215mm (8.5-inch) Ultra-light Binoculars

by The Bolton Group

No sooner were the 12-inch binos finished then Gerald looked around for a new project. The aluminium cut outs from the 12-inch binos were big enough for 8.5 inch ones so the temptation was too great. The 12-inchers are great for a start party or serious observing but for just carrying out easily for a quick view something lighter is needed. Hence these ultra-lights!


Gerald and Brian with assembled binoculars

Brian made the optics (6 mirrors) and Gerald the tube assemblies although Brian did make some of the parts including the focuser.

Progress has been rapid! The binos have been designed to be ultra-light. The total weight including optics is around 65lbs.

The binos are now finished and are awiting the aluminising of the 6 mirrors. All design goal have been met:

1. Ultra stable
2. Hold collimation
3. Simple inter-ocular adjustment
4. Easy one-man assembly
5. Easy one-man move

Being the 3rd pair made by the group the design is now much refined and improved.


Parts 1


Parts 2


Parts 3


Optics - the 6 mirrors before aluminising

The optics are f/6 and all the mirros were made Brian. They are as usual to his very high standards - if they are not perfect he will not let them out of his workshop!


Bottom cage


Top cage


Central fork mount

To save even more weight the bottom bearing uses a cirular rebate in both the upper and lower bearing plates. This is filled with over 100 ball bearings which provides a very wide track for stability plus low weight. It works a treat.

In this new design, the azimuth and altitude axis shafts have been left exposed for the possible option of fitting digital setting circles.


Inter-ocular adjuster

I hesitated to show this new idea. Our original 8-inch binos featured crayfords for adjusting the inter-ocular distance. Low and behold when commercial binos appeared a couple of years or so later they too had them! Coincidence?
This new solution was not suitable for the 12-inchers as their eyepieces came out at 45 degrees. However, with the smaller size of the 8.5 inch ones we have gone back to inline focusers so some lateral thinking took place. The adjustment only has to be 15 mm maximum - eye separation doesn't vary all that much. Brian devised this method with a single rotating knob (red) to adjust them in synchronisation. The focusing is helical - the black and brass units.
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