D i v e r g e n c e

I've been furiously looking around for my cheap telescope I had when I was a kid. When I do I'm going to try project some images up onto the clouds

If you can't find it, here are some possible telescope candidates for conversion into a beam expander using the method I used, if using a cheap green laser pointer that the focus/collimation lens can be removed from so the expander lens next to the crystal can be used instead of an eye piece, mine is an older edu science model:

Edu Science Star Tracker Telescope | eBay

50x Telescope with Tripod | eBay

Star Tracker Telescope Zoom 35x 50x50mm New in Box | eBay

Emerson Refractor Telescope 50x 100mm Tripod Astronomy Interchangable Lenses | eBay

Vivitar TEL50600 50x 100x Telescope Refractor with Tripod Black | eBay

Emerson Refractor Telescope 50x 100mm Tripod Astronomy Interchangable Lenses | eBay

This might also work with any laser pointer (blue or red too) by leaving your pointers lens on, just depends on whether the eye piece is suitable to expand your lasers beam. It's much easier to use a cheap 532nm laser and removing its collimation lens so you can stick it into the tube where the eye piece would otherwise be so the telescopes tube can hold the laser in place. Just make sure your little greenie has an expander lens deep inside it and that you can put the collimation lens back on if it doesn't work out well. This model worked great for me as you can unscrew the lens assembly and put it back on later:



5MILES 532nm Green Laser Pointerstrong Pen High Power Powerful 8000M Pointer He | eBay

The telescope should have as long an adjustable focus as you can find and without a prism in it which would cause additional loss. From what I've seen being offered on ebay, the cheapie telescopes with a right angled eye piece assembly can be removed and used without it, that part is what has the prism in it, perhaps a mirror you don't need or want as an expander. I'm not much of a telescope buff, but sure had fun with this little cheap edu science telescope as a beam expander to reduce my lasers divergence for a far longer throw before the spot gets much bigger.

I think you can use monoculars too.

There are also beam expanders on eBay for decent prices such as this pair. You get 2x in that lot, and I've seen that same model go for $200 or so for one.

Those are very nice and cheap, I haven't seen any that nice for that price before.

Edit: I bought them

You want to use telescope to change divergence of the laser beam?

I was also wondering how much better can be using 808 or 940nm instead of visible laser. Minimum point is larger, but the difraction effects are less visible, and it should help in obtaining better divergence...

I know that it may be stupid idea, but how about to use the atmosphere as a very..very big lens and try to count where to place the pointers and on what angle, to produce one beam. It will be very bad at short distance, but after 1000k it can be better than just normal lens - hard to say, just idea

Cool idea, the atmosphere around the earth does have an amount of refraction compared to space and it is curved too. I wonder how to go about investigating this, can it really produce a focus point out beyond the earth in space? My guess is the beam hitting would need to be very wide to work as a lens, otherwise it might be too flat to do anything to the beam.

I have some high power 808nm laser diodes I'd like to build a beam expander for too, I've been collecting various lenses I find cheap on ebay for that, have some nice ones now but not enough time to tackle it yet.

I think it is possible to focus it in space, but problem is that when you want good optical system whole surface of the lens should be working. When you have so big lens (atmosphere) you need big source of light. I was wondering rather to use mirror systems, something like the steam generators, but not focus it on boiler but in the space.

Sun is giving lot of energy on Earth surface and Sun is so far that rays are almost paralell. This is why taking lot of mirrors will let you to use a big surface for atmosphere lens. Huh it sounds funny but I think it is possible, but for sure, one would need a lot of mirrors

Check in the google solar steam generators. Count the power which is on the engine, then compare to the 1kW laser. Sun wins ;D

I was thinking the same thing last night, that I'd need to have a beam expander which covered most of the sky for its output. Interesting that the suns rays are mostly parallel, I've never thought of that before.

Look the problem is different, she scale you are thinking of is totally different than laser pointer.

1. What a matter if you use 200W or 500W laser diode if Sun is giving 1300W/m2. Then use 2 - 5 meters, you have kW of light (lot of infrared too).

2. What a matter if you use the best possible lens and beam expander, if there is so many material piecies in air that light will be scattered after 10km anyway.

Of course there are some military projects which have 5cm beam spot at 10km range and around 5kW power. It is used to literally cut the missle in the air. It takes around 15 seconds and it uses really advanced vision and motion detecting system. I saw this kind of "device".. let's say big truck hehe

During the daytime, laser pointers aren't very impressive due to all of the competing energy from the sun, of course. Whether you use mirrors to try to use some of the energy from the sun to send it back out, it would obviously still pale in comparison, except perhaps for the observer flying high above who sees it coming from below.

What does it matter if I use the lowest divergence I can get if the beam is scattered by particles in the air after 10km?

From my perspective, that alone is good reason to reduce divergence, with all of that scattering and attenuation going on through particles in the air, the more intensity, the more which remains in the direction your are pointing. If there were 100 percent scattering or attenuation, then, of course no contest, but from what I've been able to gather, divergence causes a loss to beam visibility from the side faster than particle scattering or attenuation in normal atmoshere, true?

I bought a 6 inch (152mm) diameter lens for 10 dollars on ebay last week which has a 4000mm (13 feet) focal length, now to match it up with the right expander
Will need a tall tripod for it. I suppose I could get two 8 foot sections of some kind of tubing and make two sections out of it which are about 6.5 feet long each for portability. I don't think I will be using a tripod tall enough to allow me to point near strait up, at 45 degrees elevation max it won't need to be so tall.