Cross-section Newtonian Parabolic Mirrors
A New and
of Mirror for Newtonians
The conical cross-section mirror has arrived for Newtonians. Light weight but no compromise in wavefront accuracy. - And no flotation cell required. Mounting is simplicity itself. The first real advance in lightweight larger aperture Newtonian telescopes since thin mirror, big telescope craze.
While this type of mirror has long been used as primary mirrors for commercially manufactured Schmidt-Cassegrains and Maksutovs as well as research grade compound telescopes of all types, it has essentially been ignored for Newtonian telescopes. Why this has happened is not entirely clear but it may be reasonably surmised that inasmuch as the Newtonian reflector does not require a central perforation, as does a compound telescope, the conical cross-section mirror has simply never been considered. Nevertheless, it has been considered by me and found to be, I believe, of tremendous value to the Newtonian. Elsewhere within this web page I have devoted space to discussing why I believe Newtonian reflectors, while possessing great potential, often perform less than optimally. Much of my concern involves the condition of the mirror and where it is located within the telescope. Buried down at the bottom of the tube, or even a mirror box, it is effectively isolated from free moving air and insulated on its back side, allowing only it's face to come in free contact with the ambient air. This results in reduced ability for the mirror to come to thermal equilibrium as well as remove spurious warm air currents near the face. By contrast, the conical cross-section mirror does not suffer from these defects encountered with conventional mirrors. The mirror can be mounted so as to have a considerable empty cavity of air space surrounding it. The mirror therefore radiates comparatively evenly and fans can be far more effectively applied over the entire surface of the mirror, back and sides as well as front. Also, this kind of mirror simply requires no articulated support system - the common flotation cell. Flotation systems are a complicated added expense and are part of the problem of isolating and insulating the back of the mirror. A conical cross-section mirror has only a small amount of rear area as a supporting surface (about 25% of the diameter) at it's thickest point and this needs only to rest against a reasonably flat metal or wood surface.
The mirror design is amazingly simple in concept and is based upon the principal of the cantilevered truss as shown below.
In this design glass is removed where it is not needed and thickened where it is needed. The result is a cross-section which takes on the appearance of a frustrated cone. Engineering tests have been performed on mirrors of this design and they have found to deviate only very slightly from ideal curves when the optical axis is tilted toward the horizontal. This is quite different from thin mirror blanks where essentially no engineering is applied and excess material is removed evenly across the face without consideration to mechanical consequences. A standard full thickness 12.5 inch mirror blank, obeying the classical 6 to 1 diameter to thickness ratio, has in uniform thickness ranging from 2 to 2.125 inches. With the conical cross-section mirror the center, having a diameter of 4.2 inches at the rear, is 1.75 inches thick tapering down to 1/2 inch at the edge. This results in a weight reduction of over 50 percent from the original full thickness blank. A full thickness 12.5 inch weighs approximately 23 pounds. In the conical cross-section mirror this is now reduced to approximately 10 pounds.
Here is something new and different for both smaller as well as larger Newtonians; a no compromise lightweight mirror and cell combination, the next step in lightweight telescope design.
For more detailed information regarding the mounting and testing and purchasing of these mirrors click on the images below.
Cells for conical mirrors can be found at: