APM 2.7x Coma Correcting Barlow
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 our Barlow at all even when used with types other than Newtonians.
The design has been optimised for both visual and imaging purposes from the beginning. The fully corrected flat field image circle of our Barlow is 26mm making it suitable for use with a variety of imaging sensors up to APS-C when used at the recommended back focus distance of 105mm. To provide the maximum number of options for users, our Barlow can be adapted for both visual and imaging use as follows; For eyepiece use there is a 1.25” adapter using our rotary clamping system. This provides a much better fixing for your eyepiece than single screw based clamps. This attaches to an extension section which allows you to achieve the optimal 105mm back focus and a 30mm image circle. The design also features options for both 1.25” and 2” mounting to your telescope. Both mounting options feature safety undercuts to prevent the Barlow falling out of your telescope!
The eyepiece clamp and extension can then be removed to reveal a T2 thread. This allows for direct mounting of a DSLR T2 mount or CCD camera. This provides the approximate correct back focus distance which can be optimised by using spacers depending on your cameras flange to focal plane distance
Above image: one of the two T2 threads providing a direct connection to cameras and other accessories
Specifications of APM Coma Correcting Barlow
Type Coma Correcting ED Barlow
Lens Doublet Cell with ED Element
Coating Broadband Multicoated
Camera Connection T2
Eyepiece Connection 1.25”
Telescope Connection 1.25” / 2” Eyepiece
Clamping Rotary Brass Clamp
Max Length 120mm
Back Focus 105mm
Fully Illuminated Field Minimum 30mm
Optimised Imaging Field 26mm
Focal Length -62.9mm
Weight 200 grams