20.06.2022 Triple OTA skeleton/truss combination?

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 Monday 20th June.  I have been thinking about an English cross-axis mounting for ages. However, the extreme difficulty of building one so far off the ground has finally put an end to such thoughts.

 The [slightly more] practical alternative is a multi-instrument OTA. Three instruments built into a single framework. 

 So that they can all be fixed onto a single [Large] German Equatorial Mounting. [GEM] Which can then be balanced by conventional counterweights. 

 A longer DEC axis will be required to achieve balance with manageable sized disks. One person, working alone, has severe weight lifting limitations while working from a stepladder!

 A GEM will not tolerate instruments on opposite ends of the Declination axis. They would each strike the pier at some point. A combination of instruments would avoid having to remove and reload individual OTAs as they are needed. The unused instruments would be temporarily capped to protect the optics.

 The 10" mirror at the bottom of the OTA would help to balance the refractor objectives at the top.

  There is no need for the instruments to be stacked vertically. [As shown in the image above.] The optical axes could be arranged on a triangle. Built around a central axis for a more compact arrangement. Albeit at the cost of greater complexity. 

 Particularly if a Porsa tube system was employed. The Porsa joints are strictly meant be built into rectangles. The joints are a very tough plastic. Reinforced with internal, aluminium, skeletal structures.

 I used the Porsa system for my folded 7" f/12 refractor with some success. Collimation was the problem. Not the Porsa system. There are many combinations of Porsa joints available. To build a very stiff, 3-dimensional structure to personal taste.

 It might be thought that interrupting a long tube with joints would weaken the structure. This is not the case here. All thanks to the stiff and reinforced  joints. Which are [rubber] hammered into place. There being an interference fit in the square tubing.

The Porsa system is often used to support massive fish tanks (aquaria.) So the Porsa load bearing capacity and stiffness are not in doubt. The increased depth of a multiple OTA would add enormous beam stiffness to the structure. Though it would need to be very well anchored to the mounting's saddle. No flimsy dovetail plates! I use a long and heavy, channel section for the saddle. I also have rectangular sections. Which would be even stiffer.

 The 180mm/ 7" refractor = 216cm f/l. Measuring directly from the tubular OTA: This would need 168cm maximum distance between front and back plates. The objective cell collimation screws would hold the objective cell to the front plate. The focuser would be screwed to the rear plate as normal. Most likely to be used for white light solar imaging. The camera to be mounted on the 2" Lacerta, solar wedge. For visual use an eyepiece can replace the camera.

The 150mm/6" refractor needs only 116cm front-rear, plate spacing. 52cm shorter than the 7". Using the same arrangements as the 7". It would need a sub baffle for the focuser.  The objective cell would  be mounted on the front plate. The H-alpha filter system extends 40cm to the rear of the focuser base. So the camera would lie within the multi-OTA structure. Offering plenty of potential for lateral support. Rather than relying only on tubular extensions cantilevered from the focuser. For visual use [highly unlikely] a solar wedge can be used. To bring the image outside the framework.

The 25cm/10" f/8 Newtonian = 200cm focal length.
Offset from axis to camera sensor = mirror radius = 12.5cm
+ distance to focuser baffle = 2.5cm
+ focuser minimum depth = 10cm
+ camera extension = 2.5cm
So 200 - 27.5 = 175cm. 
Very close [4cm] to the 7" refractor's front and rear, plate spacing.The mirror cell could be mounted on the rear of the back plate to make up the slight difference. There is no need to try and reach an eyepiece. The 10" will, like the 6"&7" refractors, largely be used for imaging. Lunar and planetary.

 A combined multi-OTA structure uses the same length of tubing as a single instrument. Only the difference in the length of lateral tubes adds weight. The three optics and focusers are included weight of course. Since three separate OTAs, all on the same mounting, would each have them. Though each would normally have a separate main tube. There would be some savings in weight in not needing six tube rings.

 I would imagine the bare, Porsa frame would be mounted first. Then the optics added afterwards. Lifting the complete, multi-instrument OTA would require the chain hoist. At potentially, much greater risk of damage or injury.

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