Telescope Diagonals – When and Why to Use them?

As an unprofessional and novice astronomer, it is pretty exciting to look at the sky through a telescope. However, if you want to pursue it as a profession or a hobby – you will have to buy your own telescope with Telescope Diagonals.

Once you buy your telescope, you might notice that it shows a mirrored view – like, for example, the night view appears left to right or upside down. Or, in some cases, it might appear both laterally and vertically inverted.

To add to your knowledge, it’s not an anomaly or problem. There are quite a lot of examples in our everyday lives where the images appear inverted – take the reflection in a mirror or rear mirror of your car.

In the case of a telescope, every telescope has a different configuration of optics. Optics are the lenses or mirrors that help us see the magnified images.

After a little while, you might get used to your microscope, and your brain automatically starts processing the inverted images. Hence, things in the night sky will begin to make sense.

Suit yourself with a little Experimenting

It’s quite right that there is no right, left, or up or down when you are in space. These terms are all relative. What is right from the North, is left from the south and vice versa. However, if you are using more than one microscopes, you need to adjust them with each other.

The difference in orientation is generally a result of how the telescope focuses the light. Typically, a telescope shows an upside-down image if it has an even number of optics such as a Newtonian reflector and two mirrors.

On the other hand, if your telescope has an odd number of optical elements – like a Nasmyth-Coude and its three mirrors, you might view a laterally inverted image – that is left to right.

Correcting an Upside-down View of the Telescope

There are two ways to correct an inverted image:

  1. Addition of a Star Diagonal
  2. Addition of a Prism Diagonal

Star Telescope Diagonals

Many refractor telescopes employ the addition of a star diagonal, which gives you a mirrored image, but it’s not inverted. However, one limitation is that you can not use it with reflector lenses as they add on extra focal length, thus pushing the eyepiece away from the point of

Prism Telescope Diagonals

Alternatively, adding a prism diagonal to your set up can produce an image that is both vertically and laterally correct.

Limitations to Altering Optical Orientation

Before changing your telescope orientation, you should look into the fact that the more accessories you add between your eye and the object, the more distorted will the image appears.

The reason behind this is the fact that as you push on more equipment or optics, they tend to scatter light. Hence, the image would not be focused, and there would be greater chances of optical aberrations.

Accessories to Correct the Orientation

To correct the orientation of the telescope image, we use equipment that is known as Diagonals. Generally, two basic types of diagonals are used in most of the telescopy – the star diagonal and prism diagonal.

Mirror or Reflective Telescope Diagonals

Also known as the “star diagonals,” mirror diagonals are available in different sizes, such as 0.965, 1.25, and 2 inches, as well as different price ranges, depending upon the type of telescope and the quality of diagonals.

Corrective Strategy

The Star diagonals or mirror diagonals turn the image at an angle of 90 degrees. They do so by using a mirror at an angle of 45 degrees. Hence, they produce an image that is vertically correct but laterally inverted.

Advantages of Mirror Diagonals

  • Mirror diagonals are less expensive as compared to prism diagonals.
  • They do not cause errors in the colors of the image.
  • They work better with short focal length instruments.

Disadvantages of Mirror Diagonals

  • They need a proper reflective coating; otherwise, they scatter the light resulting in a distorted and lesser in contrast image.
  • The reflective coating oxidizes over time, leading it to become less effective.

However, the newer dielectric mirrors are also present, which scatter less light than a star prism and can be used effectively instead.

Prism Telescope Diagonals

Generally, the prism diagonals use a 90-degree prism instead of a mirror to bend the light’s path. In addition to this, they also use a pentaprism or an Amici roof prism in place of a regular prism.

Simple 90 Degree Angle Prism

It provides the flipped or mirrored image of the real image.

Pentaprism Diagonal

Pentaprism diagonals are very difficult to find, and they show an inverted image.

Amici Prism

It typically divides the image into two parts, thereby allowing a vertically correct image – without the lateral inversion. Thus, what you see in the telescope in the exact and magnified replica of what you see without it.

Advantages of Prism Diagonals

  • It is the optimum choice for instruments with a longer focal length.
  • They offer the highest image contrast.
  • The prism improves the undercorrected refractor objective lens due to its dispersion properties.
  • It transmits more light as compared to a mirror diagonal.
  • There is significantly less probability of scattering of light.
  • A prism diagonal is resistant to oxidative stress. Hence it will never deteriorate with time.

Disadvantages of Prism Diagonals

  • Chromatic aberration is a common problem with prism diagonals.
  • The Amici roof prism diagonals make the path of light bounce around the glass piece, thereby limiting the total amount of light being transmitted. Hence, this results in optical aberrations.

Alignment of Corrective Accessories in a Telescope

Alignment is an essential aspect for the proper functioning and performance of a telescope. No matter how expensive your diagonal is, it will still not be able to give the optimum performance unless it is correctly aligned with the telescope’s optical axis.

The alignment is a more significant concern when using the star diagonals as they tend to increase the focal length between the eye and the image in question.

The Verdict

Before finally planning to change your telescope’s orientation, you might want to understand if you really need it. It’s a fact that you can go perfectly fine even if you do not use any of the diagonals.

Sometimes, an inverted or mirrored image is not a matter of concern – the moon will still be round and will be a moon even if it is inverted horizontally or vertically. Hence, it all falls on how you experiment with your telescope and make yourself comfortable with it.