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A Parallelogram Binocular Mount

by Graham Wood

This article describes the design and construction of a Binocular Mount showing the design considerations.

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Definition of parts

Main Beams
The two long beams that start at the binocular assembly and finish at the counter-weight. Each of these has a name for the section on each side of the centre pivot.
Viewing Arm
The section of the main beam from the binocular assembly to the centre pivot.

Counter-Weight Arm
The section of the main beam from the centre pivot to the counter-weight.
Spreader Blocks
The short horizontal blocks that are attached across each end of the Main Beams - one for the Counter-Weight and the other for the binocular assembly.
Parallel Arm
The upper arm that connects the binocular assembly to the centre assembly to maintain the orientation.
Vertical Arms
The short uprights that support the parallel arm.
Azimuth Pivot
The vertical pivot point at the top of the Tripod.
Schematic View of mount

General view of mount with binoculars

How far should the binoculars travel up and down?
The amount of swing should cater for the lowest to highest viewing requirements without having to adjust the height of the tripod. The lowest I used was to be seated in a picnic chair. But, it is possible that young children may have a lower eye height and this should not be forgotten when evaluating the minimum height. The highest requirement I used was to be standing and viewing a celestial object that is overhead.
How far should the binoculars stick out sideways?
When standing, the observer is relatively mobile so that the lateral position is not too important. However, when seated, the amount of lateral offset from the binoculars to the tripod legs at ground level must be large enough to allow enough space for a chair to be placed alongside. Since the mount will be at, or close to, its lower limit do not underestimate this lateral clearance.

How far should the counter-balance arm stick out?
The design considerations here include the weight of the counter-weight and the length of the counter-weight arm together with the risk of other people walking into the arm. See Safety Note at end.
The weight of the counter balance depends on many factors:
  • The weight of the binoculars and its local pivoting mechanism.
  • The distance that the centre of mass at the viewing end is from the centre pivot.
  • The distance that the counter-weight arm is to stick out.
  • The overall weight of the assembly.
Here is a guide to how all these factors add up. Remembering our schoolday physics lessons on mechanics, such a counter balanced system will be in balance when the binocular end mass times the length of the viewing arm is equal to the counter weight mass times the length of the counter weight arm and the two centres of mass and the pivot are in a straight line.
So after fixing a length for the viewing arm and weighing the binocular end mass, we have established the 'mass times length' figure for the counter weight side. We can use a long arm with a light weight at the end, or a short arm with a heavy weight on it.
So what do we actually use? I decided that the counter-weight arm should not stick out farther than the binoculars at the viewing end and should be a few inches less. This would allow me to gain an appreciation of the amount of room needed when setting up the tripod by only concentrating on the viewing end. This resulted in the counterweight mass being slightly heavier than the binocular end mass. In my case the counter-weight came out at 4.25 lbs of lead.
I calculated the volume needed and found that I could use a sardine tin as a cast to pour the molten lead into.
I played safe however, by suspending the anticipated amount of lead scraps from the counter weight arm to check that all was well. In fact, as a design point, I made the weight a few ounces heavier so that another, much smaller weight that slides on the parallel arm at the viewing side could be used to fine tune the balance.
Vertical Arms
The parallel action of the mount ensures that an object centred in the field of view by one observer will still be in the field of view when the binoculars are raised or lowered for another observer
To help prevent errors in construction from affecting this quality, it is best to set the length of the vertical arms (between pivot points) to not less than about 25% of the length of the viewing arm (between pivot points).

Main Beam Spacing
The horizontal distance between the two main beams should not be less than about 30% of the length of the viewing arm (between pivot points). This value gives a respectable size for the azimuth plate which should be approximately square.
I like wood for most things but some people may have better facilities for metalworking than woodworking. If you do make the parts from metal, make sure that it is thick enough or has an angle or box section to help prevent flexing and torsional twisting.
If using wood, do not skimp on the width and thickness of the main parts. We want to achieve a structure that is inheritantly stiff and does not flex. For the main beams use a hardwood that would be suitable for a deck chair. The parallel arm can be much lighter since it is in tension but, since both ends of it have 6mm holes in them for pivot bolts, it too should be hardwood.
I used a mixture of timbers for the different parts:
  • Beech - Main Beams, parallel arm, vertical arms.
  • Pine - Spreader Blocks at Counter-Weight and binocular ends. Also 'L' bracket.
  • Mahogany - Clamp for binoculars.
  • Marine Ply - Azimuth plate and cheeks - 3/4 inch thick.
All the pivots use 6mm machine screws or bolts. I prefer to use these rather than wood-screws since a much finer control over the tightness of the pivot point may be achieved. The pivot bolts have penny washers under the heads and between the pivoting members and are screwed through 6mm clearance holes in the front pivoting member into tapped holes in the member that supports it.

