how to make a telescope?

The fact you have 3 lenses means it was probably a pretty good projector with red, green and blue CRTs. Projectors and telescopes have very different requirements when it comes to lenses, and about the most effective way you could turn those lenses into a useful telescope is to sell them and buy something with the proceeds.

Taking individual elements from each of the lenses to make one lens would be a remarkably bad idea. Each of the elements in one barrel is needed to form a complete lens.

If you put an eyepiece at about the same distance in back of the lens as the CRT was on the projector, you'd have a refracting telescope.

But the most impressive thing about it would be that it worked at all.

It'll be low power, and you'll find a purple halo around just about any bright object. A shorter focal length eyepiece would allow you to get somewhat higher power, but with a projector lens, I'd expect apparent sharpness to go down quickly as power goes up.

Your lenses didn't need to be color corrected because each one was only transmitting one primary color of light. With a telescope, they'll be dealing with the entire color spectrum, and different colors will be in focus when the eyepiece is at slightly different distances from the objective. Normally what happens is it'll look sharpest when the distance between the objective and eyepiece has you focused on green light, and the red and blue light will be a bit out of focus and combine to form the purple halo.

If you're into experimenting and learning and enjoy frustration, you could replace the eyepiece with a monochrome CCD camera and red, green and blue filters. Shoot one exposure of the same object through each filter, and combine them for a full color image. But astronomical imaging is the most challenging aspect of the hobby. A mount with tracking accurate enough for that sort of imaging is likely to cost at least $1000, and the learning curve is very steep.

So my advice is play with the lenses for a little while, but keep them in good condition so you can sell them and buy a telescope.

First of all, the main lens is probably not suitable for use as an astronomical objective. These lenses have very short focal ratios, as shown by the thickness of the lens. This lens will have very severe aberrations (optical faults) leading to very poor images. The basic formula for the length of the tube is very simple: it is the total of the focal length of the main lens (objective) divided by the "viewing lens" (eyepiece). You can find out the focal length of the lenses by using them to focus the Sun on a piece of paper.

I'm a little worried that you have three lenses, since that may mean you've disassembled the original lens. It would have been much better to have left it assembled, as most compound lenses are designed to function as a complete group. I'd strongly recommend that you reassemble the lens if at all possible. I hope you kept track of which lens goes where and which way they were facing. Otherwise you will have a hopeless mess on your hands.

I've never heard of anyone building a telescope from a TV projection lens, though it's quite common to use the lens from a photocopier. Good luck on your project, but if you've already disassembled the lens, you may not be able to proceed. If you still have the assembled lens, you will need to purchase an eyepiece from a telescope company. Orion sells a series of inexpensive eyepieces which might be suitable: I'd use a 20mm:

http://www.telescope.com/Accessories/Telescope-Eye...

seem, human beings assist you to comprehend that that is a new child's toy, and in evaluation to telescopes some human beings have used or equipped, it is. in spite of the undeniable fact that that is a lot extra suited than the bare eye. i could say while you're thinking some telescope that length, you may desire to evaluate a pair of binoculars. i don't comprehend what budget you're speaking approximately, yet you need to have the means to discover some good 2d-hand ones with a minimum of 50 mm lenses for the comparable value. examine it out on e-bay it sluggish. I even have considered a large form of them there.

Rear Projection Tv Lens

lens have a focal point , you need to know that, for each lens,

then space them to that length