Editing Dobsonian telescope (section)

==Origin and design==
It is hard to classify the Dobsonian telescope as a single invention. In the field of amateur telescope making most, if not all, of its design features had been used before.{{citation needed|date=July 2018}} John Dobson, credited as having invented this design in 1965,<ref>{{cite book |url=https://books.google.com/books?id=Thgzo8q3GJUC&dq=dobsonian+amateur+telescope+makers&pg=PA351 |author1=DiBona, Chris |author2=Stone, Mark |author3=Cooper, Danese |title=Open Sources 2.0 |date=21 October 2005 |page=351|publisher="O'Reilly Media, Inc." |isbn=9780596553890 }}</ref> pointed out that "for hundreds of years, wars were fought using cannon on 'Dobsonian' mounts".

Dobson identified the characteristic features of the design as lightweight objective mirrors made from [[porthole]] glass, and mountings constructed from [[plywood]], [[Teflon]] strips and other low-cost materials.<ref name="levy">{{cite journal |authorlink=David H. Levy |title=Walden of the Sky |journal=[[Sky & Telescope]] |pages=84–87 |date=September 1995|bibcode=1995S&T....90c..84L |last1=Levy |first1=David |last2=Aguirre |first2=E. |volume=90 |issue=3 }}</ref> Since he built these telescopes as aids in his [[avocation]] of instructional [[sidewalk astronomy]], he preferred to call the design a "sidewalk telescope".<ref>{{cite book |url=https://books.google.com/books?id=qnN_KRFgITQC&dq=dobsonian+dobson&pg=PA17 |author=Fred Watson |title=Why is Uranus Upside Down? |date=October 2007 |publisher=Inc NetLibrary |page=17|isbn=9781741762761 }}</ref> Dobson combined all these innovations in a design focused towards one goal: building a very large, inexpensive, easy to use, portable<ref>{{cite book |url=https://books.google.com/books?id=liOMrb227N8C&dq=dobsonian&pg=PA25 |author=Knoll, Peter |title=Induced Seismic Events |date=30 July 1996 |page=25|publisher=Springer |isbn=9783764354541 }}</ref> telescope, one that could bring [[Deep-sky object#Observations and activities|deep-sky]] astronomy to the masses.<ref>{{cite book |url=https://books.google.com/books?id=I7qnSOAVa90C&dq=dobsonian&pg=PA92 |author=Albert, David H. |title=And the Skylark Sings with Me  |date=January 1999 |page=93|publisher=New Society Publishers |isbn=9780865714014 }}</ref>

===Dobson's design innovations===
[[File:Two dobsonians.jpg|thumb|right|Two [[Amateur telescope making|Amateur]] built Dobsonians at [[Stellafane]] in the early 1980s]]
Dobson's design allows a builder with minimal skills to make a very large telescope out of common items. Dobson optimized the design for observation of faint objects such as [[star cluster]]s, [[nebula]]e, and [[galaxies]] (what amateur astronomers call ''[[deep sky object]]''s). These dim objects require a large objective mirror able to gather a large amount of light.  Because "''deep sky''" observing often requires travel to [[Dark-sky movement|dark locations]] away from city lights, the design benefits from being more compact, portable, and rugged than standard large Newtonian telescopes of times past, which typically utilized massive [[Equatorial mount#German equatorial mount|German equatorial mounts]]. John Dobson's telescopes combined several innovations to meet these criteria, including:

