5 1/2 inch Photo-visual refracting telescope by T. Cooke & Sons Limited

5 1/2 inch photo-visual refracting telescope with manual slow motion, made by T. Cooke & Sons Limited, early 20th century. The telescope is fitted on a ‘class b’ equatorial mounting. Original pedestal and clock drive are missing.

This telescope, previously owned by Giggleswick School in North Yorkshire, is an almost complete example of a large Cooke refracting telescope on equatorial mounting. The instrument was manufactured by T. Cooke & Sons, a York-based English instrument-maker, at the beginning of the 20th century. It appears to be almost unaltered. According to the name plate on the mount, Broadhurst Clarke & Sons at 63 Farrington Road, London, supplied the instrument.

A refracting telescope, or refractor, is the earliest type of telescope. It uses lens as its objective to focus the light to form an image, making distant objects appear brighter, clearer and magnified.

However, this design suffers from false colour. Also known as chromatic aberration, false colour is a colour distortion that creates an outline of unwanted colours along the edges of objects in an image.

To avoid chromatic aberration, this model is fitted with an apochromatic objective lens, specifically designed for both photographic and visual use (H. D. Taylor’s Patent).

Apochromatic lenses, or apos, are designed to correct for both spherical aberrations and false colour. They are made of a combination multiple lenses made of different types of glass. An apo can be made with as few as two or three elements.

This design was invented in 1892 by Harold Dennis Taylor, at the time employed by T. Cooke & Sons, while he was working to reduce chromatic aberration. Patented in 1893, it consists of three lenses formed of three different types of glass. Tailor’s design was the first lens system that allowed elimination of most of the optical distortion or aberration at the outer edge of the image.