Short Focus Achromatic Refractors

The performance of an 80-mm f/5 refractor can be greatly improved at high magnification by the use of a V-Block filter which effectively blocks blue and violet wavelengths while allowing 95% transmission at other wavelengths. With a V-Block filter combined with a 2.8x Klee Barlow lens, a good 80-mm f/5 refractor can be useful at a magnification as high as 150 x. The photograph of the Moon in Figure 4.1 was taken with an 80-mm f/5 telescope using a V-Block filter with a 2.8x Barlow lens.

Figure 4.1. A digital photo of lunar craters taken with a 90-mm f/5 refractor.

For visual observations and digital photography of variable stars, comets, asteroids and star clusters, a short focal length refractor is more useful than an f/11. With a 25-mm Plossl eyepiece an 80-mm f/5 refractor will provide a field of view in excess of 3°. All of the pictures of variable stars, asteroids and star clusters in this book were taken with that type of telescope.

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