The excitement of buying a telescope is unmatched. From selecting the right telescope according to your budget, finding and ordering it, and finally holding it in your hands, the entire journey is an exciting ride.
Finally, when the time comes to point your telescope at a celestial object that you are most looking forward to seeing, you notice the image that you see is upside down.
Our 14-point guide to using a telescope (opens a new tab)
The disappointment is understandable, but it is quite normal for certain telescopes to invert the images so what you see is upside down compared to what you can see on the star maps. So don’t worry, your telescope manufacturer is not fooling you. Neither is your telescope broken nor functioning sub-optimally.
Why do the telescopes do this? That’s what this article is going to be about.
Why Does My Telescope Show Everything Upside Down?
Short answer: it all boils down to the exact design of your telescope.
You can think of the function of telescopes similar to our eyes. When you see something, the rods and cones in our eyes invert the image of the object, and it is the brain that orients them back so we know what we are looking at. If not for our brains, we would be seeing the world upside down!
Like rods and cones in the eyes, telescopes also have a high number of optical elements that function in harmony for the telescope to work.
The images produced by different telescopes could be upside down, left-right mirrored, or both upside down and left-right mirrored.
Which Telescopes Give Which Views?
Short answer: telescopes that contain an even number of optical elements give rise to upside-down images of objects. An odd number of optical elements give way to images that are reversed left to right.
A Newtonian reflector, for example, has a concave mirror that gives way to an image that is upside down. A Cassegrain telescope has a parabolic and a hyperbolic mirror, whose shape gathers light from celestial objects and produces an upside-down image.
Other kinds of telescopes have different mirrors that orient the object from left to right, so it will be akin to a mirror image.
Can Upside-Down Telescope Images be Corrected?
Short answer: Yes. Star diagonals and prism diagonals will help you orient your images back to normal.
Observing objects in space, where there is no direction as such, relies heavily on the observer’s ability to orient themselves. This is why although telescopes produce incorrect images, there are solutions available to correct them so that you can enjoy views from your telescope the way you intended to.
One of the solutions is a star diagonal, which is a diagonal mirror that is used in telescopes. This mirror deflects the incoming light by 90 degrees so that you can view from above the line of the telescope tube rather than parallel to it.
To use a star diagonal, you need to insert it into your refractor’s focuser tube. Your eyepiece then goes into the other end. The video below from Celestron shows how these work.
A variant of a star diagonal is a prism diagonal, which is the same concept except in the shape of a prism. The base of this prism is placed at a 45-degree angle, and the shorter faces of the prism stand perpendicular to the incoming and outgoing light. This way, the prism deflects the light by 90 degrees, similar to what a star diagonal does.
An ‘erect image diagonal’ contains a special kind of prism in the ball shape at its right angle (see 1:52 in the video above). The Amici prism inside not only turns the image upright but also turns mirrored images back the correct way. Using one of these in a Schmidt-Cassegrain or refractor will make the image appear as it would to the naked eye.
Using star and prism diagonals will rectify your upside-down images, so you can better enjoy your observations. However, these are not required for reflectors like a Newtonian reflector because they are not designed to have a large focus travel, which is the total distance an eyepiece moves in after it is inserted.
Consequence of Correction
To correct the upside-down and left-to-right images so that we can view them better (which also helps with star hopping) we are essentially adding more mirrors and objects in the telescope. This increases the path light has to travel before reaching our eyes, which means the brightness will be reduced.
Especially in prism diagonals, light travels longer, which means you will always need a high-quality prism to avoid chromatic aberration, which happens when a lens fails to focus all the colors at the same point, giving rise to a blurry and discolored image.
Star diagonals need high reflectance to orient your view of objects properly without losing any of the incoming light. A recent improvement came about with dielectric mirrors, which are said to reflect as much as 99% of the light, so the object that you are viewing will still retain its shape and color.
Seeing upside-down images can be frustrating when you’re not used to it. However, it’s important to take the time to ensure your fixes, such as a star diagonal, don’t make the image worse.
In some ways, you may prefer to learn to understand and enjoy upside-down images! That’s perfectly fine, too. There is no direction in space, anyway. You can simply turn our star charts upside down as well so that what you see on your map and what you see in the sky are in alignment.
If you wish to add star or prism diagonals, we hope this article has provided you with adequate information to do it carefully enough to retain your image’s quality.
Written by Sharmila Kunthuner