Non-Magnifying Telescope Lenses: Components & Effects

[mufan]

The components of a telescope are essentially two convex lenses? Out of curiosity (boredom) I am trying to figure out if the addition of another lens could make the image non-magnified.



For instance, what type of lens/other would you add in order to make the image look as if you were looking through a paper towel cardboard tube. In other words, what type of lens would cancel out the magnifying effect of a telescope if it were put in line with the other lenses?



It's how my mind works...I can't help it!

 

[DaveC426913]

The components of a telescope are essentially two convex lenses?

Only a refracting scope.

A reflector uses one mirror and one convex lens.

 

[Chronos]

I assume you mean the eyepiece, Dave.

 

[DaveC426913]

I assume you mean the eyepiece, Dave.

Well, I wasn't being specific but yes, it is the eyepiece.

 

[pixel01]

Hi all,

By the way, I've got a small question relating the telescope. Can I use only one concave lens for the eyepiece?. Why do they have to use a stack of lenses instead of just one lens for eyepiece?

Thanks

 

[Garth]

Hi all,

By the way, I've got a small question relating the telescope. Can I use only one concave lens for the eyepiece?. Why do they have to use a stack of lenses instead of just one lens for eyepiece?

Thanks

You can use one concave lens as the eyepiece, Galileo did!

A typical Galilean telescope with which Jupiter's moons could be observed was configured as follows. It had a plano-convex objective (the lens toward the object) with a focal length of about 30-40 inches., and a plano-concave ocular with a focal length of about 2 inches. The ocular was in a little tube that could be adjusted for focusing.

Garth

 

[pixel01]

You can use one concave lens as the eyepiece, Galileo did!



Garth

Thanks for your reply. The thing is why do they have to use some other lens stack but not only a concave as eyepiece like Galileo? There must be some drawbacks mustn't there? What are they?

 

[turbo]

Thanks for your reply. The thing is why do they have to use some other lens stack but not only a concave as eyepiece like Galileo? There must be some drawbacks mustn't there? What are they?

Some drawbacks with lenses include the fact that simple lenses do no bring all wavelengths to the same focus. This is called chromatic aberration. Reflectors usually feature a concave primary mirror, and a flat secondary mirror, and that secondary mirror aims the light to the eyepiece. In this basic design (Newtonian), there is no chromatic aberration aside from that induced by the lenses in the eyepiece, but it is very likely that off-axis images will suffer from coma, which is radial deformation. Apochromatic refractors can offer highly-corrected views with minimun chromatic aberration if they are designed with several objective lenses with a variety of curves and correspondingly tuned refractive indexes. Roland Christen is a master of this art as was Thomas Back (recently departed).

 

[Chronos]

The mulitply lensed eyepieces, as noted by Turbo, correct for both chromatic aberation and, to a lesser extent, coma. They also narrow the light cone of the primary aperature to the size of the pupil of the human eye. The eyepiece is just as important as the primary aperature for visual observation. Plossl's, for example, require several individual simple lenses to produce their visually impressive images. Multiple lens are economical because simple correctors are much easier to produce than complex shapes. They are also very efficient, thanks to anti-reflective coatings, wasting only a tiny fraction of the light that passes through them.