Why do telescopes have long barrels?

[Pranav Jha]

why do telescopes have long barrels? Binoculars use pairs of prisms to lengthen the light path between lenses, but why? What advantage does the lengthening the path of light have?

 

[davenn]

why do telescopes have long barrels? Binoculars use pairs of prisms to lengthen the light path between lenses, but why? What advantage does the lengthening the path of light have?

it has the advantage of making the binocular barrel shorter but it comes at a cost...
it adds extra glass into the optical path which can decrease quality.
So as you can imagine, in a pair of hi quality binoculars, manufacturers work hard to produce very pure optical elements, use special coatings etc.

Dave

 

[Pranav Jha]

why does it have long barrel on the first place?

 

[Dadface]

Angular magnification=Fo/Fe and in normal adjustment the lenses are separated by a distance given by Fo+Fe.
(Fo= focal length of objective lens,Fe= focal length of eyepiece lens)
It follows that for high magnification Fo is large.

 

[davenn]

why does it have long barrel on the first place?

the length of the barrel of a telescope is directly related to the focal length and the diameter of the Objective (primary) lens. The secondary lens, the eyepiece is placed at that focal point.

Large Refractor telescopes (lens type scopes) have long focal lengths 2 metre and up and hi f - ratios f10 and upwards

Dave

 

[Pranav Jha]

Angular magnification=Fo/Fe and in normal adjustment the lenses are separated by a distance given by Fo+Fe.
(Fo= focal length of objective lens,Fe= focal length of eyepiece lens)
It follows that for high magnification Fo is large.

could you simplify please

 

[Dadface]

could you simplify please

As Dave explained the length of the barrel is related to focal length.The magnification is given by Fo/Fe therefore to get high magnification Fo should be large and Fe should be small.In normal adjustment the lenses are separated by the sum of their focal lengths(Fo+Fe) and therefore if Fo is large the length of the telescope will be large.

 

[Andy Resnick]

It's not magnification- it's aberrations. The primary lens/mirror is large to collect as much light as possible. The focal length must then be long to ensure the f/# is large (numerical aperture is small) to decrease the various aberrations introduced by using spherical surfaces.

Note, well-designed telescopes often operate at lower f/#, and they are often rather expensive.

 

[Pranav Jha]

i don't get any of these stuffs.I was reading about total internal reflection in binoculars and while reading the question popped up in my mind. Could anyone of you suggest the reading required to understand the answers that have been posted

 

[sophiecentaur]

Did you try Wikipedia for the basics of telescopes and total internal reflection?

 

[Dadface]

i don't get any of these stuffs.I was reading about total internal reflection in binoculars and while reading the question popped up in my mind. Could anyone of you suggest the reading required to understand the answers that have been posted

May I suggest that first you study the action of lenses,particularly convex lenses and that you become familiar with using the lens equation and drawing ray diagrams.Familiarise yourself with the nature,position,orientation and magnification of the image formed when the object is placed
1.Between the lens and the principle focus(F)
2.At F
3.Between F and 2F
4.At 2F
5.Beyond 2F with distance increasing
6.At infinity
Having become familiar with single lenses you can then study how to use them in combination to make optical instruments like telescopes.The simplest telescope theory uses a weak objective lens(long focal length)and a powerful eyepiece lens(short focal length) separated by a distance equal to the sum of their focal lengths.This is what I was referring to in my notes above.
You should also study concave lenses and curved mirrors.
That will give you the basics and cover most of the content of GCSE and AS level courses(UK exam system).Of course there's a lot more to lenses and optical instruments and if you want to go further you could study topics such as:
1.Lens defects and how they can be reduced.
2.The "eye ring"
3.Diffraction" and "resolving power"
4.Different types of optical telescope and telescopes that operate in other regions of the electromagnetic spectrum(eg radio telescopes)

 

[turbo]

In keeping with Dadface's #4, have you looked at catadioptric telescopes, like the common Schmidt-Cassegrain types? These 'scopes have very short tubes. Light enters the tube through a glass corrector plate, hits the primary mirror at the back, bounces forward to the secondary mirror mounted on the corrector plate, then back through a central hole in the primary mirror to the focuser. Such telescopes tend to be slow (long focal length). The tube is short, but the focal length is long because the light traverses the entire length of the tube 3 times, plus the length of the focuser.