Jack Gelfand Telescopes and Astronomy Diagonal


The figure above is a schematic drawing of the modified Novak diagonal holder in the Flying Dobsonian. The vanes are fastened to the top of the diagonal holder with angle brackets. Loosening the four screws on top allows rotation about the axis of the mirror. The screws at the sides allows movement in and out of the telescope tube. The shaded portion represents the mounting brackets of the vanes on both sides of the diagonal cage. Loosening screws A and B allow rotation of the entire diagonal holder around the axis of the diagonal mirror at C. Note that the mirror is securely held in the holder and all adjustments involve the movement of flat surfaces that can be tightened securely against each other. Though it is a bit complex, this design results in a diagonal holder that assures that the mirror will be protected during transit and will hold its adjustment.

My diagonal holder is derived from the diagonal holder in Coulter Odessey telescopes. I am very impressed with the simplicity of adjustment of the Coulter system. A schematic of their diagonal holder is shown below.

A 45 Deg. angle bracket attaches the mirror to the diagonal vane. The mirror is glued to the bracket and the bracket is attached to the vane with a bolt. The vane has two right angles at each end. A hole in each of hese allows you to bolt the vane to the vane to the telescope tube. Though it is a single piece we would refer to this configuration as a two vane diagonal system.

There are two degrees of freedom for adjustment. One along the axis of the vane and one along the axis of the bolt holding the bracket to the vane. If the bracket is exactly centered on the vane, one only needs two degrees of freedom to get sufficient adjustment for the diagonal, so this works quite nicely. The important aspect of this approach is that if the bolts are tightened well, it is very stable and does not easily become loose. The only aspect of this design that isn't suitable for a portable scope is that the mirror is just glued to the bracket. If it falls off during transit, bye-bye mirror and bye-bye anything else it hits in the suitcase.

If you study the diagram of the my diagonal holder above, you will see that the system is topologically quite similar. The Novak 45 Deg. shell around the mirror and the 90 Deg. angle bracket in my design serve the same function as the 45 Deg. angle bracket in the Coulter system.

There is one drawback to the Coulter arrangement in that the vane must be quite thick in order to be stiff enough to hold the diagonal mirror without vibration. The vane on the Coulter 17" weighs a few pounds ! I tried to use my design with a single vane and found that it had to be unacceptably heavy or too thick. This is why I added the third vane in my system. It allows me to use much lighter vane material and, of course, weight is at a premium in the diagonal cage. I believe, however, that if one used curved vanes, one would only need two vanes, because the curvature would add strength in the direction at right angles to the plane of the vane.

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