Rebored - more eyepiece diameter

Is the switch to 2-inch eyepieces worthwhile? Find the answer quickly with a sheet of squared paper.

Bigger is usually better, but what options does an increase in the focuser plug diameter offer? Except for at extreme focus positions, the telescope focal plane lies within the focuser.  A larger focuser thus offers more space to observe a larger section of the sky. Larger eyepiece lenses can also show larger areas, and thus an eyepiece with a 2-inch plug diameter can also be designed to observe significantly larger sections of the night sky. At a focal length of 40mm, the image circle of a 1¼ inch eyepiece shrinks to less than 45°, this is known as tunnel vision which many star lovers find less visually appealing. However, a 2-inch eyepiece with a focal length of 40mm can reach 70°, making larger focal lengths attractive – up to a focal length of around 55mm.

More eyepiece focal length does not only mean a larger section of the sky is visibly, but you also get a brighter image – as long as the magnification does not fall below the minimum. Faint objects in particular benefit from this. Whether a larger eyepiece diameter could be useful depends primarily on whether the telescope would illuminate a noticeably larger image circle. To easily determine whether this is the case, remove all eyepiece attachments and point the telescope at a section of bright daytime sky (but never into the Sun). A sheet of squared paper held at the aperture will show the illuminated image circle. The field stop diameter of the eyepieces you are considering should not exceed this size. Many telescopes with an eyepiece connector designed for 1¼ inch eyepieces have a T2 thread camera connection with around 38mm free clearance. Well-stocked retailers will have 2-inch eyepiece connectors that are suitable.

Author: Sven Wienstein / Licence: Oculum Verlag GmbH