Refractor Vs. Reflector Telescopes

Can of Pie
Can of Pie
Paper Towel Tube
Paper Towel Tube

The Two Types

The two types of telescope that you mainly want to compare: refractor vs reflector telescopes. The difference is easy to keep track of: refractor telescopes use glass lenses similar to eye glasses. Reflector telescopes use mirrors - you see your reflection in a mirror... That's how I keep it straight.

Pretty simple, right? I always think so until I look a little more into it, then decide things aren't as they seemed.

You can always tell the difference between the two types just by looking at them. Refractor telescopes are long and skinny like a tube from a paper towel roll. Reflector telescopes are usually short and wide like a can of pie filling. Another way to tell is that the eyepiece is always on the back of a refractor telescope and always in the middle-front of a relfector telescope.

What the Difference Is

Why are there two types? One company said theirs was better? No. What the difference is often depends on the purpose of the telescope. You see, advances were made with glass lenses first so a lot of telescopes were made with glass lenses. It wasn't until Newton that they were really practical for anything but looking. I'm not sure if it was Newton that discovered this coming property or not, but it gave rise to reflector imaging.

Refractor lenses don't focus all colors on the same point. Mirrors do.

I think of light like most scientists do: a collection of wavelengths blended together to make the colors we see. There are many types of light you know by name but don't associate with light. Microwaves, radio, infra-red, visible light, ultraviolet, X-ray, cosmic, and gamma rays. The visible light you see with your eyes actually spans a very narrow window of the light that is out there. The light that comes from the sun and lands on the earth's surface is mostly visible light (with a little IR and UV mixed in). Thus, we took longer to discover that there are more types of light out there.

Most people think of radio waves in terms of frequency. I tend to think of all light in terms of wavelength - the two are very related, but I opt for wavelength. The shorter the wavelength, the higher the frequency and energy. Blue light has not quite twice the energy of red light.

What does this have to do with the lenses? Well, when you split an image into the colors and then focus the images, people were finding that when red was in focus, blue would be slightly out of focus. They would focus the blue and suddenly the red would come out of focus. This problem only occurred in refractor telescopes.


It is a Big Deal!

For small-scale operations it's all a matter of preference and is not a big deal. When you go snap a picture with your friends, the red and blue are so close together in focus that you can't tell - so it doesn't matter. But when you got a telescope as big as the Hubble or any that has an observatory built around it, then it will most probably be a reflector telescope.

When I said that visible light is a narrow window in the spectrum, that means that the red and blue won't be far out of focus from each other. How about when you look at X-Ray Vs. Microwave? It is a big deal! If you were trying to take a picture of an event with both wavelengths, one would be so far out of focus that you wouldn't be able to identify what you were looking at. But with a reflector telescope, the microwave will be just as in-focus as the X-Ray. That is why it is a much sharper image when using a reflector to look at a wide range of colors.

Tricky Logic

When I first started looking into telescopes and saw a diagram of a reflector telescope, I almost blew it off as bullshit. Why would anyone stick a mirror in the middle of oncoming light like that, especially in the center of attention? It would be like waving a hand in front of a camera - it would block the image you're trying to take a picture of.

Then I started wondering why your iris contracting in your eye doesn't create a dark circle in the edge of your vision. Or the aperture in a camera?

Then I realized that if you wave a hand ten feet in front of the camera while focused at a hundred feet, the image can still be seen with a very blurred hand in the middle. The image can still be seen in focus. The smaller the object in front of the camera and the closer it is to the camera, the more it will dim the image as opposed to blurring it. When waving your hand in front of a large-aperture telescope, the entire image can still get through. Tricky logic, eh? You will not have an image of a hand stuck in the middle of an image of the moon - the hand will be so out of focus and dim that you might not be able to tell the hand was there at all. That's the same with the mirror - it might block ten percent of the light, but it will not create a void in the center of your image like I'd previously thought. Since the mirror in the telescope is small it will only dim the image as opposed to blurring it or creating a void in it.

Comments 26 comments

Rob 7 years ago

Great hub! Very informative. It gave me a bit more info that I didn't already know so thanks :)

Sarah 7 years ago

Thanks for that. I've always wanted to know how to tell the difference between the two.

CV 7 years ago


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Science Guru 7 years ago Author

One other note about why refractor telescopes aren't used much in astronomy:  glass is transparent to visible light but not transparent to some other energies of light. You might be trying to watch an object/event in the IR spectrum and just see blackness if using glass lenses. You don't have that problem with reflector telescopes.

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Aidan James 7 years ago from Sydney

Nice hub - I have a couple of similar ones if you would like to check them out sometime

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Science Guru 7 years ago Author

Aidan does have a good hub on telescopes. Mine is designed to help understand the science while his does that as well as gives advice to consumers.

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Science Guru 6 years ago Author

I can always tell when finals week at universities comes around because the number of hits to this site jumps by around 300%. Just remember that sites like this are designed to help you understand concepts and to inform. Like wikipedia, it is not a site you can cite in your thesis for a PhD without pissing people off.

Stargazer 6 years ago

Not all reflectors have the eypiece in the middle. Those are typically Dobsonian designs. Schmidt Cassegrain designs have eyepieces at the back just like refractors.

Wrong 5 years ago

Stargazer's comment is incorrect. I think Stargazer meant secondary mirror, not eyepiece, in the middle. All reflector (Newtonian) telescopes will have the secondary mirror in the middle. Dobsonian is a type of mount that houses a Newtonian reflector. SCT designs are not considered reflectors, but are a hybrid of reflector and refractor.

Jamal khan 5 years ago

i am a beginner and want to buy an astronomical telescope. i also have a bit knowledge about stars and planets so i want to buy that one by which i can see galaxies and invisible stars.please tell me which one will be most suitable for me.

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Science Guru 5 years ago Author

Well truly I cannot tell you which one to buy. Really all I can say is that if I were to buy a telescope meant to view stars that are not visible to the naked eye, I would go with a reflector telescope ("Can of pie" or Newtonian). It all boils down to personal preference, comfort while gazing, affectiveness... how well the telescopes fit to you. But a few key points to remember is that you want to be able to tilt it and rotate it while viewing, that doesn't take a college education to use, and also if where you live is going to have a clear view of the sky. I live in Portland, which means for six months a year the sky is cloudy and thus no stars. Talk about a dreary winter season here.

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Science Guru 5 years ago Author

So another cool little bit I found on this subject... While searching for ways to increase solar panel efficiency, I was reading up on solar concentrators: setups that use mirrors to concentrate the light before it reaches the photovoltaic surface. It dawned on me that reflector telescopes have almost the exact same setup.

Turns out, a power station in Southern California did just that! They turned a solar concentrator power station into one massive telescope! Wikipedia has an article on it:

Good food for thought. Enjoy!

Ahmetcan Caglayan 5 years ago

I am studying for a Planetary Astronomy test and this article simplified what was confusing at first. Thanks.

Mike 4 years ago

The explanation for the secondary mirror doesn't make sense to me. The example of the eye doesn't work because our brains do so much extra processing to create an image that it can't really be used. We actually only look at a small area in the centre of our vision and our lens can change focal length anyway to utilise the smaller pupil diameter.

The hand in front of a far away focussed camera also didn't work for me as it blocked the image quite well. (In front eyes doesn't work because of stereo vision of course) I could see how because some light can get around that isn't travelling parallel (or is hitting a different part of the mirror) and that this would allow an image formation if the object isn't at infinity but I can't see how that works with a secondary lens with focal length on the actual focal length. I think I need to draw some ray diagrams...

King David 4 years ago

Great info!

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Science Guru 4 years ago Author

The image doesn't reach a perfect focus on the second mirror in a reflector telescope. The light bouncing off that mirror reaches a perfect focus just outside the telescope, which is where you want to put your eye.

In regards to the hand in front of the camera concept, you need an aperture larger than your hand because if it's smaller than your hand (like your eye) you will block all light entering it. If you have a large aperture and focus on the moon, moving your hand in front of the aperture will dim the image of the moon, but the image of your hand might be so blurry that you won't even be able to identify it as a hand at all.

look2theskys 4 years ago

My girlfriend bought me a Celestron 21037 powerseeker 70eq telescope it will be arriving in about 10 days my question is how well will i be able to see the moon

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Science Guru 4 years ago Author

look2theskys, I honestly can't say. I'm not familiar with that particular telescope. I don't know your level of experience, but make sure you read up on how to use the telescope to maximize it's effectiveness. You'd be amazed at how many people I've talked to that don't understand why a star moves out of the scope and how to adjust for this.

Martin Long 4 years ago

Hi, I am considering purchasing, for the first time, a telescope. I have two boys (aged 13 and 11 yrs) and we live in Johannesburg, South Africa.

We are complete 'beginners' and the purpose would be more for fun and enjoyment.

The model I am looking at is the Celestron Skyprodigy 130 which appears to have received particularly good reviews.

I would appreciate your thoughts.

Kind regards

Martin Long

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Science Guru 4 years ago Author


I'm not familiar with that particular telescope. To be honest, I have very little hands-on experience compared to how much time I've spent studying the theory of how telescopes work. Good luck looking for the right scope! Have fun stargazing.

Aditya Apte 4 years ago

It is really very interesting... :)

Abhijit Khatu 4 years ago

thanks, really thanks

gayathry 4 years ago

thanks, informative

vidhya 4 years ago

that was a lot of information.nobody expains in that detail.thanks

miles 3 years ago

Great. Liked it a lot. Minded only one thing. There is just one kind of light and that is visible light. Microwaves, radio, infra-red aren't light.

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Science Guru 3 years ago Author


It's all part of the electromagnetic spectrum. Tomato, Tomahto.

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