Poncet Platform
Encyclopedia
Equatorial platform
An equatorial platform or equatorial table is an equatorial telescope mount in the form of a specially designed platform that allows any device sitting on it to track astronomical objects in the sky on an equatorial axis. They are used to give equatorial tracking to any device sitting on them, from...
(a telescope mount
Telescope mount
A telescope mount is a mechanical structure which supports a telescope. Telescope mounts are designed to support the mass of the telescope and allow for accurate pointing of the instrument...
that adds an additional polar axis to non-equatorial mounts
Equatorial mount
An equatorial mount is a mount for instruments that follows the rotation of the sky by having one rotational axis parallel to the Earth's axis of rotation. This type of mount is used for astronomical telescopes and cameras...
) that uses a simple polar pivot and an inclined plane. The motion of the mount allows any device sitting on that platform to track the apparent motion of the stars in the night sky (Diurnal motion
Diurnal motion
Diurnal motion is an astronomical term referring to the apparent daily motion of stars around the Earth, or more precisely around the two celestial poles. It is caused by the Earth's rotation on its axis, so every star apparently moves on a circle, that is called the diurnal circle. The time for...
). Its low profile and extremely simple design has made it a perfect complement to Dobsonian telescope
Dobsonian telescope
A Dobsonian telescope is an alt-azimuth mounted newtonian telescope design popularized by the amateur astronomer John Dobson starting in the 1960s. Dobson's telescopes featured a simplified mechanical design that was easy to manufacture from readily available components to create a large, portable,...
style altazimuth mount
Altazimuth mount
An altazimuth or alt-azimuth mount is a simple two-axis mount for supporting and rotating an instrument about two mutually perpendicular axes; one vertical and the other horizontal. Rotation about the vertical axis varies the azimuth of the pointing direction of the instrument...
ed telescopes since it follows that designs philosophy of being easy to build using common materials, without the need for specialized tools or machined parts.
Origins and usage
The Poncet Platform was invented in the 1970s by Adrien Poncet. Poncet's original design was a very simple type of equatorial platform that uses pivot as one support and an incline plane in line with the Earth's equatorEquator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....
along which two other supports slide. It was publicized in the January 1977 issue of Sky & Telescope
Sky & Telescope
Sky & Telescope is a monthly American magazine covering all aspects of amateur astronomy, including the following:*current events in astronomy and space exploration;*events in the amateur astronomy community;...
magazine. The model Poncet demonstrated was very simple plywood construction, using a nail pivot and a Formica
Formica (plastic)
Formica is a brand of composite materials manufactured by the Formica Corporation now based in Newcastle, Tyne & Wear, a division of the New Zealand company Fletcher Building. In common use, the term refers to the company's classic product, a heat-resistant, wipe-clean, plastic laminate of paper or...
covered incline plane with plastic 35mm film canisters as the platforms bearing feet, on which he mounted a 6" newtonian telescope
Newtonian telescope
The Newtonian telescope is a type of reflecting telescope invented by the British scientist Sir Isaac Newton , using a concave primary mirror and a flat diagonal secondary mirror. Newton’s first reflecting telescope was completed in 1668 and is the earliest known functional reflecting telescope...
. The mount in its basic form is very simple, requiring just hand tools and common materials to build, with the only precise calculation being setting the angle of the inclined plane to match the angle of the Celestial equator
Celestial equator
The celestial equator is a great circle on the imaginary celestial sphere, in the same plane as the Earth's equator. In other words, it is a projection of the terrestrial equator out into space...
. It has been used to add equatorial tracking to everything from small cameras to entire observatory buildings. Its simple design and low profile has made it a useful "retrofit
Retrofit
Retrofitting refers to the addition of new technology or features to older systems.* power plant retrofit, improving power plant efficiency / increasing output / reducing emissions...
" for altazimuth mounted telescopes such as the popular Dobsonian telescope. Users simply place their telescope on top of it to get the added feature of tracking in the direction of right ascension
Right ascension
Right ascension is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. The other coordinate is the declination.-Explanation:...
accurate enough for work at higher magnification or astrophotography
Astrophotography
Astrophotography is a specialized type of photography that entails recording images of astronomical objects and large areas of the night sky. The first photographs of an astronomical object were taken in the 1840s, but it was not until the late 19th century that advances in technology allowed for...
.
Poncet Platforms do have design limitations. They are usually designed to track for 1 hour (15° of tilt) since longer tracking could cause the instrument on top of it to topple off. After that hour the mount has to be pivoted back to the east to reset the clock drive
Clock drive
In astronomy a Clock drive is a regulatory mechanism used to move an equatorial mounted telescope along one axis to keep the telescope in exact sync with the apparent motion of the celestial sky...
mechanism. Since the Poncet platform has no roller bearing surfaces that can be driven, the clock drive mechanism itself has posed some design difficulty for telescope makers. Straight line drives such as threaded nut/bolt drives change drive rate when converted to a circular motion. It is also time consuming to reset via spinning the nut back to the starting point. Amateurs have tackled this by employing curved bolt designs and even using specially shaped cam
Cam
A cam is a rotating or sliding piece in a mechanical linkage used especially in transforming rotary motion into linear motion or vice-versa. It is often a part of a rotating wheel or shaft that strikes a lever at one or more points on its circular path...
s to convert the straight line motion to a variable speed motion. High mechanical loadings from heavy telescopes or using them at low geographic latitudes can cause the mount to bind up, requiring more complicated improved bearing surfaces to overcome this. This has led to equatorial platform
Equatorial platform
An equatorial platform or equatorial table is an equatorial telescope mount in the form of a specially designed platform that allows any device sitting on it to track astronomical objects in the sky on an equatorial axis. They are used to give equatorial tracking to any device sitting on them, from...
variations based on the Poncet design include Alan Gee's platform mount using a more complicated cylindrical bearing surface in place of the inclined plane, and Georges D'Autume's platform design which uses a sophisticated conical bearing system.