# Optical Forums > Ophthalmic Optics >  home made telescope\binoculars

## ashish

Hi everybody,
Got to help my son in in this school project of making a telescope or a binocular. A very basic one is easy to make,but any sugestions on fixing up something fairly good at home with lenses at our disposable ?
Thanks

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## Darryl Meister

There are two basic types of "refractor" (or lens-based) telescopes: 1. The _Keplerian_ design, which uses a plus-powered objective (front) and a plus ocular lens and creates an inverted image, and 2. The _Galilean_ design, which uses a plus-powered objective and a minus-powered ocular lens and creates an erect image. This means that everything would appear upside down through a Keplerian (or "astronomical") telescope since the image is inverted, while everything would appear normal through a Galilean (or "terrestrial") telescope. It is also possible to use additional optics to re-invert the image through a Keplerian telescope.

The two principal concerns for constructing a basic telescope would be the tube length (or length of the telescope) and the magnification.

To determine your tube length, you must add the _focal lengths_ of the objective and ocular lenses. Focal length (f) is given by

f = 1000 / F

where f is the focal length in millimeters and F is the power of the lens. Now, a Keplerian telescope, which uses two plus lenses, will have two positive focal lengths. A Galilean telescope, which uses a plus lens and a minus lens, will have a positive focal length and a negative focal length. Consequently, for the Galilean, adding the two focal lengths will be the same as subtracting the size of the focal length of the minus lens from the focal length of the plus lens. This also means that Galilean telescopes are shorter than comparable Keplerian telescopes.

For instance, consider a Galilean telescope with a +2.00 objective and a -8.00 ocular. The focal length of the objective (f1) is

f1 = 1000 / 2.00 = 500 mm

While the focal length of the ocular (f2) is

f2 = 1000 / -8.00 = -125 mm

Which give a total tube length (l) of

l = f1 + f2 = 500 + (-125) = 375 mm

The magnification can be determined by simply dividing the power of the ocular by the power of the objective. For instance, given our previous objective (f1) of 2.00 and ocular of (f2) of 8.00, the magnification (m) would be

m = f2 / f1 = 8.00 / 2.00 = 4x

Or four times larger than with the unaided eye. A weaker objective (front) lens will allow for greater magnification for a given ocular lens, but will require a longer tube length. Conversely, a stronger objective will produce less magnification for a given ocular lens, but will require a shorter tube length.

You can build a basic telescope using a PVC pipe the tube, as well as some pipe caps to hold the lenses in place after some creative sizing and cutting. (You can find PVC pipes in the plumbing section of hardware and home improvement stores.)

Best regards,
Darryl

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## Jeff Trail

Be careful when you put it together though, tried the PVC type and used some heavy glue and it ended up messing up the lens, fumes made the surface of the lens all cluttered with dust that stuck :-)

  Not to change the subject but you do any tinkering with laser's darryl? Since Edmond Sci. now "owns" one of my credit cards.. I was doing some tinkering with blasting some lasers through ophthalmic lens, nice and bright all along the inclusions and stress fractures in the surface :-) ..also did some work for Radon (military stuff) and swapped out some math and prism mounting work on some laser sighting if they would "freeze" a bunch of blanks for me at different temps and types of materials so I could play with them and see if the matrix changed in the lens, what happened to the stress and what it did to the optics.. Didn't have anything to measure tensile strength but any idea's there? (with in a budget please)..
   I think the couple of OD's that came around and tinkered in my garage with me were more interested in my keg and shooting pool though and using this as an excuse, but it was pretty interesting .. Can't wait till my daughter wants to do some thing for the science fair! :-) the only first grader with a working laser and splitters ..BTW we lit up the path smoking cigars since we didn't have the correct light conditions.."ole blow the smoke" and see the path trick..another lame excuse for everyone to fire up a good cigar :-) 
    Pretty neat stuff.. kind of expensive hobby to explain to the wife, but interesting..

    I have a great book on optical experiments and how to do them at the house, I'll see if I can find it and post the name of it.. tons of things to tinker with explains the whole thing from formula to construction ...

Jeff "no Mr. Wizard but still likes to tinker" Trail

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## chip anderson

The Boy Scouts of America used to have a kit for hand grinding and making your own scratch Telescopes,  you even got a merit badge if you completed it and it worked.  You truly "hand ground and polished" the lenses with no automated equipment.

Chip

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## Darryl Meister

> *Jeff Trail said:* 
> Not to change the subject but you do any tinkering with laser's darryl? Since Edmond Sci. now "owns" one of my credit cards... ... so I could play with them and see if the matrix changed in the lens, what happened to the stress and what it did to the optics.. Didn't have anything to measure tensile strength but any idea's there? (with in a budget please)..


I haven't played much with lasers personally, but we did a bit with them in my physics class at school. (Shining them through diffraction gratings and that sort of thing.) Our engineers at SOLA work with lasers quite a bit for a variety of applications, but I haven't really messed with them, myself. I'd probably just shoot my eye out! ;)

I definitely agree that Edmund Scientific is a great resource for this kind of stuff. I bought an inexpensive optical experiment kit from them a few years ago that included an optical bench and a light source along with several lenses, mirrors, and filters. I definitely got a lot for my money.

A basic polariscope will help identify the regions of the lens under stress, as well as indicate the relative levels of stress. (Lenses under stress exhibit birefringence caused by "optical activity.") Some companies make instruments specifically for measuring stress in parts, which often use crossed polaroid filters (like a polariscope does) and even have spiffy scales and such for measuring the birefringence.

Best regards,
Darryl

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## Optical Plumber

> *ashish said:* 
> ... any sugestions on fixing up something fairly good at home with lenses at our disposable ?
> Thanks


Here are a few more things you may want to consider:

1. Use two tubes, one for the object lens and one for the eyepiece that (friction) fits just inside the main tube. This is very important to allow adjustment of the focus.

2. Paint the inside of the tubes with a flat black paint (preferably spray paint). This helps ro reduce unwanted reflections off the inside of the tube.

3. Consider using a compound object lens consisting of a convex lens of crown glass (1.53 index) and a concave lens of higher index glass (such as flint glass with a 1.6 index). This will virtually eliminate the chromatic abberation that you will experience otherwise.


Terry
:)

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## Bob Stern

It sounds like the suggestion of two lenses of different indices joined together for the objective lens would create an achromatic doublet. You might want to think about grinding the facing surfaces of this doublet in equal but opposite signs and then mount them together. I would like to know how it turns out.

Good Luck.
Bob

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## Optical Plumber

I believe you are correct, that is an achromatic doublet, and after thinking about this it seems a bit more involved than the original project called for. 

Another way to lessen (but not eliminate) the chromatic abberation is to increase the focal length of the object lens. A one diopter object lens will require a 1 meter main tube and will give reasonable optical performance with a relatively inexpensive off the shelf achromatic eyepiece. 

If this is to be used as an astronomical starter scope, then go for the biggest object lens you can get, preferably in crown glass, and the lowest power (longest focal length) eyepiece. This will give you a nice wide field of view and lots of objects that you can't see with the naked eye, such as moon craters, individual stars in the milky way, etc.

Above all have fun!

Terry
:)

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