200mm (8-inch) Binoculars

by The Bolton Group

These Binoculars are not suitable for people with ears that stick out!

Assembling the Binoculars.
Captive bolts and wing nuts secure them together

These binoculars were made by Gerald Bramall to his own design in polished aluminium with twin 200mm f6 mirrors. They are altazimuth mounted and hold collimation perfectly. They perform as good as they look!

We have debated several times as to whether to build larger ones - but the 8-inch is a very practical size - easy to assemble - easy to transport - easy to collimate and easy to use - no ladders required! Our experience is that they are the optimum size.

26mm and 15mm plossl eyepieces are used and they provide stunning deep sky views from our dark site in the Lake District. The 26mm fit the Double Cluster in beautifully. There is always a long queue to try them. Until you have tried two eyes words cannot describe the experience they provide. It's as if there was no telescope there and you were actually out in space!

User's eye view

The tubes use the same technique as the 16 inch, ie diaphragms and tubes with through threaded rod and bolts. Unlike the 16 inch the top section, comprising three diaphragms, separates for transportation. The binoculars have been modified so that they can be collimated single-handed by having mirror cell adjusters in reach from the eyepieces. These adjuster rods are just visible on the top photo on the right hand (as we look) telescope.

Close-up of eyepieces
To set them up, first the inter-ocular distance (ie for the distance between your eyes) is adjusted by means of crayford type focusers between the tubes. The telescopes do not move! The actual focusing is individual and helical at the eyepieces. However, we have discovered for collimation that it best to aim at a bright star and defocus one telescope. The reason for this is that if the two images are the same, the brain will snap them together long before perfect collimation is achieved. By defocusing one image, this is prevented and the two telescopes can be perfectly aligned.

The image below is in response to several queries as to how the telescope tubes are assembled. The individual longitudinal tubes are in effect spacers between diaphragms with the through bolts and nuts (just visible) compressing everything together. This type of construction is probably limited to about a 16-inch telescope - beyond that conventional truss types are stronger.

Last updated: January 2007
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