Why are modern telescopes reflectors rather than refractors?

Question

Why are modern telescopes reflectors rather than refractors? What are the benefits of relfection telescopes compared to refraction? Thanks!

Answer 1

because of the ability to create inexpensive yet high quality mirrors. Reflectors are easier to build and maintain and are more cost effective.

Answer 2

Observatory telescopes are exclusively reflectors now because it not feasible to build refractors in the sizes now being used. The largest refractor in the world is only 40 inches (~1 meter) in diameter, while a 2 meter telescope is considered small in a world of 6 and 8 meter reflectors.

There is a limit to the practical size of a refractor because of the difficulty of supporting a large piece of glass so it doesn't sag. This isn't a problem with a large reflector, because the back of the mirror can be supported all the way across without blocking the optical path. Also, the types of glass required for a refractor would be very difficult to make in such large sizes. Glass type and quality for reflectors is much less critical, because light doesn't go through the glass.

Refractors are regaining popularity as amateur telescopes, due to the availability of well-corrected apochromatic lenses. But refractors are very expensive for their size, so reflectors rule in sizes over six inches.

Answer 3

There are a few reasons that most modern telescopes are generally reflectors or catadioptrics rather than refractors.

1. Large flawless lenses are difficult to make and the larger they are, the more prone they are to field distortion. Also, lenses by themselves suffer from something called chromatic aberration, where different colors focus to different points, and the only way to correct for this is using more lenses to make the system free of chromatic aberration...or apochormatic as it's called.

2. In large apochromatic refractors, multiple lenses must be made greatly increasing the cost and weight of the telescopes optical tube. Also, refractors typically have long focal lengths and a large 12" refractor may have to be many feet long to accomodate this.

3. The sheer weight and size of a large refractor means that they must have very heavy duty mounts which raise them well off the ground. Mounts are typically the most expensive part of the telescope so a 12" refractor ends up costing many tens of thousands of dollars.

In comparison, a 12" reflector or catadioptric such as a schmidt cassegrain can be had for under $2000. Only a primary mirror, secondary mirror, and in the case of catadioptrics, a corrector plate has to be made which is far less work intensive than making multiple large lenses. Reflectors and catadioptrics can be made with folded designs so they can have long focal lengths in short tubes, and can also just be made with short focal lengths without introducing too many errors. Also, even with large apertures, such as 12", the tubes typically don't exceed 50lbs or so and the mounts can be much smaller and transportable.

One exception as far as catadioptrics go are Maksutov Cassegrains. Their corrector plates are fairly thick lenses and these lenses have the same attributes which limit the size of refractors.....they get more difficult to make and overtly heavy if they're made to large so Maksutov Cassegrains tend to be smaller.

Answer 4

You may know large lenses are required for the objective lens.
The light is refracted and
refraction absorbs light so that there is appriciable reduction of the quality of image.
While in the reflector, the light has to pass through no such media, which maintains its quality.
:)

Answer 5

You can make larger reflector telescopes much more easily than refractor telescopes.

Answer 6

Another benefit is that mirrors don't suffer from chromatic aberration. But the main benefit, as others have said, is mainly a matter of weight.

When Isaac Newton invented the reflecting telescope his main motivation was to avoid chromatic aberration.

Answer 7

Go partway down this page to read more about it, although the answers above are all correct.

http://www.telescope.com/content/learningcenter/content2main.jsp?iCategoryID=29&iContentID=623&CCNavIDs=19,22,29

Answer 8

It is easier to build huge mirrors than it is to build huge lenses.