Simple Telescope making Help ?

[Venus2008]

Hi , Friends this is my first post in this Forum hope you will help me to the point.
I am a beginner and my Question is very simple , I found a tutorial of making an Ultra Simple Telescope at:
http://amasci.com/amateur/teles.html

Now I want to Know about the Power of Convex Lense ?

Two convex lenses:
Eyepiece:2 or 3 cm in focal length:
what is is the power of 2 is it - or +?)

objective lens: 25 to 50 cm in focal length :
what is is the power of 2 is it - or +?)

Please Help me .....
Venus

[Kaptain K]

Welcome to BAUT!

The "power" of a telescope is the focal length of the objective lens divided by the focal length of the eyepiece lens.

[RickJ]

As a kid I remember finding two magnifying glasses in our house. One a big but low power one and the other a small, higher power one. Using the low power one for the objective and the high power one for an eyepiece I made a 2 power scope. Turned out the low power lens had twice the focal length of the high power one -- see Kaptain K's post for the math.

It made great rainbows and I could tell the moon was larger. But it showed little more detail. Problem is simple lenses have severe spherical and chromatic aberrations. They have other defects as well but these are the two big ones. Its fun to get a bunch of lenses of different types and see what they do. Even a negative lens can be used for an eyepiece making a scope of the type Galileo used. But don't expect to see much. A pair of binoculars you probably already own will show you far far more in the night sky. They make a great first scope. See Dave's sticky binocular post for details.

Rick

[JustAFriend]

You can get lenses mounted and ready to go:

Here's an 80mm lens already mounted for less than $30-
http://www.surplusshed.com/pages/item/l2196.html

Look around their site, they have lots of stuff in many different price ranges.

[Venus2008]

Respecable Froum members thanks for your response


Venus

[Dave Mitsky]

Now I want to Know about the Power of Convex Lense ?

Two convex lenses:
Eyepiece:2 or 3 cm in focal length:
what is is the power of 2 is it - or +?)

objective lens: 25 to 50 cm in focal length :
what is is the power of 2 is it - or +?)


I don't fully understand what you're asking. What do you mean by 2? A biconvex lens? A doublet lens?

Be that as it may, the objective lens will cause light to converge so it is a positive lens.

http://en.wikipedia.org/wiki/Lens_(optics)#Types_of_simple_lenses

In a simple refractor, the eyepiece can be either convex or concave. If it is convex, it is also a positive lens and the resulting image will be inverted.

Dave Mitsky

[Venus2008]

Sir , Thanks for your Reply , I wanted to ask about the detail information about the Lenses , used in the Ultra simple telescope ,

Iin it Two convex Lenses are used .

One is Eye Piece and the other is Objective .

Eye piece: 2 to 3 centimetres in focal length.
objective lens: 25 to 50 centimetres in focal length .

My question was very simple , as you replied in your response is that What will be the Power of Eye Piece , Will it be +2 or for Objective Lense , Will it be 25+, I dont know the basics of all this .

Your Page of Wiki is also very informative .

In the last I must ask an other Question , as I think I will recieve your kind response , What is the Diffrence between Lense and Mirror ?

Thanks for your kind Reply Again ,

Venus

[RickJ]

To repeat what Kaptain K said at the start of this thread; the power of a telescope is the focal length of the objective divided by the focal length of the eyepiece. So if your eyepiece has a 2 cm focal length and the objective has a 50mm focal length you have a 25x telescope. Using a single simple lens for each, the image will be highly color fringed and distorted, less so at the center and getting rapidly worse toward the edge of the field of view. It is fun to play with a set of simple lenses to get the principal down but don't expect such a telescope to actually be of much use. They are a teaching tool only.

Now if you use a 25 cm objective with that 2 cm eye lens then the power is 12.5. Put it in my 356 cm focal length telescope and you'd have a 178 power scope.

So for telescope objectives and eye lenses (eyepieces) we speak in terms focal lengths not powers.

As to lens and mirror; a mirror reflects light while a lens passes light through it. Either can focus light if curved properly. Neither focus light if flat. Each has advantages and disadvantages depending on the purpose. I guess I don't understand your question.

Rick