Editing Newtonian telescope (section)

==Description==
[[File:Newtonian telescope2.svg|thumb|Newtonian telescope design]]

A Newtonian telescope is composed of a [[primary mirror]] or [[objective (optics)|objective]], usually [[parabolic reflector|parabolic]] in shape, and a smaller flat [[secondary mirror]]. The primary mirror makes it possible to [[etendue|collect light]] from the pointed region of the sky, while the secondary mirror redirects the light out of the [[optical axis]] at a [[right angle]] so it can be viewed with an [[eyepiece]].

===Advantages of the Newtonian design===
* They are free of [[chromatic aberration]] found in refracting telescopes.
* Newtonian telescopes are usually less expensive for any given objective diameter (or [[aperture]]) 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 ([[Gregorian telescope|Gregorians]], [[Cassegrain reflector|cassegrains]], and early refractors had two surfaces that need [[figuring]]. Later [[Achromatic lens|achromatic]] refractor objectives had four surfaces that have to be figured).
* A short [[focal ratio]] can be more easily obtained, leading to a wider [[Field of view#Astronomy|field of view]].
* The eyepiece is located at the top end of the telescope. Combined with short f-[[ratios]] this can allow for a much more compact mounting system, reducing cost and adding to portability.

=== Disadvantages of the Newtonian design ===
[[File:Telescope trailer 22.jpg|thumb|right|A large Newtonian with ladder to reach the eyepiece]]

* Newtonians, like other reflecting telescope designs using [[parabolic reflector|parabolic mirrors]], suffer from [[coma (optics)|coma]], an off-axis aberration which causes imagery to flare inward and towards the [[optical axis]] (stars towards edge of the [[field of view]] take on a [[comet]]-like shape). This flare is zero on-axis, and is [[linear]] with increasing [[field angle]] and inversely proportional to the square of the mirror [[focal ratio]] (the mirror [[focal length]] divided by the mirror diameter). The formula for third order [[tangential coma]] is 3θ / 16F², where θ is the angle off axis to the image in [[radian]]s and F is the focal ratio. Newtonians with a [[focal ratio]] of f/6 or lower (f/5 for example) are considered to have increasingly serious coma for visual or photographic use.<ref>{{cite web |first=Vladimir|last=Sacek |date=2006-07-14 |title=8.1.1. Newtonian off-axis aberrations |url=https://telescope-optics.net/newtonian_off_axis_aberrations.htm |quote=off-axis performance of the paraboloidal mirror drops so quickly with the increase in relative aperture beyond ~ƒ/6 |access-date=2009-09-29}}</ref> Low focal ratio primary mirrors can be combined with lenses that correct for coma to increase image sharpness over the field.<ref>{{cite web |last=Knisely|first=David |title=Tele Vue Paracor Coma Corrector for Newtonians |work=Cloudy Nights Telescope Review |year=2004 |url=http://www.cloudynights.com/documents/paracorr.pdf |access-date=2010-11-29}}</ref>
* Newtonians have a central obstruction due to the secondary mirror in the light path.  This obstruction and also the [[diffraction spike]]s caused 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 diffraction [[sidelobe]] intensities 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 Newtonians [[Collimated light#Collimation and decollimation|collimation]] can 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 (specifically [[Maksutov telescope|Maksutov 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 on [[equatorial mount]]s,<ref>{{cite book |first=Alex|last=Hebra |title=The Physics of Metrology: All about Instruments: From Trundle Wheels to Atomic Clocks |url=https://books.google.com/books?id=mds4BpM3hdYC&pg=PA258 |year=2010 |publisher=[[Springer Science+Business Media]] |isbn=978-3-211-78381-8 |pages=258–259}}</ref> tube orientation can put the [[eyepiece]] in a very poor viewing position, and larger telescopes require [[ladder]]s or support structures to access it.<ref>{{cite book |first=Antony|last=Cooke |title=Make Time for the Stars: Fitting Astronomy into Your Busy Life |url=https://books.google.com/books?id=ARS84_3BMTMC&pg=PA21 |year=2009 |publisher=[[Springer Science+Business Media]] |isbn=978-0-387-89341-9 |page=14}}</ref> 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.