Telescope of Dobson

In Astronomy amateur, the telescope of Dobson is a Télescope of Newton assembled on a simplified azimuth Monture. To indicate such an instrument, one intends sometimes the Anglicism telescope dobsonien (which comes from Dobsonian telescopes ), but more usually and simply, one uses rather the term Dobson or, more affectionately, “ Dob ”. As these names indicate it, it is astronomer amateur American John Dobson, born in 1915, which is at the origin of this concept which finds followers more and more, especially in America and Europe.

Original philosophy

Before the years 1970, the astronomers amateurs had hardly other choice but to put the hand at porte-monnaie to find material allowing to observe the sky thoroughly. Often, the amateurs of visual observation were attached by the limited diameter of the instruments which they were able to offer itself (all things being equal, the price of a telescope grows roughly speaking exponentially with the diameter), and the only manner of further going was summarized to launch out in the astrophotography, more tiresome and which does not get the same feelings.

According to the dires of John Dobson itself, this last cut its first mirror S of telescope at the end of the years 1950 in the greatest secrecy of its monastery Californian vedist. He would have thus built its first telescope (of 130 mm in diameter) in 1958 starting from materials of recovery available on the spot, including the emery S necessary to the polishing of the mirror. One decade later (after being expelled of its monastery for lack of assiduity), John Dobson and his club had a telescope of 600 mm in diameter.

Noticing from the start the interests inherent in the system (a telescope of large diameter at a reasonable cost), the astronomers American amateurs quickly adopted it: Dobson of all sizes manufactured in one weekend containing boards, of nails and other ends of pipe became currency then. Since then, no matter who can reach the beauties of the deep sky with the help of a few hours of do-it-yourself. Even if it is not the first approach “ Do it yourself ” which was born in astronomy amateur, it is undoubtedly one of most popular still today.

Diversification of the concept

Industrialization of mass

The relative simplicity of Dobson allowed industries optics to adapt an inexpensive concept to manufacture and thus being able to be proposed to the customers with competitive prices. With orée of the 21e century, Chinese and Taiwanese factories crossed the step by proposing telescopes of Dobson (with a metal tube) of 150, then 200,250 and 300 Misters One cannot deny that the appearance of these instruments in the trade contributed to the rise of modern astronomy amateur: the beginner of today can start with a telescope of 200 mm in diameter where the former generation was to be satisfied with glasses of 60 mm or of a telescope of 115 Misters.

A transportable telescope

Vis-a-vis the increasing urbanization of their territory and more generally with the recrudescence of luminous pollution, the astronomers amateurs had to be solved, for the majority, to move to observe. Until the end of the years 1980, the only transportable instruments were inevitably limited in diameter since the length of the tube is at least double of this one. The instruments the largest commercial but nevertheless transportable that one could then find were Schmidt-Cassegrain of 14" (305 mm), not inevitably obvious to set up by only one person (more 20 kg to be hoisted on mounting).

Since then, some American amateurs and craftsmen had the idea to replace the traditional full tube out of paperboard by triangulated metal stems less cumbersome, less heavy, and especially dismountable, taking as a starting point the “Serrurier tube” of the Hale telescope 5 m of the Observatory of the Mount Palomar. Thus Dobson could be dismounted in four distinct parts:

the cradle (“ rocker ”) which is used as mounting,
the box with mirror which contains the primary education Miroir with its Barillet, and on which the Tourillon S are fixed which rest on the cradle,
the cage of the secondary mirror in which the secondary Miroir is and the carry-eyepiece intended to receive the instruments of the observer, and
tubes (in general eight) intended to connect the box to mirror and the cage of the secondary and possibly wrapped, after assembly, of a “black fabric sock” allowing to protect the mirror of the bad weather and the parasitic lights.

The main difficulty of realization holds in the good definition of the rigidity ratio/weight not only in the structure of the telescope, but also in its optical part. Glass used for the primary education mirror is very dense, and it is often the heaviest part. The race with the superfluous kilograms necessary to the success of the telescope amounts decreasing the thickness of the mirror, knowing that if it is too fine it will tend to become deformed under its own weight and thus to degrade the quality of the images. It is thus often necessary to have recourse to a complex barrel: of the three traditional contact points one will have to often pass to six, nine, eighteen, twenty-seven points even more, as long as it is more advantageous to have a telescope complexes (it is necessary to preserve the isostatism barrel!) that difficult to transport.

Today, many are the amateurs who do not hesitate any more to call upon composite materials (carbon fiber tubes…) or with the services of a mechanic. There exists even company which manufactures spare parts, so that the amateur can concentrate on the polishing of the mirror or work of joinery.

Proof of the compactness which this type of telescope can reach, a French amateur managed to make return Dobson of 760 mm in diameter in the trunk of a Peugeot 106.

Towards more sophistication

Less and less of amateurs balk to call upon the miracles of electricity (even of electronics) to improve their telescope. Principal defect of Dobson resides in absence of compensation of movement relative of observer compared to vault of heaven, this is why it exists systems of motorization of two axes (azimuth and height) which not only make it possible to follow the stars observed, but also to point an object automatically (system “ go-to ”). Another system called “equatorial table” or “equatorial platform” allows, once slipped under the mounting of the telescope, to ensure the equatorial follow-up without rotation of field during approximately one hour; certain amateurs equipped with telescopes of Dobson thus could make imagery even if it is not the vocation first of this type of instrument.

The amateurs wishing to save time can also equip their instrument with a system of coders connected to an interface allowing to select the object to be observed. Once this selected object, the racket posts the differential coordinates of the object compared to the current position of the telescope; once the observer turned the tube of its telescope so as to post zero in azimuth and height, then the desired object must be in the field of the eyepiece. This system is called “ push-to ” (literally “to push towards”) in opposition to “ go-to ” (“to go towards”) which supposes a motorized displacement and not handbook.

Finally, Dobson of today does not have very often much any more of common points with the original idea of John Dobson. Even if this spirit remains long-lived at many astronomers amateurs, it does not remain about it less than the time of the boards, the four nails and the mirrors cut with goes-quickly is completed. The talented craftsmen and handymen showed, since, that the telescopes of Dobson produced carefully are the ideal instruments for the visual observation: light, dismountable and with an optical good quality in spite of their size, it can appear perfectly usable with the daily newspaper, as well for the observation of the deep Ciel as for that of the Planet S. Of course, such an approach can require a rather important investment, as much in terms of time that of money.

The telescopes of Dobson are also the occasion to leave free course to its ideas: thus of many genious amateurs built of composite material Dobson (carbon and/or glass fiber), of Dobson binocular, or of Dobson which are folded up in simple a valisette…

See too

Telescope
Astronomy amateur
John Dobson
Construction of the telescope of amateur
Quality of the mirror of a telescope
Astrophotography

External bonds

Altaz : site dedicated to the manufacture of Dobson
German giant Dobson: the largest telescope of the Dobson type in the world (107 cm diameter)

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