Author Topic: Cheap telephoto lens?  (Read 5878 times)

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Offline frenkyTopic starter

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Cheap telephoto lens?
« on: November 02, 2016, 07:42:48 am »
I'm looking for alternative to this:  :P
http://therm-app.com/product/35mm-lens-for-therm-app/

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:

https://www.aliexpress.com/store/product/High-Quality-GaAs-Focus-Lens-CO2-Laser-Engraving-Cutter-DIa-20mm-FL-101-6mm-4-Free/1513187_32267113994.html

or this:


https://www.aliexpress.com/store/product/High-Quality-ZnSe-Focus-Lens-CO2-Laser-Engraving-Cutter-DIa-12mm-Focal-50-8mm-2-Free/1513187_32546386499.html

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

Offline Fraser

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Re: Cheap telephoto lens?
« Reply #1 on: November 02, 2016, 09:21:12 pm »
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.

Fraser
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Offline frenkyTopic starter

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Re: Cheap telephoto lens?
« Reply #2 on: November 02, 2016, 10:08:23 pm »
Tnx. I have found it:
https://www.eevblog.com/forum/thermal-imaging/yet-another-cheap-thermal-imager-incoming/msg721312/#msg721312
https://www.eevblog.com/forum/thermal-imaging/yet-another-cheap-thermal-imager-incoming/msg721327/#msg721327

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.
 

Offline Fraser

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Re: Cheap telephoto lens?
« Reply #3 on: November 02, 2016, 10:30:47 pm »
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.

https://en.wikipedia.org/wiki/Refracting_telescope

https://depts.washington.edu/hssexec/committee/hss_galileo.html

http://astro.uchicago.edu/yerkes/docs/AstronomerTalks/AlHarper-ElementaryOptics-v5.pdf

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

http://fisherka.csolutionshosting.net/astronote/astromath/htmatkit/Handout%20Keplerian%20telescope%20how%20to%20make.doc

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.

Fraser
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Offline frenkyTopic starter

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Re: Cheap telephoto lens?
« Reply #4 on: November 02, 2016, 10:50:45 pm »
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:
 

Offline Fraser

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Re: Cheap telephoto lens?
« Reply #5 on: November 02, 2016, 10:54:47 pm »
Just checked, the distance between the lenses is the sum of the FL's so in your case 100mm + 50mm = 150mm. Distance from rear lens to camera objective needs to be found by experimentation. Start at 50mm and move closer.

Magnification is larger FL / smaller FL of rear lens. In your case 100mm / 50mm = X2

Do bear in mind that the front lens is normally larger diameter than the rear lens and both are matched in diameter so as to correctly illuminate the eye or camera objective in your case. You will need to experiment and I will be interested to hear how you get on. I own several Germanium and ZnSe lens elements but have not had time to experiment with them yet. One of the Germanium lenses is 12mm diameter with a nice short 15mm FL  :)

Fraser
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Offline Fraser

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Re: Cheap telephoto lens?
« Reply #6 on: November 02, 2016, 10:58:15 pm »
Frenky,

I did not realise you were thinking of removing the camera odjective. Yes a single lens will produce an image in the same way as a pin hole camera works. I use auxiliary lenses that fit in front of the original objective.

Nice picture  :)

Fraser
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Offline Bill W

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Re: Cheap telephoto lens?
« Reply #7 on: November 02, 2016, 11:55:45 pm »
Fit a single lens in place of the original is the simplest option.  However beware of the loss of 'brightness', the lens quoted is 100mm f/l and 20mm diameter.  That is f/5 so things will be 10% of the normal brightness, or 10x the noise.

An alternative is to add another identical lens to the original which creates a second image plane.  This is either a thermal microscope as the image plane is the same as the sensor.  To that you can then add any other lens focussing to that new plane.

technically none of these are 'telephoto' as that implies shorter than its' focal length

Any single lens will not be that good off axis, although less of a problem with longer focal lengths.  I have had OK results with a 25mm CaF single element onto a 35um pitch 320x240 sensor but nothing like a 3 element Ge lens would be.  Now where is my 150mm f/0.7 hiding.... it should not be able to hide  :-DD


Bill

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Offline frenkyTopic starter

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Re: Cheap telephoto lens?
« Reply #8 on: November 03, 2016, 11:36:15 am »
Thank you guys for sharing your knowledge.  :)

I'll try a few different setups and also watch ebay for cheap big diameter thermal lens.
 

Offline Fraser

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Re: Cheap telephoto lens?
« Reply #9 on: November 03, 2016, 12:41:11 pm »
I bought a complete lens from a chap in the USA so I can state that he is a great supplier. He is currently selling a decent Germanium lens element that is cheap for what it is, but maybe not so cheap in hobbyist terms

http://www.ebay.com/itm/Thermal-imaging-germanium-meniscus-positive-lens-diameter-43mm-F-aprox-50mm-/282230605709?hash=item41b641478d:g:PjUAAOSw8gVX50Db

Fraser
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Offline frenkyTopic starter

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Re: Cheap telephoto lens?
« Reply #10 on: November 03, 2016, 08:08:44 pm »
That is really a good price for the 44mm dia Ge lens. :-+
But to get it to EU it would cost me nearly twice of the asking price and currently I can't justify that expense just to see if I could use it as telephoto lens.
 

Offline Fraser

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Re: Cheap telephoto lens?
« Reply #11 on: November 03, 2016, 08:36:29 pm »
Frenky,

I bought a much larger complete lens from him and it was $16 postage to the UK. The seller is a member of this forum since I told him about it. Drop him a message on ebay and tell him Fraser sent you from EEVBlog. He might be able to do you a good deal on shipping ?

I attach a picture of the lens I bought from him. 15 Degree FOV and its a real beauty from Raytheon. It appears new and unused.

http://www.ebay.co.uk/itm/282193094536?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

Fraser
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Offline frenkyTopic starter

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Re: Cheap telephoto lens?
« Reply #12 on: November 04, 2016, 07:39:24 am »
I know it's really a good deal and lens do look perfect, but it's still (100$ + shipping)*1.22 VAT + 10€ postal handling fee = approx 150$ for a single lens element. Way to much.
I will rather make a simple telephoto lens from a 20mm dia (FL 50mm) lens that I already have.

And then put some €€€ and effort into making good calibration box which I really need.
« Last Edit: November 04, 2016, 09:40:04 pm by frenky »
 

Offline Uho

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Re: Cheap telephoto lens?
« Reply #13 on: November 04, 2016, 09:16:41 pm »
When the production  the lens is necessary to consider  of the physical size of the matrix. For example the thermal lens with an angle of 50 degrees working for Lepton has an angle of 5 degrees.
« Last Edit: November 04, 2016, 09:18:35 pm by Uho »
 


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