Astro-Tech 2X Telecentric Extender Barlow for 1.25″ Eyepieces

Description

A standard Barlow uses a single negative lens group (typically 2 elements) that diverges the light cone. It works, but it shifts the exit pupil, can cause vignetting with wide-field eyepieces, and its magnification changes depending on the spacing between the Barlow lens and the eyepiece. A telecentric extender adds a second positive lens group (for a total of 4 elements) that corrects the exit pupil back to its original position. The result is a constant 2x magnification, no vignetting, and no change to the eyepiece’s eye relief or apparent field characteristics.

Can I use this for astrophotography?
Yes. Unscrew the eyepiece holder section to expose the 42mm T-thread. Attach a T-ring for your DSLR or mirrorless camera, or thread on a dedicated CMOS/CCD astronomy camera directly (most have 42mm T-threads built in). This doubles your telescope’s focal length for planetary, lunar, and double star imaging.

Does this work with a binoviewer?
Yes, and it’s one of the stronger use cases for this product. The constant 2x magnification means the longer light path through the binoviewer doesn’t shift the magnification the way it would with a standard Barlow. You can also unscrew the top section and use an M42-to-1.25″ adapter to attach the optical section directly to the binoviewer body, saving weight and nosepiece length.

Will this vignette with wide-field eyepieces?
The telecentric design specifically addresses the vignetting problem of standard Barlows. Because it corrects the exit pupil back to its original position, the light path entering the eyepiece is the same as it would be without the Barlow just at twice the magnification. Users report no vignetting even with 82 apparent field eyepieces.

Accessories

• Astro-Tech UWA 82 Eyepiece Series: Parfocal wide-field eyepieces in 16mm, 13mm, 10mm, and 7mm. Pair with the 2X telecentric to fill the gaps between focal lengths a 13mm becomes a 6.5mm, a 10mm becomes a 5mm.
• 1.25″ Eyepiece Filters: The filter-threaded nosepiece accepts standard 1.25″ filters. A #80A blue filter sharpens Jupiter, a #23A red helps Mars, and a variable polarizing filter tames the Moon.
• T-Ring for your camera: Required for DSLR and mirrorless astrophotography. Thread it onto the 42mm T-thread after removing the eyepiece holder section. Available for Canon, Nikon, Sony, and other mounts.

Final Thoughts

The math on a 2x Barlow is simple: every eyepiece you own becomes two. But the math only works if the Barlow doesn’t degrade the view on the way through. Budget 2-element Barlows compromise vignetting, ghosting, wandering magnification, altered eye relief. The TeleVue Powermate solved those problems years ago with a 4-element telecentric design, but it costs several times more. The AT2XTB brings that same optical approach four elements, constant 2x, corrected exit pupil to a price that makes it a practical purchase rather than a long debate. If you’ve been using a shorty Barlow and wondering why the view isn’t quite as clean as it should be, or if you’ve been avoiding Barlows entirely because of past experience, this is the one that changes the calculation.

Tech Details: