I love the way that objects like M31 in Andromeda, the Orion Nebula, distant galaxies, and beautiful star clusters stretch across the huge field of these eyepieces. I feel more connected with the universe at large and as I study the treasure of stars in my eyepiece I am reminded of the distances and age of the cosmos and the constant discoveries that are being made by professional and amateur astronomers alike.

You can see from the illustration above that an eyepiece with a larger apparent field of view (AFOV) will produce a larger true field of view (TFOV) if we are comparing different eyepiece designs of the same focal length.

Large apparent field eyepieces give you what I call "full- immersion" viewing. With eyepieces that have small apparent fields (like Kellner ("K"), Modified Achromatic ("MA"), Orthoscopic ("OR"), or Huygens ("H") designs) if the eypieces are of good optical quality, the resolution, contrast, magnification, and sharpness will be there, but the observer tends to be more "removed" from the experience of observing the object. You feel like you are looking down a dark tunnel to see the object at the end of the eyepiece, and your eye tends to strain a little as you lock your eye straight down the eyepiece. In visual astronomy, the harder you try to look as you strain your eye, the less you see.

But when you use eyepieces of larger apparent field, you have a much wider visual angle to scan your eye across the image inside the eyepiece. By scanning your eye from edge to edge you can more comfortably and naturally look into the eyepiece, and as you relax your eye, you begin to see finer and fainter details. This "immerses" you into visual experience of astronomy.

The experience for many who first use extreme wide angle eyepieces like the 82° Series is often quite overwhelming as they start to really connect with the universe. The cost of such eyepieces is generally more, but experienced observers often feel the expense is worth it.

To calculate the true field-of-view with any eyepiece, first calculate the magnification the eyepiece is producing by dividing the telescope's focal length by the eyepiece's focal length. Then divide the apparent field of view of the eyepiece by the magnification. This will give you a number in degrees of the actual section of the sky that you are seeing through the eyepiece, the true field of view.

Scott Roberts