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GizmosBalancing a Lightbridgeby Don Van Akker, Edmonton Centre (don@knappett.com)The big new Lightbridge was pretty exciting. We wereseeing things we had never seen before and seeing thethings we had seen before in a whole new way. Especiallythe stars around the edge of the field. Who would have thoughtthey would be shaped like that?It took a while, and an expert opinion on our mirror, beforewe learned that if you were going to have a mirror that big (16˝)and that fast (f/4.5), then you were going to have coma as well,and that you could either learn to live with it or you could get acoma corrector.We got a Paracorr and the difference was astonishing.Combined with a big Nagler eyepiece, we got pinpoint starsacross the entire field and resolution we’d never dreamed of.We also got a nose-heavy telescope that dropped like a stone.Without a stick to hold it up, you almost couldn’t use it at all.I thought the answer might lie in a 2 ½˝-long slice of4˝diameter steel rod that I’d pulled out of a scrap bin once, andwhich has been getting in the way ever since. I thought that if Icould mount it behind the mirror it just might be heavy enoughto balance the scope. The question was how.When I looked at the back of the Lightbridge, it seemedas if it had been designed with me in mind. Ringed aroundthe perimeter of the tube end were three collimating knobs,three lock knobs, and three rubber feet, all equally spaced, allarranged in triangles. The rubber feet unscrewed to leave three8-mm threaded holes and the rest was obvious.I cut a triangle from 3/16˝ aluminum because that’s whatI had. You could use something a bit thinner or thicker, inaluminum or steel, or even plywood. I drilled the centre of thecounterweight and tapped it for a 3/8˝ bolt and made someoffset spacers by drilling out three ¼˝ coupling nuts to slidesmoothly over 8-mm bolts. I put the whole works together likeyou see it in the photograph and bingo, the scope was balancedand I was a hero.This project came together for me a little more quicklythan most. I lucked out on the size of the counterweight and Ilucked out on the fact that there was room enough under theend of the tube when vertical to install this assembly, but youcan use some variation of this idea on almost any nose-heavyDob. You probably don’t have a handy slice of 4˝-diametersteel rod, but look around at what you do have. If you can’tfind anything at all, Metal Supermarkets is a last resort, buta project like this is most fun if you can manage it withoutpaying for anything.Don Van Akker didn’t remain a hero for long because he couldn’tget into the tights. He observes with his wife, Elizabeth, from SaltSpring Island, and they are members of the RASC EdmontonCentre. Don is happy to answer questions on this or any Gizmosproject. Email don@knappett.com.April / Avril 2008JRASCBuilding for the International Year of Astronomy (IYA2009)71

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