APM’s unique photo-visual apochromatic 2.7x Barlow with true ED glass, which is calculated for the correction of Off-Axis Coma in Newtonian telescopes, can now easily be combined with a second APM 2.7x Barlow to increase the magnification factor to 6.25x. Not only is the magnification higher, coma correction is retained along with the corrected flat field.
On its own, APM’s 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 as 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 their 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 extension so there is a lens at each end. This configuration yields a corrected field of 14mm at a distance of 83.1mm from the shoulder of the lens cell (84mm from the center of the last element).
The double Barlow permits very high magnification with Newtonian telescopes for planetary imaging while still retaining a respectably low F ratio needed for high frame rates. Simply set your camera sensor back 83.1mm 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 lens group using the Chinese ED glas FK61 (similar to Ohara FPL51). The lenses are soft cemented with a broadband multi-coating with over 99 % transmission for wavelength from 400 nm to 700 nm.