The big lens lights up

[Ultrapurple]

I had some spare time and the weather was fair so I thought I’d give my big lens a trip into the garden to see if I could finally get some images out of it. If you’ve been following  the saga you’ll know I acquired this lens last year but haven’t managed to decent pictures from it yet; a forum member said he was pretty sure a diverging ‘interface’ (my word) lens was required to make it work, and that lens was built into the body of the camera this behemoth was intended for.

The few tries I gave it definitely showed some promise, but no clear and well-focused images resulted. Until now.

I tried all the various lenses I had to hand, to no avail, until I hit upon the combination of my unknown-source 40 or 50mm lens, reversed, placed in front of the Therm-App 13mm f/1 lens. Success! Bad vignette, to be sure, but the images I saw today definitely indicate that, assuming I can get the interfacing right, this is going to be one heck of a lens.

These images variously show the big lens front and back; the thermal view using the regular 13mm germanium lens and a view through the big lens showing the chimney stack from the centre of the previous image. Finally, a diagram of how things were set up. This was all hand-held, so highly unstable, but I did manage to capture an image to prove it works.

It’ll be a while until I can return to playing with this beast. Meanwhile, has anyone got any odd diverging lenses of 1.5” diameter or more that they’d consider lending (or selling) so I can try that approach?

Edit 2 Apr: I've just checked on Google Earth and the chimney is 60m (190ft) from the lens.

[Vipitis]

Knowing how you managed to craft your telescopes before - I am sure you will be able to do this.

So right now you use the telescope, a corrector lens and the camera lens?

My initial idea is to reduce the amount of elements. Either take the middle lens adapter and reduce it to one element - or use the thermapp without a lens.

[Ultrapurple]

I have tried using the Therm-App without a lens in this and similar applications. Unfortunately the long lens barrel that protrudes in front of the sensor means you can't get close enough to the sensor to couple light into them with any hope of having it in focus.

One thing I have not yet tried is removing the Therm-App case, which would mean I could get as close as I like. But I think I will have to contrive some kind of optical adjustment stage for precision control  - I do NOT want to bang the front of the sensor on things...

[Fraser]

You need a concave lens to bring the lens output into a parallel beam , as in the camera the lens was intended for, namely the AGEMA Thermovision 880 LWB and 870 LWB. The cameras are scanning types that have a concave lens built into their lens mount.

Fraser

[Bill W]

The back to back lens pair (in this case 13mm / 40mm) are working as a simple x3 adaptor / relay with the 'big lens' focal plane just outside of the rear.

There might be a need for a negative lens for aberration correction on a larger area sensor, or something specific to the Agema camera design as Fraser notes.

The big lens looks like a straight Cat (-adioptric) to me.

Would not another ThermApp 13mm lens be the solution ?
That would 'sample' a ThermApp sensor sized portion of the big lens' image plane.

Bill

[Ultrapurple]

Thanks Bill

I figured that there was something relay-like going on; I have done other experiments with back-to-back lenses at thermal and visible wavelengths and something felt familiar, though I would have been hard-pushed to name it.

The 13mm lens vignettes on the Therm-App Pro - its image circle isn't quite big enough. The 19mm works fine. I have two 19mm but only one 13mm so perhaps I can try that.

I also have an x3 Keplarian telescope and an x3 Inframetrics (I think) lens. The latter appears to produce a real image at its exit point, so it may have some scope, but the (rectangular) exit aperture is small, about the size of a micro SD card.

I shall continue playing. I presume there are no components of an Argus 1 lens that would warrant experimentation? I have a couple of these cameras, of which only one works, and plan to scrap the other in due course.

[Bill W]

The Argus 1 lens is an 18mm focal length, f/0.7 with a 20mm image circle.  It is though all-spherical so might lose resolution on a more modern small pixel sensor.  However with the iris you may be able to deal with the worst of that and not lose too much signal to noise.

A bit big but the lenses will fit front to front nicely.

[Ultrapurple]

Hmmm - 18mm f/0.7 might brighten up a Therm-App somewhat! The stock 19mm lens is f/1.1, so the Argus lens would be considerably more sensitive, albeit at the cost of depth of focus.

I suspect the Therm-App sensor (17µm 384x288) will use only the centre of the image circle (which was presumably matched to the 18mm dia target on the Pevicon). That should give about a 20° wide FoV, roughly equivalent to 100mm on a 35mm SLR. What it would do to the thermal sensitivity is anyone's guess.

[Ultrapurple]

The story continues here.