Which Part Of The Telescope Reflects Light To The Eyepiece?

Have you ever wondered which part of the telescope reflects light to the eyepiece? If so, don’t worry because we have all the information you need about reflector telescopes.

Which Part Of The Telescope Reflects Light To The Eyepiece?

In this article, we’ll tell you everything you need to know about reflector telescopes and how they differ from their refracting counterparts. 

About Reflector Telescopes

The first reflecting telescope that worked for astronomy was created in 1668 by Isaac Newton. Since then, it’s become one of the most popular telescope types.

A reflecting telescope can help astronomers see distant objects in space by using mirrors. The mirror collects lights from different objects and forms an image. 

The primary mirror is wide and reflects the image of the object into another, smaller mirror.

The smaller mirror reflects the light from the primary mirror into the eyepiece. The eyepiece magnifies the image of the celestial object. 

Reflector telescopes can be much larger and more powerful than refractor telescopes. This is because refractor telescopes use lenses to collect light. At larger apertures, the lenses become very heavy. They also become very expensive to produce.

About Refractor Telescopes

Refractor telescopes are made solely with lenses, so the light bends as it passes through the lens. This works similarly to the glasses you wear to make your sight less blurry.

However, in telescopes, it allows you to see faraway objects as if they were closer to you.

We know that anyone with poor eyesight should have thick lenses in their glasses, which is also similar to telescopes. The more powerful your lens, the more you can see through your telescope.

However, the downside of this is that a large lens is heavier.

Heavy lenses are difficult to make and hold securely, making them more difficult to find. The light will be affected as it passes through them if not made precisely.

Likewise, if the lens isn’t extremely smooth, flaws will affect the image, appearing as though you’re looking through a blurred window.

Even though refractors use lenses mirrors are often used in conjunction with them. Due to the design of these telescopes observing through them can be uncomfortable. To improve comfort a telescope diagonal can be used. These use a mirror that reflects the light at 90 degrees to the telescope tube and into the eyepiece. They are fitted to the telescope’s focuser.

Why Reflector Telescopes Are Better

Unlike lenses, mirrors can be thin, and a bigger mirror doesn’t need to be thicker to become more powerful.

The light becomes concentrated by bouncing off the mirror, so the mirror will only require the correct curve to get the best results.

While lenses are more difficult to make, a larger near-perfect mirror is easier for manufacturers to produce. After all, mirrors are one-sided, so they’re simpler to polish and clean.

Mirrors suffer from fewer errors than lenses. If not corrected lenses cause aberrations due to the nature of the materials used and that of light. As a mirror does not transmit light, these errors are not present.

Despite mirrors being better, it doesn’t mean they don’t have issues. For example, imagine yourself looking into a spoon and finding yourself reflected upside down.

Much like a spoon, a telescope’s curved mirror will reflect like this. The mirror will flip the image, and the secondary mirror will flip it to appear as it should.

While mirrors do not suffer aberrations that lenses do they may be affected by coma. This results in stars on the edges of the field of view not being round. It is possible to correct this. For example, a field flattener can be used. However, they are not required for observational purposes (as opposed to astrophotography).

The main benefit of mirrors is that they don’t weigh much. They’re lighter than lenses and are thus much easier to launch into space. They are also cheaper to produce.

These properties are why the Hubble Telescope, Spitzer Telescope and more recently James Webb Space Telescope have been able to capture shots of nebulas and galaxies so far away from our solar system.

How Telescopes Work

There are three main principles to how any telescope works.

Regardless of how powerful a telescope is, you’ll find that they all work like this: 

  • They collect lots of light with a large aperture
  • Focus the light into a small yet sharp image
  • Magnify the object using an eyepiece

When you use a reflecting telescope, the light enters the front of the telescope tube and hits the primary mirror at the end of the tube.

The objective mirror is placed at the back end of the optical tube assembly (OTA). The location of the primary mirror is why the eyepiece is not on the rear of the telescope on Newtonian reflectors.

Instead, you’ll find the primary mirror is curved to reflect light to a secondary mirror. This then diverts and focuses the image of the object to your eyepiece on the side of the telescope. The light is diverted at 90 degrees to the telescope tube.

The Downsides Of Reflector Telescopes

There are some downsides to using reflector telescopes. These factors are the result of the design and imperfections in their manufacture. The most significant issue reflectors have is collimation.

Over time, the primary mirror will misalign with the secondary mirror, blurring the image.

Which-Part-Of-The-Telescope-Reflects-Light-To-The-Eyepiece (1)

Collimation is the process of realigning your primary with your secondary mirror. This is not the most straightforward process. However, that doesn’t mean it’s impossible.

It can be a challenge at first, but it’s a simple process when you find the right method that works for you. You can get details on collimating a reflector in our article called How To Collimate A Telescope.

Due to their design, most reflector telescopes aren’t sealed. The front of the telescope is left unobstructed to allow as much light as possible to hit the primary mirror. This means that the mirrors will need to be cleaned periodically.

The next issue is coma. Coma is an optical aberration that happens with reflectors. You’ll find the results of the aberration towards the edges of your view. The stars are stretched a little to resemble a coma.

Unfortunately, you can’t prevent a coma like you can collimation. Still, you can get out of it using a Schmidt-Cassegrain telescope, which offers the best reflector and refractor telescopes. 

Final Thoughts

In summary, light enters the front of a reflector telescope and hits the primary mirror at the back of the tube. The primary concave mirror reflects and focuses the light towards the flat secondary mirror. The secondary mirror reflects the light at 90 degrees to the telescope tube and towards the eyepiece.

All mirrors must collect light with a large aperture, focus it to a small image, and then magnify it using the eyepiece. This is the principle of all telescopes, but refractor telescopes work differently as they utilize lenses.

Reflectors have advantages over refractors. However, they have their disadvantages.

If you have any more questions about telescopes, please review our other articles to keep yourself informed. 

Frequently Asked Questions 

What Telescope Reflects Light?

A telescope that uses mirrors is a reflecting telescope, which uses mirrors instead of lenses. A reflecting telescope reflects light towards the eyepiece. 

What Are The Parts Of A Reflecting Telescope?

A reflecting telescope comprises an optical tube, mount, eyepiece, finderscope, primary mirror, and secondary mirror. 

Jason Anderson
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