Double telescopes?

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Having spent so much  time and effort building a German mount I now find myself rather wishing for a fork. A fork mounting is far more compact and can be centralized in "the dome." This non-offset support system would allow my long telescopes to function in a smaller dome. Any offset demands the dome be bigger to house the object hanging off  to one side of the mounting's center point. My GEM even has a big fat cylinder on the PA. Just right for carrying the extended, cantilevered loads via rollers for a fork.

Let thee think all this is unimportant I beg to disagree.  Every extra foot of dome diameter costs serious money. Making the telescope physically smaller helps. A refractor and a reflector are as physically different to house as you could wish for. The reflector wants a low opening to reach lower sky altitudes. The refractor needs a tall pier or it gets all huffy and demands the user grovel in abeyance when looking overhead.

With the refractor literally laid tightly over the reflector I'd have a dual purpose, reasonably well-balanced monstrosity to fit in a minimum observatory diameter. The idea is really quite simple. The heavy refractor objective balances the equally heavy mirror in the same tubular[?] framework. Two evenly sized kids on the seesaw if you missed the point. Though one is hunched down and clinging on while the other is [unaccountably] sitting high on a box. Don't ask me why but that's the gist of the arrangement.

The closeness of the optical paths removes the need for dual OTAs. Allowing the reduction of duplication of support tubing in a single frame of modestly rectangular cross section. And avoiding a telescopic pier of epic proportions and change in height. 

The problem now is that their combined balance point will be somewhere in the middle of the hybrid's tube. Demanding a very long fork if northern pointing is desired. Though this is not really essential.

Offset forks are used, but this would demand considerable counter-weighting for such a weighty "double" telescope. Despite the potential problems this whole idea really appeals because I already have both objectives. Far better [surely] than swapping heavy OTAs on the mounting, at intervals, whenever the whim takes me? Increasingly rarely I would imagine with my now, rapidly increasing age.

The most obvious alternative is closely mounting the folded refractor on top of the reflector instead of the long straight one. This lifts the refractor's eyepiece and star diagonal to a potentially far more comfortable position for most pointing altitudes. The downside is the much greater depth of the dual instrument to house both optical paths without conflict. The further the center of mass from the PA the more counter-weighting is required. Unless you use a normal fork instead of a GEM or offset fork.

The upside is the potential to place the folded refractor slightly lower on the reflector tube. Which would move the balance point lower on the combined OTA. Requiring a much shorter fork. Though this ruins the chance to have the refractor eyepiece at a reasonable height from the floor. Just to avoid unsightly grovelling to the gods of the overhead sky.

A 90cm [3'] high lowest eyepiece level, when pointing at the zenith seems about optimum for a normal height of my folding wooden chair. If the refractor is being used for imaging the EP height becomes largely irrelevant but should still be kept in mind. A stepped height seating 'box' offers room for variation in eyepiece height from the observatory floor. At the risk of increasing clutter and something else to fall over in the dark.

Even with a very squat mounting and pier height the rather long 10" f/8 reflector needs a stepladder to reach the eyepiece at higher pointing altitudes. But again, is completely free of such problems when imaging. It is not intended that one OTA be used for guiding the other for imaging. Any manual guiding will be monitored on the computer screen. Though occasional
visual comparisons through each OTA might be very interesting. In order to compare the seeing conditions, resolution and optical quality in the two very different instruments. It is not very likely I would avoid visual observation having gone to all the trouble of building or obtaining an observatory.

The next question is whether I should use the same Porsa, tubular build system for the reflector which I used for the folded refractor. It would make some sense to combine the tubes which lie between the separate, but closely arranged, instruments. Duplication adds more weight without adding very much extra stiffness. Though two individual instruments could be simply bolted together with the option of their easy separation later on. Combining them in one framework makes their separation far more difficult. It also makes for a very "lumpy" load to dismount in one piece. Removing one OTA is bad enough and has needed a chain hoist.

The image alongside shows the relatively low balance point of the 10" f/8 reflector alone. I later lowered the pier by another six inches to allow the mirror cell to just clear the ground when pointing at the zenith.  This reduced the height of the necessary stepladder to reach the eyepiece.

The freedom to move the folded OTA, relative to the long reflector, would greatly simplify finding the best combined balance point when both are fixed together. The two complete instruments need only be clamped temporarily together until the ideal balance point is found. The diagrams above shows the general idea. With the arrows indicating how the folded refractor should be  moved up and down to find the best combined balance point while matching the ideal eyepiece height.

Mounting the OTAs on opposite ends of the GEM's declination axis is possible but even more bulky. Requiring a much larger dome to be able to move safely around the instruments in a cramped space in the dark. There are also pointing positions where one, or the other instrument, will collide with the pier. A fork avoids such collisions while keeping the OTAs central in the limited diameter of a round observatory.

A wider fork would allow both instruments to be mounted alongside each other. I'm not sure I can see any advantages in this arrangement. Though a fork mounting does safely avoid the necessary counterweights of the GEM dangling invisibly in mid air. Just waiting for the observer to walk into them in the dark. Or [worse] stand up from crouching or crawling under them to cross the observatory floor.

Click on any image for an enlargement.

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