200mm Minimalist Binoculars

by The Bolton Group

Our previous light-weight binoculars had weighed in at a very reasonable 65lbs. But could we go lighter? Gerald had a cunning plan!

Going back to basics meant the answer was yes and the these 200mm Binoculars weigh in at 44lbs total - an easy one man lift. Yet no stability has been sacrificed and performance will be just as good if not better.

To go with these new binos Brian has produced his finest pair of matched mirrors ever. Under autocollimation (double light pass doubling any errors) they exhibit absolutely straight Ronchigrams - no zonal error and no turned edge.

finished binos 2012

The finished binoculars at Kelling Heath 2012. Gerald is tweaking the collimation. Brian is left and Luke, the new owner, is on the right. We had some great views through them and Luke's adjustable chair made viewing a real pleasure.

All design goals have been met, ie:-

1. Ultra stable
2. Hold collimation
3. Quick inter-ocular adjustment - our simplest yet!
4. Easy one-man assembly
5. Easy one-man move

Note although the side-bearings look like simple Dobsonian bearings they have real bearings - no teflon here.

They dis-assemble into 3 components for easy transport and storage. What assembly there is, is by thumb-screws - no allen keys required.

top end

Tripod legs look like they are off the Apollo lander!



Close-up of the side bearings - real bearings mounted internally. The little felt pad being held provides adjustable friction.

Finished but un-coated mirrors installed

interocular adjuster

Focusing is push-pull using rubber O-rings for friction.
The brass knob is the simple interocular adjuster ie it sets the eye spacing correctly.


This image shows the basics behind the new inter-ocular adjuster. Underneath is a spring which holds the eyepiece holders against a travelling wedge.


All the optics were made by Brian ie the 2 matching parabolic mirrors, 2 elliptical secondary flat mirrors and 2 rectangular flat tertiary mirrors. The main mirrors are 200 f/5 (1000mm focal length) and were made from low expansion glass. Their focal lengths are within 4mm ie less than 0.5% difference. They were tested using autocollimation with a high quality test flat. In this test the light strikes the mirror under test twice. This is therefore a severe test as any errors present are doubled. In the rochigram below they both exhibit straight diffraction lines ie no detectable errors or turned edge to the limit of the test. They are certainly better than 1/10 wavelength accuracy probably nearer 1/20 wave. Note the hole in the centre is caused by test flat having a central hole..

Autocollimation ronchigram

Gerald with the near-finished binoculars at Kelling Heath 2011. Mirrors not installed yet.


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