector examples: Single-mirror systems – Jones-Bird
^"TELESCOPES – OVERVIEW AND TELESCOPE TYPES, CATADIOPTRIC NEWTONIAN".Archived fromthe originalon 2020-08-11.Retrieved2014-11-18.
^Fred Watson (2007).Stargazer: The Life and Times of the Telescope.Allen & Unwin. p. 108.ISBN 978-1-74176-392-8.
^The Galileo Project > Science > Zucchi, Niccolo
^Derek Gjertsen (1986).The Newton Handbook.Routledge & Kegan Paul. p. 562.ISBN 978-0-7102-0279-6.
^Michael White (1999).Isaac Newton: The Last Sorcerer.Basic Books. p. 169.ISBN 978-0-7382-0143-6.
^Michael White (1999).Isaac Newton: The Last Sorcerer.Basic Books. p. 170.ISBN 978-0-7382-0143-6.
^Newton thought little could be done to correct aberration short of making lenses that weref/50or more. "the object-glass of any telescope cannot collect all the rays which come from one point of an object, so as to make them convene at its focus in less room than in a circular space, whose diameter is the 50th part of the diameter of its aperture”
^Stephen Parkinson (1870).A Treatise on Optics.Macmillan. p.112.
^Henry C. King (1955).The History of the Telescope.p. 74.ISBN 978-0-486-43265-6.Retrieved2013-08-01.
^Raymond N. Wilson (2007).Reflecting Telescope Optics I: Basic Design Theory and its Historical Development.Springer Science & Business Media. p. 9.ISBN 978-3-540-40106-3.
^telescope-optics.net Reflecting Telescopes: Newtonian, two- and three-mirror systems
^amazing-space.stsci.edu – Hadley’s Reflector
^The complete Amateur Astronomer – John Hadley's Reflector

References

Smith, Warren J.,Modern Optical Engineering,McGraw-Hill Inc., 1966, p. 400

[books.google.com-1] Hall, A. Rupert (1992).Isaac Newton: Adventurer in Thought.Cambridge University Press.p. 67.ISBN 9780521566698.

[ingalls1935-2] Ingalls, Albert G.,ed. (1935).Amateur Telescope Making(4th ed.). Munn and Co., Inc.[1]

[3] Sacek, Vladimir (2006-07-14)."8.1.1. Newtonian off-axis aberrations".Retrieved2009-09-29.off-axis performance of the paraboloidal mirror drops so quickly with the increase in relative aperture beyond ~ƒ/6

[4] Knisely, David (2004)."Tele Vue Paracor Coma Corrector for Newtonians"(PDF).Cloudy Nights Telescope Review.Retrieved2010-11-29.

[5] Hebra, Alex (2010).The Physics of Metrology: All about Instruments: From Trundle Wheels to Atomic Clocks.Springer Science+Business Media.pp. 258–259.ISBN 978-3-211-78381-8.

[6] Cooke, Antony (2009).Make Time for the Stars: Fitting Astronomy into Your Busy Life.Springer Science+Business Media.p. 14.ISBN 978-0-387-89341-9.

[telOpt-7] "Schmidt-Newton telescope".telescopeOptics.net.Retrieved28 August2012.

[8] Rutten, Harrie G. J.; van Venrooij, Martin A.M. (1988).Telescope Optics: Evaluation and design.ISBN 9780943396187.

[9] Mollise, Rod (28 February 2009).Choosing and Using a New CAT.p. 101.ISBN 9780387097725.

[10] 10.1.2. Sub-aperture corrector examples: Single-mirror systems – Jones-Bird

[11] "TELESCOPES – OVERVIEW AND TELESCOPE TYPES, CATADIOPTRIC NEWTONIAN".Archived fromthe originalon 2020-08-11.Retrieved2014-11-18.

[12] Fred Watson (2007).Stargazer: The Life and Times of the Telescope.Allen & Unwin. p. 108.ISBN 978-1-74176-392-8.

[13] The Galileo Project > Science > Zucchi, Niccolo

[14] Derek Gjertsen (1986).The Newton Handbook.Routledge & Kegan Paul. p. 562.ISBN 978-0-7102-0279-6.

[15] Michael White (1999).Isaac Newton: The Last Sorcerer.Basic Books. p. 169.ISBN 978-0-7382-0143-6.

[16] Michael White (1999).Isaac Newton: The Last Sorcerer.Basic Books. p. 170.ISBN 978-0-7382-0143-6.

[17] Newton thought little could be done to correct aberration short of making lenses that weref/50or more. "the object-glass of any telescope cannot collect all the rays which come from one point of an object, so as to make them convene at its focus in less room than in a circular space, whose diameter is the 50th part of the diameter of its aperture”

[18] Stephen Parkinson (1870).A Treatise on Optics.Macmillan. p.112.

[19] Henry C. King (1955).The History of the Telescope.p. 74.ISBN 978-0-486-43265-6.Retrieved2013-08-01.

[20] Raymond N. Wilson (2007).Reflecting Telescope Optics I: Basic Design Theory and its Historical Development.Springer Science & Business Media. p. 9.ISBN 978-3-540-40106-3.

[21] telescope-optics.net Reflecting Telescopes: Newtonian, two- and three-mirror systems

[22] zing-space.stsci.edu – Hadley’s Reflector

[23] The complete Amateur Astronomer – John Hadley's Reflector

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