Why Is Your Telescope Image Upside Down?

Your telescope is working correctly. Astronomical telescopes produce an inverted image on purpose, and it is one of the most common reasons a new owner believes something is wrong.

Any simple optical system that brings light to a focus produces an inverted image. Turning that image the right way up requires adding an extra mirror or prism, and every additional optical surface scatters a little light and softens the image slightly.

For a terrestrial spotting scope that trade is worth it, because an upside-down bird is useless. For an astronomical telescope it is not, because in space there is no up. Nobody looking at Jupiter cares which way round its cloud belts appear, and everybody cares about the light.

The mirrored view from a star diagonal is the one that catches people out, because it looks correct at a glance and then a star chart will not match. It is the standard configuration for a refractor and it is not a fault either.

Less than you would think. After a couple of sessions the brain adapts, and most observers stop noticing entirely.

Where it does matter is star-hopping with a chart, because the pattern in the eyepiece does not match the pattern on the page. Two practical fixes: rotate the chart 180 degrees for an inverted view, or use a planetarium app with a mirror-image mode, which most have.

Only if you will use the telescope for daytime terrestrial viewing. For astronomy the answer is no, and it is worth being specific about why.

• It costs light. Two more glass surfaces, both of which reflect a small percentage rather than transmitting it.
• It costs sharpness. Cheap erecting prisms in particular introduce visible aberration at high magnification.
• It costs focal travel. Adding one changes the focus point, and some telescopes will not reach focus with one fitted.
• It solves a non-problem. The inversion costs you nothing at the eyepiece once you have got used to it, which takes about two sessions.