This astronomy hints post is going to double cover much of what we covered in our Telescope buyer’s guide but we hope to go into a bit more detail for those considering their first telescope – and that is the raging battle between reflectors (telescopes that use mirrors) and refractors (telescopes that use lenses).
Now we are just concentrating on the telescope’s optical tubes. We won’t talk too much about mounts right now. First let’s take a look at the two designs:
Now odds are, if you are looking at these two designs and don’t know a thing about telescopes beyond what TV & movies may show your first instinct will probably be to think of the refractor (the one on the left) as being what you think of as a telescope: a tube pointing at what you want to look at while on the opposite end of the tube is the place you look into. Its very simple, and very intuitive. It even invokes the old brass collapsing telescopes every pirate move seems to be require to show by law.
Conversely, the reflector (on the right) can be a bit odd-looking. First of all, the front end is open and empty! You have to look all down the tube to see any sort of optics (i.e the primary mirror). Then you notice that there is no place to look through at the end of the tube. The eyepiece just kind of sticks out of the side, and the finderscope (a little telescope used to help find things in the night sky) isn’t in the same place as the refractor. What is going on here?
Well, what is going on is that while the result is the same (light gathering and magnification), and some of the fundamental optics are doing the same thing, the fact is they are both doing it very differently.
First up the refractor employs a large lens, located at the front of the tube, to refract (hence the name) or bend the light. This bent light is designed to come to form an image very close to where the eyepiece is located (you can get an idea of how this works by making an image of an object on a wall with a magnifying lens – just remember the telescope is designed to have the image form much further away!). The eyepiece then focuses on this image. In a way this is like having a magnifying glass focus on a magnified image – sort of.
Conversely, the reflecting telescope doesn’t bend the light, it reflects it, or bounces it if you prefer. This is similar to what the mirror on your car or in the bathroom does, except that the mirror in a telescope is curved. So it also forms an image not very far from the telescope’s eyepiece:
So now that you know how each is different, the question becomes:
WHICH IS BETTER
The fact is: There is no simple answer for this. Let’s just go over some basics.
Let’s start with the advantages
- Tend to have sharper images
- Have more traditional designs
- Do not require much maintenance or collimation
- Are less expensive to build at larger sizes
Seems like Refractors are the winner, yes? No, not really. You see that first advantage of reflectors is a killer. It is simply much easier to make a larger mirror than a large lens for a telescope. With a mirror, you need to polish only one side and coat it. With a reflector, you need to polish both sides at least two lenses (most refracting telescopes use air-spaced achromats (multiple lenses) and will probably need coat them with an anti-reflection coating as well. That’s four lens sides, each one with a different curve.
The major selling point of refractors at small sizes is their traditional designs & lack of need for collimation. A small refractor will cost about as much as a small reflector and will give a beginning astronomer fewer things to worry about. The other advantage, the sharper images, tends to only come with higher-quality refractors or even what are known as apochromatic (3 or more lenses, or two very well designed ones) objective lenses. These can get very expensive, but at small sizes (i.e. easy to transport) the refractors can easily win out with image quality. Many astronomers, not wanting to haul around heavy reflectors, may opt for these advanced refractors.
Let’s look at the other side of the coin:
- “Chromatic Abberation” – where the lens breaks up the light like a prism
- More expensive than reflectors at mid and large sizes.
- Requires occasional maintenance/collimation for ideal viewing
- “Secondary Shadow” from secondary mirror will cause some loss in the light gathering
Chromatic Abberation can be annoying when viewing bright objects (the Moon, Jupiter, Venus, etc) as it results in what is known as a “violet fringe” around the object. This can be filtered out, and it doesn’t show up much on dim objects (light galaxies, nebulea, etc.) . But it can be frustrating to some novice viewers. More critical to refractors is the expense of making medium and large-sized models. We already covered why in the advantages section.
Reflectors on the other hand have the problem with collimation. Most large reflectors are probably going to need collimation if they’ve been moved around a lot. They will still work if they aren’t precisely collimated, but the images will appear streaky. Collimation can be a bit tricky to master and so can frustrate novice astronomers.
The secondary issue is bit of a misnomer. What it essentially says is that any reflector telescope is not going to gather as much light as its full surface area as a portion of it will always be in the shadow of the secondary mirror. This is mostly just a nuisance as if one needs more light gathering power one simply makes a larger primary mirror., but then weight does become an issue.
So to answer the question of ‘which is better’, there really isn’t much of a straight answer – at small sizes (for beginners) there is little reason to not get a refractor. But once the sizes get larger the reflector very quickly becomes much more economical. These rules are in no way ‘hard and fast’ as there are many exceptions to them. But as an overall guide they can give you an idea of what design to look at.