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APM Coma Correcting 2.7x Barlow Cell and Extension for 6.25x

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The APM Coma Correcting Barlow re-defines what is possible with a Barlow lens at a surprisingly low price. It has been designed by Gerd Düring in Germany following much discussion with users as to what would represent the ideal Barlow lens. This new fully Apochromatic design provides a magnification factor of 2.7x and uses a 1.25” lens cell. This lens cell is a 2 element cemented doublet incorporating an FK61 ED element (similar to O’Hara FPL-51). The elements are fully broadband multi-coated and provide 99% light transmission of wavelengths from 400-700nm.

A unique feature of the design is that it is optimised for use with f4 Newtonian telescopes and provides full coma correction for this type. This allows users to view detailed objects at the edge of field with the same clarity as if they were centred. Dobsonian users will find this particularly useful as often objects tend not to be centred on axis with this design.

Despite the optimisation of the optics to correct for coma that is present in Newtonians, the optical quality of our Barlow also allows it to be used with any other type of telescope including our own Apochromatic Refractors. Due to its coma compensating design it will therefore introduce coma to none Newtonian designs. However following testing with a number of telescopes types, we have found the image to still be almost perfect to the very edge of field. Colour correction and optical quality are equal to Fluorite Barlow’s costing many times more. In use we believe you will detect no loss of quality with this Barlow at all even when used with types other than Newtonians.

This kit includes an APM 2.7x Barlow plus a specially made extension. When combined with a second APM 2.7x Barlow cell, the resulting magnification will become 6.25x with a 14mm flat field. This is achieved at a specified back focus of 84mm from the centre of the last lens element (83.1mm from the shoulder of the lens cell). With a Newtonian telescope this retains the coma correction of the single lens cell. This kit is intended for users who already own an APM 2.7x barlow.

This special kit allows owners of Newtonian telescopes the opportunity to maximise the performance of their scope for planetary imaging. A typical F4 8" Newtonian enjoys 5000mm of focal length with an focal ratio of only F25 allowing for high capture speeds. A 12" F4 becoming a very powerful 7500mm focal length.



Type Coma Correcting ED Barlow

Lens Doublet Cell with ED Element

Coating Broadband Multicoated

Magnification 6.25x

Back Focus 84mm (83.1mm from lens cell shoulder)

Optimised Imaging Field 14mm


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