By Frédéric Géa many thanks to Luc Arnold and David Vernet.
After few years of making Dobsonian telescopes, I understood that their is always something to optimize to have a better instrument and one should not hesitate to completely returning on points which however appeared to be immutable principles. One of the characteristics of Dobsonian ‘scopes, is the way the primary mirror is laterally supported. It is generally held in a sling of a high strength (standard auto safety belt for example) that maintains the mirror so that it does not slip off its supports when the optical tube is tilted. This very basic system has the advantage of being simple to realize and effective. At least it is what I believed until an observer told me to see the astigmatism that can be generated by the strap. The mirror was then moved on its support, the astigmatism decreased but did not disappear. (see also astatic cell )
The sling is indeed one of the elements that generates this phenomenon and is so bad for the image quality. This night there, I had the opportunity to observe in a telescope, which has an astatic mirror cell with a lateral support really different to what I knew. This type of support described in the thesis of Couderc of 1932 and has been used for years on instruments of the Observatory of High Provence. Those lateral supports were improved thanks in particular to the work of Luc Arnold, professional at the OHP, in order to allow a still better support of the mirror in its mirror cell wherever we aim the scope at the sky. These supports replace the sling without deformation or generation of astigmatism on the mirror. Of course, if the scope is pointed close to the horizon, the astigmatism can be present but it disappears as soon as the scope pointed higher in the sky. The mass of the primary mirror no longer deforms any more than using a sling and the optical axis is kept true while moving the scope around the sky. What surprises first is that these supports do not deform the star image of Polaris with a power beyond 1000 X. Impressive compared to what I’m use to seeing using a sling.
With a closer inspection of these supports, it appears that Luc incorporated an enchased piano wire in a hole in a threaded rod. The other end of the piano wire is firmly inserted in a small steel or aluminum part with a disc of Teflon as a pad at its end to contact the mirror edge. The threaded rod is adjustable in its support (this support is interdependent of the mirror cell) and this adjustment makes it possible to bring the perimeter in contact with the mirror and thus to center the mirror on its position. This piano wire resists the pressure remarkably thus preventing any movement of the mirror. Some " side play " is permitted by the small size of the piano wire in its threaded pin and allows the accompaniment and the optimum transmission of the force generated by the swing of the mirror at the time of the movements of the optical tube around the sky. The supports pad contacts the mirror at half of its thickness.
If the parts are too complex to realize for an amateur, it is possible to order this parts at Valmeca.
Frederic's personal lateral supports.
Ci-above, 3D vision of these supports.
Thank you in Luc Arnold to have brought to our attention this system, which represents a very interesting and needed improvement.