* '''Nontraditional [[altazimuth mount|alt-azimuth]] mount''': Instead of a standard mount using axial bearings, Dobson opted for a very stable design that was simple to build, and which had fewer mechanical limitations when used with large and heavy telescopes. He modified the classical fork mount into a free-standing three-piece construction, which holds the telescope steady on seven discrete support points and allows for easy and safe repositioning of a large and heavy telescope.
** The classical Dobsonian mount (refer to Fig.1) consists of a flat horizontal "ground board" platform (Fig.1, black) on top of which are attached three of the seven supports (Fig.1, bottom yellow). Upon these three supports rests a box construction called a "rocker box" (Fig.1, dark blue). A loose center-bolt (Fig.1, dark green) keeps the rocker box centered and allows it to pivot above the ground board. On opposing sides of the rocker box, semicircular depressions are cut out from the top edge of each wall (the rocker box is open on the top and on the back). Each depression has a widely spaced pair of supports installed in the cut (Fig.1, top yellow). The telescope optical tube assembly (OTA, Fig.1, light blue) has two large round [[trunnions]] (or arc-shaped rails for larger telescopes) secured on the left and right sides (Fig.1, red). Their common axis intersects the [[center of gravity]] of the telescope OTA. The trunnions (commonly known as altitude bearings) rest atop the aforementioned four supports in the top cutouts of the rocker box. To raise the telescope (altitude), just lift the tube and the trunnions will slide over the four supports. To move the telescope left or right (azimuth), push or pull the top rim of the OTA (some have a dedicated handle) so that the pivoting rocker box slides over the ground board's three supports. [[File:Dob.png|thumb|right|Fig.1 Dobsonian mount schematics]]
** Classical Dobsonian mount parts are typically made from plywood and other cheap materials which are glued, screwed, or even nailed together. In contrast to other telescope mount types, no precision-machined mechanical parts are required. For smooth sliding motions, small [[Teflon]] (PTFE) blocks are used for the seven supports. Their surface sizes can be precisely calculated for the particular OTA weight. To improve the smoothness and steady position-holding, the bottom of the rocker box is typically covered with micro-textured [[Formica (plastic)|Formica]]. The altitude trunnions often have a large diameter, and can also be covered with textured material. For larger telescopes, semicircular wood pieces or arc-shaped rails can be used instead of round trunnions.
** The use of Teflon over textured material combines with gravity-produced wedging forces to create a unique smooth action, transitioning from rock steady to smooth motion and back. Thus a clamp mechanism is not needed to prevent unintentional motion of the telescope, unlike most other telescope mounts. The steadiness of the classical Dobsonian is unparalleled, as the telescope is actually not rotating on two axes as other mounts but instead statically standing on seven solid blocks (until pushed to a new position). 

* '''Thin mirrors''': Instead of costly [[Pyrex]] mirror blanks with the standard 1:6 thickness ratios<ref name="books.google.com"/> (1&nbsp;cm thick for every 6&nbsp;cm in diameter) so they don't flex and sag out of shape under their own weight, Dobson used mirrors made out of glass from surplus ship porthole covers usually with 1:16 thickness ratios.<ref>{{cite book |url=https://books.google.com/books?id=l2TNnHkdDpkC&dq=dobsonian+amateur+telescope+makers&pg=PA286 |author1=Newton, Jack |author2=Teece, Philip |title=The Guide to Amateur Astronomy |date=19 January 1995 |page=288|publisher=Cambridge University Press |isbn=9780521444927 }}</ref> Since the telescope design has an alt-azimuth mount, the mirror only has to be supported in a simple cell with a backing of indoor/outdoor carpet to evenly support the weight of the much thinner mirror.

* '''Construction tubes''': Dobson replaced the traditional aluminum or fiberglass telescope tube with the thick compressed paper tubes used in construction to pour concrete columns. "Sonotubes", the leading brand employed by Dobson, are less expensive than commercially available telescope tubes and are available in a wide variety of sizes. For protection against moisture, the tubes were usually painted or coated with plastic. Sonotubes are claimed{{fact|date=March 2025}} to be more rugged than aluminum or fiberglass tubes which can dent or shatter from impacts during transport. They have the added advantage of being thermally stable and non-conductive, which minimizes unwanted convection currents in the light path caused by handling of the tube assembly. 

* '''A square "mirrorbox"''': Dobson often used a plywood box for the tube base and mirror housing, into which the tube was inserted. This gave a rigid flat surface to attach the mirror supports, and made it easy to attach the trunnions.