New Life for Old Mirrors

[NigeB]

Hello All,

I wasn't sure whether this should go in Science or Photography, but it's a demonstration of a phenomenon related to a class of techniques in wavefront sensing (which lies at the heart of adaptive optics systems), so I thought it was reasonable to put in Science.

Here's a physics experiment you can try with an old telescope mirror (or perhaps your prize Newtonian, with the spider removed)   You can do it with lenses as well, but mirrors make for easier and more compact optical arrangements.

This is a short sequence of "Schlieren imaging" - some early tests I did while preparing a demonstration for some optics classes. The mirror is an old 6" reflector mirror, something around f/10, mounted on a tripod. The mirror is a parabola, but spherical figures work better. A video camera is set up on-axis, at a distance equal to the mirror's radius of curvature, and pointing at the mirror. Immediately underneath the camera lens is an LED mounted behind a pinhole. A sharp knife edge is placed on the front of the camera, exactly bisecting the aperture.

As the diagram indicates, the image of the pinhole is brought to a focus on the knife edge. You need to co-mount the camera and light source, so they move as one, on a tripod that gives you very precise vertical motion. The key is to adjust the height so that almost all of the (tiny) pinhole image is on the knife edge, not hitting the lens. (The illuminated spot on the knife edge should be as close as possible to the middle of the camera lens, i.e. so that if you were to remove the knife edge, the spot would fall exactly on the centre of the camera sensor).

If you set this up just so, the system becomes extremely sensitive to changes in refractive index near the mirror (think of Snell's law acting as a lever arm, moving the position of rays arriving at the camera). For example, if you warm the air near the mirror, you change its refractive index, and "spoil" that knife edge blocking arrangement so that the rays which pass through the warm air are refracted by a different amount to the others, miss the knife edge and are recorded at the focal plane. Candles (or soldering irons) make this easy to demonstrate, but if you get the alignment just right, you can see the refractive index changes caused by heat rising from your hands, or in your breath. Cold objects also work - one of the videos below shows a cup of CO2 ice. 

This technique is used in all sorts of fields - aerospace engineering for example, where you can visualise shock waves around aircraft in supersonic wind tunnels, or the sound coming from a trumpet. Or the rapid expansion of gas from a fired gun, accompanied by the spherical sound wave carrying the BANG, etc. Our setup has improved since these early trials, and for my demos now I make a G&T - the cold ice, CO2 vapour from the tonic bottle, the alcohol vapour, and the heat from the candles to set the mood, work rather well.  

Don't blame me for broken telescopes though...

Nigel

[skybadger]

Sounds suspiciously like the foucault test to me. Use a co-moving slit by using a single knife-edge across the led and lens. Don't really need the pinhole in this case. you will see exactly this phenomenon in a garage where the air hasn't settled.

The distance between the source and detector is normally minimised though to prevent astigmatism (check!)  in the readings.

Mike

[NigeB]

Sounds suspiciously like the foucault test to me. Use a co-moving slit by using a single knife-edge across the led and lens. Don't really need the pinhole in this case. you will see exactly this phenomenon in a garage where the air hasn't settled.

The distance between the source and detector is normally minimised though to prevent astigmatism (check!)  in the readings.

Mike

"Suspiciously like" might be taken to imply "the same as", and that's not the case,  but certainly, they are very closely related and both stem from a method demonstrated by Huygens. Toepler refined the foucault geometry to provide significantly increased contrast and improved sensitivity to disturbances in front of the optic - hence the constraints on the size of the light source and the advantage of the longer focal length, which makes the arrangement more sensitive to those disturbances. But your point is taken - mirror makers and people who are familiar with the Foucault test, will quickly recognise what's going on here.

[barkis]

    You see the same effect under the Foucault Knife edge as a mirror cools after a polishing or figuring session.

Air currents passing through the reflected beam are also revealed.

    A thumb pressed against a mirror on test for a few seconds, will raise the glass sufficient to see it under the knife  as a small hill,

which disappears gradually as equilibrium is attained.

    Very interesting to observe these phenomena, and a good example of why a Telescope should be permitted to reach ambient

temperature, prior to observing, or Imaging sessions.

Ron.