APM 1.25" ED Coma Correcting Barlow Lens 6.25x

APM Coma correcting 1 1/4" ED Double Barlow 6.25 x

Designed by Gerd Düring / Germany

APMs worldwide unique photo-visual apochromatic 2.7x Barlow with real ED glass, calculated for the correction of coma at Newtonian telescopes, can now be combined with a second APM 2.7x Barlow at a specific distance to increase the magnification factor to 6.25x. Needless to say that coma correction is retained as is the corrected flat field.

On its own, a single barlow lens cell has a magnification factor of 2.7x and will provide a fully illuminated imaging field of 26 mm at a back focus of 104.1 mm (measured from the shoulder of the lens cell). It is optimized for F/4 Newtonian type telescopes and when used with such also offers coma correction. This is particularly useful for manual Alt-Az Dobsonian users who can enjoy a coma free view of objects as they drift across the field of view.

To use the barlow in a 6.25x configuration, simply add a second 2.7x lens cell to the other end of the 64.5 mm long extension tube so there is a lens at each end. This configuration yields a corrected field of 14 mm at a distance of 83.1 mm from the shoulder of the lens cell (84 mm from the centre of the last element).

The double Barlow permits very high magnification with Newtonian telescopes for planetary imaging while still retaining a respectably fast F ratio needed for high frame rates. Simply set your camera sensor back 83.1 mm from the shoulder of the last lens cell to have the correct back focus for imaging.

The design of each single Barlow element follows APM’s usual principles of outstanding optical quality. It features a double lense group using the Chinese ED glas FK61 (similar to Ohara FPL51). The lenses are soft cemented and applied on both sides with a broadband multicoating with over 99 % transmission for wavelength from 400 nm to 700 nm.