Why won’t my erect image eyepiece or diagonal always give right side up images with my scope?

Image erectors won’t work in all circumstances. The orientation of the image produced by your scope depends on its optical design and its orientation. Though optics of many scopes create inverted images, an image may actually be right-side-up, sideways, or at an angle depending on how your tube is oriented, how your head is oriented and looking in the eyepiece, etc.

An erect image eyepiece is a series of prisms combined with eyepiece lenses designed to rotate the image from a telescope’s main optics by 180°. It’s most commonly used with the Newtonian reflector. It simply replaces whatever eyepiece you would normally use. When used with the reflecting scope where the image is presented upside down, it will give a right side up image that’s correct right to left.

An erect image diagonal is a right-angle or 45° star diagonal that replaces a regular prism with an Amici or roof prism. When used with a refractor (where the straight-through image is inverted), it will give a right-side-up image that’s correct right to left. A regular prism star diagonal also gives a right-side-up image, but it’s mirror-reversed.

Image erectors--whether eyepieces or diagonals--are also affected by orientation. They will work properly only in certain scope and viewer orientations.

An erect image eyepiece with a Newtonian will give the erect views only when the tube is in certain positions and you are looking into the tube from a certain position relative to the tube’s axis. The combination works when the main tube is level and the eyepiece drawtube is level on the right-hand side and you are looking directly into the eyepiece. The corresponding position on the left-hand side will also work (tube is rotated 180° in the tube rings). A third position that gives an erect image is with the tube level, the eyepiece drawtube on top, and looking into the eyepiece from behind (standing back towards the mirror end of the tube). Any other position will give a tilted or even upside down orientation, despite the use of the erecting eyepiece.

For Newtonian telescopes with dovetail mounting systems that place the eyepiece at a convenient 45° position for astronomical observations (like the NexStar 114SLT and 130SLT and the 114LCM), you can't get an erect image, only a tilted one. So these scopes will show landscape scenes and terrestrial views at an angle.

An erect image diagonal has similar limitations. It will work with a level refractor tube when looking straight down into the diagonal and the eyepiece you’ve inserted. It will also give an erect image when rotated 90° right or left and you are looking in from the right or left side. Other orientations of the diagonal and your viewing angle will give slanted views.

Note: A simple trick will work with a Newtonian reflector without any erecting optics. Point the level scope at what you want to see and rotate the eyepiece drawtube so it is pointing straight up. Now stand in front and to the side of the scope (not blocking light from getting in the tube), looking back towards the mirror end of the tube. Bend over and look into the eyepiece from this position; you will see an erect image.

Updated 12/27/13