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Cheap telephoto lens?

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I'm looking for alternative to this:  :P

I was thinking... Cheap ZnSe work great for closeup images if you add them in front of existing lens.

What If I would buy a lens that are better than ZnSe but still cheap. Could I use it as single lens element for diy Telephoto lens?

Something like this:

or this:

What do you think guys, would this work?  ;)

An auxiliary telescope lens requires at least two lens elements and will be inverting. To obtain a lens that is non inverting you need a third erecting lens. All lenses need to be matched which is the challenge as such are normally expensive no matter what thermal transmisive material is used. ZnSe is actually an excellent material to use and more than adequate for the task.

A chap has built auxiliary telescope lenses for the SEEK camera so have a look at that thread for more info. His lens was inverting.


Tnx. I have found it:

I would like to try it with 100mm and 50mm ZnSe lens that I have at hand.

Should I position it like this?
100mm lens | 100mm gap | 50mm lens | 50mm gap | sensor

I don't mind if I get mirrored image, because I can easily fix that in software.

Hi Frenky,

I regret I am not very knowledgeable on optical assembly design. Someone else may be able to provide better advice. I use Google and lens simulation programs when investigating the elements used in a complete lens assembly. The simple two lens element telescope is called the Keplerian Refracting Telescope. There is a lot about it on the internet as it is what children build as their first telescope at school.

Take a look here for how to build Keplerian telescopes. There are many such guides on the NET.

Note that the Galileo (Galilean) Telescope also exists and just needs a different rear lens for a non inverting type. Sadly it has some drawbacks though and you can read about those on the NET.

I attach a document that discusses the Galilean telescope Vs the Keplerian.


Thank you. I'll study the links. :-+

Meanwhile I've made a single element telephoto lens.  ;D

It's really hard to find focus but it works sort off.
I just unscrewed the original lens from ThermalExpert and held ZnSe lens by hand in front of it.

Subject is LED light bulb on the ceiling (housing gets pretty hot.)

With original 13mm lens:

With 50mm ZnSe handheld 50mm away from the sensor:

with 100mm ZnSe handheld 100mm away from the sensor:


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