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Explore Scientific 1.25" 2x 4-element focal extender # FE02-125 with compression ring.
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Increase Your Magnification Without Sacrificing Magnification Factor or Eye Relief
A Focal Extender does exactly what the name implies; it extends the effective focal length of the lens that it is used in conjunction with, thus producing higher magnifications for imaging and visual applications.
Focal extenders are similar to a Barlow or tele-negative lens, but there is one important difference between the two that you will want to take into consideration, especially if you plan to use the Explore Scientific Focal Extender for imaging and astro-photography.
When you use a standard Barlow or tele-negative lens to increase the magnification of your optical system, your results will be dependent on where you place the Barlow in the optical chain. For instance, if you put a 2x Barlow lens in front of your diagonal instead of behind it (Barlow-Diagonal-Eyepiece as opposed to Diagonal-Barlow-Eyepiece), you won't get 2x magnification in both set-ups. The magnification varies with placement. However, with an Explore Scientific Focal Extender, even if the distance is changed, the magnification remains the same. You can place your Focal Extender anywhere along the optical chain between an eyepiece or a camera, and if you are supposed to get 2x magnification, you will get 2x magnification.
When it comes to imaging, this is a critical difference, because you may want to introduce an accessory like a flip mirror or filter wheel in between the Focal Extender and a CCD camera, which would increase the distance to the focused image. This difference in design allows users to rest easy, whether they are pumping up the magnification of a lower power eyepiece for visual observations or using their Explore Focal Extender in imaging applications.
Explore Scientific Focal Extender Highlights