The Tele Vue 4x 2" Powermate is a top-quality lens designed as an excellent replacement to Barlow lenses. This Powermate will quadruple the magnification of any eyepiece, even the Nagler 31. Simply insert the eyepiece into the top of the Powermate, and get an instant power boost. The focal length of the eyepiece is divided by 4, so a 25mm eyepiece becomes 6.25mm. The field of view is also quartered.
Tele Vue insists on nothing less than the best fully multicoated optics, using high-index glass to deliver the highest light transmission and best contrast, with less off-axis false color than 3-element Barlows.
What is the difference between the Powermate and Barlow lenses? There are a few shortcomings in the Barlow design, most notably unwanted increases in the eye relief of low-powered eyepieces, edge of field aberrations, and vignetting of the field using "shorty" Barlow lenses. The Powermate is designed to be superior, avoiding these effects.
Unlike Barlow lenses which diverge field rays, the Powermate features telecentric operation, meaning the field rays leave parallel to the optical axis. This is ideal for use with Hydrogen-alpha solar filters.
The Powermate is also perfect for photography. The optional T-adapter fitting provides a solid, compact mounting for any T-ring or T-threaded camera.
Many astronomers prefer using lower-powered eyepieces with a Powermate to higher-powered eyepieces, due to being more comfortable to work with.