Not another crazy idea?

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I was thinking about the difficulty of mounting, sheltering or housing my 10' long, 7" f/12 refractor when I had a crazy idea. The Porsa build system I used for the folded form of my refractor lends itself to a very weird hybrid instrument. A combined refractor-reflector in a Porsa build frame.

The 10" f/8 is tall, when vertical, but needs only a very low mounting.  It doesn't care if the mirror cell just clears the ground. The folded refractor needs to be just high enough, on average, so that I can reach the star diagonal on the back plate when the instrument points overhead. Since the two instruments need not be removed and remounted with each use it could be built together as one permanent unit. 

By placing the refractor at a suitable level compared to the long reflector the eyepiece can be more  comfortably reached. Fixing the two optical systems together in parallel means that some build tubes can become redundant. The frame will be as stiff as required and all be mounted on one heavy equatorial mounting, down near floor level,  without the need for a near 7' high pier. The combined instrument can be lighter without loss of rigidity or duplicating two entire frameworks. The strangely shaped instrument could also be mounted in a fork to maintain overall balance.

A lower pier means greater stiffness for the same base area. It also saves some weight and material cost. It is not remotely as bulky as having two long, parallel telescope tubes. Nor adding one instrument to each end of the declination axis to balance each other out. Downside is it will be heavier and thus need double the number of heavy counterweights. Though the combined instrument can be far more easily housed in a smaller observatory. Or under a removable cover.

Let's imagine placing the folded refractor one end of the Declination axis and the long reflector on the other. The heavy mirror would help to balance the equally heavy refractor objective. Now we can dispense with all those counterweights. The problem is the dynamic balance with the heaviest components diagonally opposite each other and widely separated.

Adding extra weights to the top of the Newtonian and the bottom of the folded refractor would produce two, balanced "dumbbell" weights which each balance each other on opposite ends of the Declination shaft. The problem now is the huge moment of two equally heavy OTAs. Not only the moment about the Polar Axis but also around the Declination shaft.

The other problem is the danger of contact between each OTA and the pier. This OTA layout better lends itself to the astrograph mounting rather than the German. The astrograph mounting is where the polar axis is greatly extended before it finally meets the declination 'T.' Such a heavily cantilevered design would need a seriously large [and heavy] polar axis shaft. Certainly larger than my present 50mm or 2" diameter. Interestingly[?] both wormwheels were bored 60mm in error. Which does offer the tantalizing prospect of an astrograph mounting if 60mm was considered sufficiently large.

I would need to buy 60mm flange bearings and obtain a new and much longer PA shaft. The declination shaft is perfectly adequate at 50mm.  How long the new PA needs to be is quite another matter. It must allow the folded refractor sufficient "ground clearance" for an observer to comfortably reach a star diagonal mounted eyepiece at the zenith. About 3' or one meter is probably [just] enough. However clearance must be assured to avoid a meridian flip for both OTAs. Supporting the lower end of the PA where all the major stresses fall needs very serious, metal cross sections for stiffness. If the PA sags over its great length, under considerable end loading, the mounting will not point accurately to the Pole. 

An alternative mounting would be a fork but this offers very poor access to the eyepiece for a refractor. A fork design is not unlike an astrograph in some senses. Because the entire weight of the OTA[s] and the fork are all at a great distance above the upper PA bearing. The difference is that the astrograph mounting heavily loads the lower PA bearing while the fork loads the upper one.

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