300mm (12-inch) Binoculars

by The Bolton Group

These binoculars were judged the best at the 2005 Kelling Heath Star Party, the UK's largest and most attended event. This is the first time this award has been made.

Gerald with the almost completed binos in his workshop.

Underside of the mirror cells.

Top mounted push-pull adjuster in place
Looking down to the mirror cell

The mount features ball bearings on both axes. The single central arm means the telescopes are as close together as possible.

I know, I know - we said 8-inch binoculars were the optimum size for both use and transportation but.... aperture fever has struck again

The 12 inch binocular project took nearly 4 years! Gerald got side-tracked into building a new workshop, kitchen, bathroom and driveway. They are similar to the 8-inch except the tubes are 12-sided instead of 6.

This was a major project, around 1000 hours work, and was completed March 2005. First light was at our dark-sky observing night in March 2005.

There are just a few minor bits to add in the light of first use but nothing major.

The push-pull adjuster for the mirror cell is a new idea by Gerald using a contained (internally and externally) spring. It is adjusted from the top unlike more conventional adjusters. On one telescope they will extended upwards to make them reachable from the eyepieces so final collimatiom should be a one man job.

Unlike the 8-inch binos the eyepieces are not directly between the two telescopes. They are brought out at 45 degrees. This saves a lot of space and makes the binos much more compact ie not as wide as they would have been.

The finder is a simple red dot type and works excellently. In combination with Pocket Stars on a Pocket PC objects can be quickly located.

Ronchigram of one of the 12 inch mirrors

1/4 wave diagonal

Master optician, Brian Webber ground and figured both 12 inch (30cm) f/5 mirrors with their focal lengths differing by less than 1/4 inch (6mm). The mirrors were made from suprax blanks (a form of pyrex) and were a bit thicker than desired but by the time both sides were levelled up the thickness was down to 40mm. As you can see from the ronchigram there is no turned down edge! Eagle-eyed might spot a very very slight central "hole" but this is of no consequence being in the shadow of the diagonal.

The diagonals were also made by Brian and as can be seen from fringes (left) they are smooth and around 1/4 wave - note the slight curve is caused by the camera viewing angle not the diagonal.

All the mirrors (6) were HiLux coated at Orion Optics UK. This enhanced coating claims 97% reflectance - important when there are 6 mirrors!

The binos have been our biggest construction job but the effort has been worth it. The following pictures give some idea of the construction. Sorry but there are no drawings available - they just came out of Gerald's head!

Crayfords are for inter-pupilliary distance adjustment. They are the side-winder type.

Clever positioning of eyepieces keeps telescope separation to a minimum.

Spider/diagonal - note plastic hub and push-pull adjuster on the vanes.

Simple 9-point cell.

The binos were designed to separate into two halves from transport. The bottom cage is attached permanently to the mount and is the heaviest part to lift. Sensibly it requires two people. The top cage is much lighter and is an easy one person lift. The two halves are quickly located and and joined with three thumb-screws per tube. These need to be made captive to remove any chance of them falling on the mirror.

The two halves of the binos

Set-up and collimation is also quick and easy. The approximate collimation can be done in daylight with final alingment carried out on an object - usually a star. However, for the Haverthwaite Star Party we couldn't wait and aligned on the crescent moon long before dark. This final tweaking of alignment/collimation is easily done with the push-pull spider adjusters. They keep collimation all night.

Ready for action at last!

First testing on the very thin crescent moon was breath-taking. During the night we saw all our old favourite deep-sky objects but better than ever before as we were seeing with two eyes! The mount moves effortlessly and is unaffected by the wind.

A common question asked is what about reflections. Sure it looks like the bright ali would produce reflections but of course it doesn't. The necessary parts are blackened such as the spider and inside the focuser but everything else is sparkling ali. There are some black baffles around the mirrors and opposite the focuser.

The binoculars have proved a great success both in use and judged best homemade telescope in the UK's biggest star party at Kelling Heath 2005. They provide a first class tour of the deep-sky, all in the confort of viewing with 2 eyes and no need for ladders!

Brian and Gerald at the Haverthwaite Star Party, March 2005.

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