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Backing focal

Letters

I beg to differ with Peter J Felton's when he says: 'A hemisphere does not have a focal point and would lead to a poor signal'. (NCE 14-28 December) The focal point of a hemisphere (whether concave or convex) is on the focal plane half way between the hemisphere and its centre of radius. Satellite TV dishes are hemispherical as are car mirrors and give perfectly good signals or images.

Graham Pennells, Whitby Bird & Partners, gpenn@whitby-bird.co.uk

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