When you look through a telescope at the Moon, you might be a little confused as to why it looks flipped left to right or rotated on its side 180 degrees. Your finderscope perhaps produces an image similarly flipped upside-down. This can be a headache to understand, and many beginners often wonder why it isn’t a concern – or rather, a necessary evil – for using a telescope. In this article, we’re going to go over why telescopes rarely display a correctly-oriented image, the pains that must be taken to do so, and why they’re not worth bothering with.
How to Fix an Upside-Down Image
Generally speaking a most telescopes image will be either upside down, mirrored or a combination of the two depending on the accessories and your scopes design.
To fix an upside down image in a Refractor or Cassegrain we can add a star diagonal to our eyepiece to correctly flip the image, however the image will remain mirrored. To further correct or “erect” this to what’s known as land use, we can introduce what’s known as an erect image prism diagonal to correctly orientate the image.
To fix an upside down image in a Newtonian Reflector we can also add the star diagonal to our scope to flip the image, however this is not advised as this addition will remove the eyepiece further from the focal point of the scope. This extension makes it impossible to regain focus with the eyepiece and effectively making the scope unusable.
It’s important to note that when introducing either a diagonal or a prism into our scope is that it will always add an extra surface that will reduce light throughput and create optical aberrations (you can learn about those here), therefore it’s always better to know the reason why you want to flip your image before introducing extra equipment. Read on to find out more about optical image orientation.
Some Basic Physics
When light enters a lens or bounces off a mirror of a telescope objective, the converging light rays that hit the top of the objective continue to travel and ultimately form the “bottom” of the image. This means that, looking straight through a Newtonian reflector, or most commercial catadioptric or refracting telescopes without a star diagonal, the image is upside down.
Adding an additional reflecting surface does further wacky things to the light rays. The star diagonals we tend to use with most non-Newtonian telescopes end up flipping the image back right-side up again – but regardless of whether they use mirrors or prisms, a regular astronomical diagonal flips the image left-right and provides a correct orientation up/down. This can be a bit of a pain if you’re trying to match your image to a star chart – with a Newtonian you can just flip the chart over, but with a refractor or catadioptric you’d need to photocopy the charts and reverse them left-right for them to match (also ruining the legibility of any text).
Bringing the Image Right-Side Up Again
Terrestrial telescopes – and binoculars, which are just pairs of refracting telescopes aligned to each other – obviously need to display a correct image for reading text, quickly determining the orientation of landmarks, wildlife or people, and of course to provide an aesthetically pleasing view that doesn’t confuse the user. Thus, they use prisms.
Since you look straight through binoculars, the prisms need to flip the image right side up without also diverting the light 90 degrees. Thus, they usually use either Roof or Porro-style prisms. Porro prisms are generally used on binoculars that have objectives spaced more widely apart than the eyepieces – this is necessary with very large binoculars. Porro prisms are also easy to manufacture and don’t absorb much light going through them. Because of their generally higher light transmission, lower cost, and the fact that they make binoculars a little fatter and easier to hold you’ll generally see the majority of astronomy binoculars advertised with Porro prisms.
If you ever see a Newtonian reflector advertised with an “erect image eyepiece”, that’s simply an eyepiece with a small Porro prism installed in the bottom end, or a Porro prism attachment that inserts ahead of the telescope eyepiece inside the focuser.
Roof prisms work in the same way as a Porro prism, but are more complex optically and have more air-to-glass surfaces – meaning that a Roof prism provides a dimmer and possibly fuzzier image at the other end than a Porro prism. This is fine for terrestrial use, but a Porro binocular is easier to hold, is cheaper and of course the light transmission is a lot more important on dim astronomical targets.
While some spotting scopes are straight-through and typically use Porro prisms, most refracting telescopes and catadioptric designs designed for terrestrial use generally have a 45-degree, 60-degree, or 90-degree diagonal in them like an astronomical star diagonal. These diagonals use either an Amici prism or a pentaprism. Amici prisms are similar in design to the Roof prism, and are generally cheaper and easier to manufacture than pentaprisms. Amicis are generally found in correct-image finderscopes or astronomical telescopes designed with the possibility of terrestrial use in mind, while most good spotting scopes use pentaprisms. Pentaprisms have worse light throughput than an Amici prism and are harder to manufacture, but produce sharper images.
Why Not Correct the Image?
The simplest answer many astronomers will give to this question is simply that there’s no up or down in space. Ultimately, image orientation in a telescope is basically completely meaningless unless it has to do with aiming it at a particular target using a star chart.
A finderscope with either a correct or upside-down image will match star charts, as will a zero-power finder, and as such there’s little need to worry about the inconvenience of orientation when aiming the telescope. The exception to this might be when trying to locate dim fuzzy objects in the field of view, but given that most large telescopes are Newtonian reflectors, rotating the chart 180 degrees works as a solution.
The arguably more important reason to not bother with correcting the image, however, is that it hampers the overall quality of the view. An Amici prism diagonal will not only provide a slightly dimmer image than a regular star diagonal in a refractor or catadioptric, but it will vignette with low-power eyepieces and produces a bright spike, false color, and often glare on bright targets, which can seriously hamper viewing the Moon and planets – the few objects where you might be able to make the case for an upright image.
Newtonian reflectors won’t reach focus with many types of image-erecting prisms, and of course they dim the image like with a refractor. The amount of strain a long heavy prism with a (likely heavy) eyepiece on the far end puts on a focuser can also be an issue, as well as it throwing the balance of the whole telescope off.
Should I Correct the Upside-Down Image for Astrophotography?
This is really a matter of personal preference and geographic location. What looks right side up for someone in the United States will look upside down to someone in Australia, so you should probably focus on the overall aesthetic appeal of what orientation your image is displayed at. It’s all done in post-processing anyways.
Image Orientation Final Thoughts
Now that we’ve gone over the reasons images aren’t correctly oriented in most astronomical telescopes, how this is fixed for terrestrial use, and the detriments they cause for astronomical use, we hope you’ll appreciate why they aren’t often used. At the end of the day, the simplest explanation is really the easiest – it really doesn’t matter what orientation stuff in space is, and certainly not enough to sacrifice your telescope’s bright and sharp views for.