With a new telescope, one of the first steps is to align the finderscope. So you look through the finder scope to line it up with a distant tree, tower, or antennae. The thing is it’s a tad tricky. The image is back to front or upside down or both! You can work with it, but a simple change can save that stretching of the brain when using the scope to find night sky objects in the future. Here’s what to know.
It can be a surprise the first time you look through a telescope or a finder scope to realize the image is upside down (inverted). The image is different from what you see with a naked eye. It will be reversed left to right, upside down, at a strange angle, or all of these. Don’t worry. Optics work that way. All telescopes show a non-rectified image.
(It could be reversed left to right, if the focuser has a diagonal in it.)
Binoculars differ to this in that they are designed so that the image is the right side up.
Why is my finderscope upside down?
Your finder scope is not upside down but rather the image you see through the finder scope is opposite to what you see with the naked eye. This is because humans have this ability to invert the image received by our eyes to ‘right side’ up. More on that below.
How to fix an inverted image on a telescope
An erecting prism will flip the image the right way up and from left to right to fix an inverted image on a telescope. Other image rectifiers involve an erecting lens or a diagonal mirror (star diagonal) that will fix the upside down problem. With all of these, you simply position the accessory in the focuser of the telescope before the eyepiece. But there is a limitation, as I explain in the following.
The erecting prism is more sophisticated than the 90º star diagonal, which involves a mirror. It inverts the image all ways (left to right and right side up) whereas the mirror fixes the right side up but not the reversed left to right.
Read on for the limitation in using these in telescopes for viewing planets and other celestial bodies.
For a telescope upside down fix
An erecting prism works fine on a refractor telescope used for terrestrial viewing, or astronomy.
But for viewing through a reflector telescope for astronomy, there’s a downside.
As Dickinson, the author of Nightwatch, mentions, erecting prisms in astronomical telescopes add extra glass that reduces the light transmission. So you’re not going to get the best view and you’ll likely find problems with optical aberrations at high power.
So, for a superior optical view in astronomical telescopes, avoid using an erecting prism (other than with refractors. I cover why refractors are different from reflectors in another article).
If you do want an upside down fix for a reflector, there is the erecting eyepiece. But you might find that this gadget is not all that useful because of how it extends out from the focuser.
At the end of the day, when it comes to viewing celestial objects in the night sky, an upside image is not going to matter.
On the other hand, with a telescope viewfinder or finder scope, it helps to have the right orientation when looking for objects to view through the telescope.
Telescope viewfinder image upside down – how to solve
To solve the issue of an “upside down finder scope”, you can simply replace the straight-through finder scope with a right angle type or similar that has an image correction feature incorporated.
Most telescopes come with the standard straight-through finder scope, so you will probably need to buy an image-correction one as an accessory to get the benefit. I recommend getting a right-angle finder scope, especially, if you have or intend to get a Dobsonian. Else, you’ll find yourself contorting your body just to look through the standard finder scope.
Look out for a right angle correct-image finder to suit your brand of telescope and you’ll solve your finderscope upside down image problem.
Why don’t we see things upside down?
We see things ‘right side up’, i.e. we don’t see things upside down because our brains transform the information we receive through our lenses to rectify the image.
What you view is received as light through the eye pupil and ends up on the retina (see diagram) as an upside down image that is sent to the brain via the optic nerve as impulses of electricity. The brain receives this information and transforms it, not only to the right side up, but into the 3-dimensional image that you perceive you see. This is explained simply at bceye.com, which provides more details about this amazing process.
Why is my image upside down (Video)
If you prefer a video to understand why the image is upside down, this one explains the concept in relation to telescopes fairly well. (Don’t worry, the video is not upside down).
Nightwatch, a Practical Guide to Viewing the Universe by Terence Dickinson (available at Amazon).
bceye.com: Your Eyes See Upside Down