The main difference between refracting and reflecting telescopes is that refractor is using lenses and reflector is using mirrors.
I will explain how they actually work and all their pros and cons. So let’s have a first look on refractors because they are way older than reflectors.
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How Does Refracting Telescope Work
It all begins with the lens many centuries ago- we don’t know exactly when because scientists argue about that. Some say that the lens is thousands of years old. Archeological evidence indicates that there was some kind of rocky crystal artifact dated to the 7th century BC so-called Nimrud lens. We also have proof that even ancient Egyptians were using some type of lens based on certain hieroglyphs.
But the first oldest reference for using the lens is from Aristophanes mentioning a “burning glass” in his writings from 424BC. And it is confirmed that in the Roman period people used burning-glasses. Anyway, the lens was developing century by century until it was first used in a telescope.
It was in the 16th century when great mathematician and astronomer Galileo Galilei made his first simple telescope by using two lenses in a hollow tube. The first prototype of the refractor was created.
Nowadays, the refracting telescope is more sophisticated than the Galileos first prototype, but they work on the same principle. Its based on two lenses at the front, one for light collection and the second for magnification, and the hollow tube with the eyepiece at the end of the scope.
- Perfect for observing close celestial objects like moon and planets. They can provide a crystal clear image.
- Low maintenance because you don’t need to collimate and clean them from the inside, it is a closed system.
- Ideal for astrophotography.
- It doesn’t suffer from thermal degradation.
- Expensive if you want a bigger aperture.
- Problem with chromatic aberration, which is the effect of dispersion resulting in an issue to focus all colors of the optical spectrum to the same convergence point.
- Not good to use for deep sky objects (galaxies, nebulas, star clusters).
How Does Reflecting Telescope Work
When Galileo Galilei created his first telescope and pointed it to the stars, he started a revolution in astronomy. People wanted to see more, and they were building bigger and bigger refracting telescopes. It was not a long after when they started to realize that it is not practical and lenses suffer from chromatic aberration (rainbow colors around the observed object).
Few years after trying to resolve this problem, Isac Newton created the first prototype of reflecting telescope, Newtonian telescope. He was using a concave primary mirror at the end with a secondary small diagonal mirror at the beginning and focusing with an eyepiece. Surprisingly, we are using the same design even today. The main advantage of the mirror is that it doesn’t suffer from chromatic aberration. The light is only reflecting off the mirror, and it is not going through any lens like in refracting telescope. We can build much bigger and more efficient telescopes. The reflecting design is being used in almost all observatories on Earth. Even Hubble telescope is a reflector with an aperture of 4.2m, and it is orbiting the Earth since 1990. It was the first space telescope.
- Cheap to build, good entry level for beginners.
- It can have a big aperture for observing deep sky objects.
- No chromatic aberration problem because it’s using mirrors.
- It has good contrast and can capture a lot of light.
- Can be used on Dobsonian mount.
- It requires a maintenance, especially collimation.
- Thermal degradation.
What Should I Choose?
Every beginner is asking this question. I must say, it is tough to answer because every scope and every mount is used for different purpose. Personally, I like reflectors more- I consider them an all-rounder for everyone. That’s why I always recommend starting with Newtonian telescope first. It is cheaper, and you can buy a decent aperture on the budget. The best choice is to buy any Dobson, as big as your financial situation allows. They are great for beginners, and the Dobsonian mount is very easy to use. As an example, you can buy 8inch aperture Dobson! And that is a huge scope. You will be blown away what you can see with this decent Dobson scope.
Can I Use Telescope During The Day?
Yes, you definitely can. Many things can be done during the day. If you equip the scope with a solar filter, you can observe the sun, specifically the sunspots. You also must have noticed that sometimes you can see the moon during the day. So you are not limited to use your telescope only at night. However, sky watching at night is something completely different and satisfying in its own way.
I know that some people will ask if you can use a telescope to watch mountains, animals in the distance or spying on your neighbor 😀 The answer is, yes you can, but there are few issues that you have to be aware of. The first is that the magnification is sometimes so high that it will be useless in the real world during the day. But the biggest problem is that you will see the image upside down in the eyepiece.
Why Is The Image In My Telescope Upside Down?
Here is a very common question that people ask when they’re buying or starting to use their first telescope. Why is the image upside down? Is it normal? Is something wrong with my scope?
To give you peace of mind, your telescope is alright, and an upside-down image is absolutely normal.
First of all, an upside-down image is specific for telescopes which were made to observe the sky- extraterrestrial world. Astronomy telescopes of any type whether it’s refractor or reflector are producing a non-erected or a non-rectified image. The only telescope which is an exception is the spotting scope. Same like a pair of binoculars, it is designed to be used on Earth for the everyday terrestrial world, and it uses prism that flips the image for regular viewing.
Now, for astronomy scopes, the image is left upside down because for the extraterrestrial world which we’re observing, it is not that significant if the object is upside down, rotated or inverted from left to right. The fun fact is that the astronomy telescope works in the same way as a human eye. The view of the world through a human eye is actually upside down. The lens system in the eye projects an image onto the retina, and this image is upside down. And really, that’s the key to understanding what is going on with an astronomical telescope.
How To Correct Upside Down Image in Refracting Telescope
If you take a closer look at a refractor, you can see it has two elements at the front. It is a compound objective with two lenses and a little air gap between them. At the back end of the telescope, you have an eyepiece holder and the star diagonal. If you take those two off, all you have left is an empty tube- your telescope is just a hollow pipe all the way to the front lenses.
So just like the lens system in the human eye, a lens system in a scope produces an inverted image, and if you put an eyepiece alone on its place, even with a regular star diagonal the image will be upside down. When we replace the regular star diagonal with the erecting diagonal prism, it will erect the image and turn it the right way up.
The reason why we leave it on astronomic telescopes like this is simple. With the astronomical telescope, we always use minimum optical elements possible- we can’t afford to put an extra optic in because we need all the light we can get for the observation.
How To Correct Upside Down Image in Reflecting Telescope
The situation is completely the same as with refractors. The image is upside down, and we can’t fix it with more optic. What we can do is to use a nice little gadget called the erecting eyepiece which will correct the image. Anyway, I must tell you that even though it fixes the image, this gadget is not very practical and comfortable to use because of the weird angle a focuser is placed at on the reflector.
As you can see, it’s a lot of fixing for something that is not important for observing the sky, and it’s not even the fault. And I’m not sure it’s worth to use your telescope for terrestrial observing, but if you wish to do so, you know what you need now. Anyway, one more interesting fact about upside-down images and astronomy; it was worked around this problem in the past. The moon maps were printed opposite way with south on the top to make it easier so if astronomers were watching the moon it was easier to navigate with a map printed upside down.