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Conical Mirrors
Some things explained and some myths dispelled

 

and please see  Parsec Instruments for Conical
Mirror cells.

I have pretty much invested my reputation in conical mirrors for all types of telescopes, especially Newtonians. In 2003 I began to sell them for Newtonians as well as for the more traditional use in Cassegrain types. My contribution was the development of a mounting system that would make conical mirrors suitable and practical for Newtonians. Initially, I sold only 12.5" mirrors completely perforated through the center and held with a traditional O-ring hub. Later, in 2004, I began to make and sell 8" and 10" mirrors of my own mold design having a new mounting method that preserved the face of the mirror, the so-called continuous face system. In 2006 this was expanded to a 12.5" size made from my own mold design that saves a bit on weight. 14.5" and 16" Newtonain mirrors are still held with a through-core hub. Conical mirrors for my 8" through 12.5" Dall-Kirkhams no longer use hubs with O-rings but are actually potted into hubs using a special RTV. 

These mirrors have numerous advantages, the three most significant being: rapid thermal stabilization, light weight, and ease of mounting. Secondary advantages include the elimination of mirror clips (which are actually quite damaging) and figure-controlled thermal stabilization. This last feature is a serendipitous advantage that apparently results from the conical shape itself. The mirror does not necessarily go into significant over-correction during cooling. This is very apparent during fabrication where stabilization occurs more rapidly than with a standard mirror and the figure does not experience nearly as much change during and after transition.

As far as mounting is concerned for Newtonian mirrors you need only drill a hole through a tip-tilt plate and mount the mirror by screwing it down with the hardware provided. That's right, just screw it down. And you don't need to make a flotation system cell as is the case with thin mirrors. Some people have inquired (and incorrectly assumed) that 'the nut' in the center of the mirror will cause a certain amount  thermal or mechanical distortion and that these mirrors are somehow 'compromised'. This is not the case. The hub is held in place using a barrier layer of RTV that absorbs any thermal discontinuity. 

If one looks at a cross-sectional representation of a mounted conical mirror and a standard mirror what becomes apparent is that the conical mirror allows for a free circulation of air around the mirror. The mirror is not confined in any way in a cell that holds warm air and inhibits the cooling process. The entire back as well as the front of the mirror is free to radiate heat.

As to potential use for imaging, these mirrors have one immense advantage - they simply do not move. A standard mirror must of necessity be not held tightly or they will bend in some way due to thermal changes. Clever devises have been designed to reduce this defect but all standard mirrors shift, even if just a little. I have had extensive contact with Vincent Steinmetz of Strasbourg, France. He is a highly expert imager who is working wonders with an 8" f/4,5 mirror I made for him in 2008 (see link on home page). He has mentioned on numerous occasions the critical necessity of keeping every element in the optical system absolutely stable and collimated. The biggest problem he and others encounter is shifting of the primary mirror. In a recent e-mail he made the following unsolicited comment, "... for example, if something moves in the scope you see it at a glance, but now with the conical Royce, nothing moves at all, except the mount to track stars 8-)."

Please see Parsec Instruments where Curt Walker is making excellent cells for my conical mirrors