General view of mount showing binocular support
To get a good parallel action, it is important to make identical pieces where they have to match. The three most important items are the two main beams and the parallel arm. Actually the important bit is that the distances between the holes in them are identical. So, after cutting them to length and dressing their ends, clamp all three together and drill all the holes. IE Drill through three pieces at the viewing end, three at the centre point and two at the counter-weight end of the main beam for its spreader block
Drilling the front and rear pivot elements is best done in a pillar drill, but otherwise, at least use a drill stand. Holes that are not perpendicular to the surface will cause binding in the action. Use a 6mm drill for the front element, preferably of the modern 'centre-point' style to avoid the drill tip wandering.
For the rear element, use a 5mm centre-point drill and, since it will be tapped, the depth should be at least 5 times the diameter of the bolt.
Affix one vertical arm to the viewing end spreader block with wood-screws and then measure the distance between its pivot point and the one at the spreader block. Attach one end of the parallel arm to it and the other end to the centre vertical arm. With a spirit-level, set the base part of the centre assembly and the top face of the 'L' arm to be truly horizontal. Now clamp the centre vertical arm to the centre assembly so that the distance between the two matching pivot points is the same as previously measured. Drill pilot holes and affix with wood-screws.
The tapped holes are formed with a home-made tap made from one of the 6mm bolts.
To make a tap, first taper off the first few threads of the bolt with a file then file down half the thickness of the bolt over eight to ten threads keeping the file parallel to the axis. Blend the head side of the cutaway into the next few threads. When looked at vertically, head end down and the cut-away faced towards you it should look 'U' shaped. Then turn the bolt a quarter turn and the profile of the cut-away should look like a 'J'. Next take a block of scrap 'two by one' wood about four inches long and drill a 6mm hole in the centre of the wide face. Fit the 'tap' in the hole and lock it down very tightly with two nuts. You can now test the tap.
Only try to advance the tap two half turns at a time and then back out the same amount. Each three or four times, back out all the way and release the cuttings. It takes a little while, but the result will be a nice thread that will support the bolt and, furthermore, it will grip it sufficiently well to prevent it unscrewing itself.


Binocular Assembly
My binoculars (Zeiss 7x50) have no tripod attachment point so I had to devise a method of supporting them. I found that it was possible to support them by the centre tube without reducing their range of adjustment. I made a block and drilled a hole at one end from one side to the other of such a size that would snugly take the centre tube. Then I sawed the end off across the centre of the hole after drilling four holes to re-attach it. This block clasps the centre tube and the four screws can be set to gently grip the tube yet allow the binoculars to be levelled with a little friction to keep them in place.

General view of the working 
end showing the articulation

General view of the working 
end showing the articulation

General view of the working 
end showing the articulation

This block stands up from the centre tube and a 5mm hole is drilled vertically down through the centre of the top face of the block and tapped 6mm. This assembly is attached to an 'L' shaped arm with a 6mm bolt that provides the left-right pivot for the binoculars.
The 'L' shaped arm is attached with a horizontal 6mm bolt fitted in a 6mm hole drilled through the other part of the arm. The bolt is screwed into a tapped hole in the centre of the vertical face of the spreader block held by the ends of the main beams. This provides the up-down pivot for the binoculars.
If there are people close to the 'back' of the tripod or children running around, move your face away from the binoculars until the situation is resolved otherwise you risk 'two luverly black eyes' if someone bumps into the counter-weight end.

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Copyright (c) 1998,2000 Graham Wood.
Points arising should be addressed to the author Graham Wood at who will be pleased to assist.
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