Homemade Camera Zoom Lens and Schlieren Experiment

I think I need a bigger playhouse workspace. The mirror that I use for my Schlieren photography requires that the camera be 120 or so inches from the face of the mirror. This is of course because of the focal length of the mirror and a longer one is better for Schlieren photography (so I'm not complaining) but it takes up a lot of room. Another thing that having a long focal  length does is it requires me to have to zoom in a lot to see what I am photographing.

  

The camera that I am using is a Fujifilm FinePix J38 that has a optical zoom and a digital zoom built in but they don't get close enough. Actually the digital zoom does get really close but the image gets so grainy that I can't use it. To zoom in as far as I need to I built a zoom lens for my digital camera from an old pair of binoculars. At first this seemed like a really easy way to go but I quickly realized how sensitive the alignment of the binocular is to the camera lens.

 

Homemade Camera Zoom Lens for Schlieren Photography

Above is a picture of the zoom lens setup that I built. The binocular is now a monocular because I cut one side of it off but you get the idea. I mounted the monocular on a couple of linear stages that are position adjustable with Vernier adjustments. These Vernier knobs allow me to position the monocular right in front of the camera lens. The camera and the adjustable monocular are both mounted on a linear slide that allows them both to be moved closer together and farther apart but keeps them exactly aligned while moving them.

Also in the picture is the 'pin point light source' on a magnetic base dial indicator stand, the camera and a razor blade being used as a "Knife Edge Schlieren Stop". Click HERE for a diagram of more or less how this  type of Schlieren setup works and HERE for a picture of the parabolic mirror that I am using.

One thing that I wanted to try was to move the razor in and out of the picture while I was filming and see what effect it has on the image dynamically. The best way to do that would have been to have the razor on another Vernier stage and move it in really slow but I don't have another stage. The next best thing was to 'go ghetto' on it and tape the razor to a piece of metal and just move it in and out by hand. While I was making this first video I was again using the high intensity blue LED.
 

 
The last time I wrote about Schlieren photography I was using an homemade anamorphic lens. I mentioned that I would detail that in another post but this isn't the post that I am going to do that. For now I have set that project (and the lens) aside because I have to think a bit more about what I want to do with it (Read: it doesn't work yet). That's OK because I have a lot of other things to try.

In the above video you can see that the image gets darker in the areas where there is a lot of refraction (different temperature). Interesting but color looks a lot better so...

One of the things that I have been thinking about is using a colored filter with a hole in it. This is in line with the razor experiment that I tried above. I want to see exactly what the "Schlieren Stop" - razor, colored filter or whatever - does. I went to the local plastics store and bought some colored acetate film and cut a small piece. I poked a really small hole in it as neat and clean as I could with a razor blade (happen to have one!). I then put the filter in place of the razor in the above picture and aligned the hole in the filter so it was exactly in the image. So what I ended up with was a clear image (the light is going through the hole) and no light going through the green acetate. Below is a picture of what I did.
 

Schlieren Setup With Colored Filter and Hole

You should be able to click on the image for a bigger view. In the bottom of the picture I drew a close up of what I think is happening. The 'correct image' is focused right down to a pinpoint and passes through the hole in the colored filter unaltered. Any refracted light that is 'moved' from where it should be is focused at a different point and passes through the filter getting colored green. HERE is a post about the mirror that I am using in case you are interested.

So the setup is focused with ambient temperature air so that the image is going right through the pinhole. If there are any density changes in the air that density change causes the focal point of the light to move and it's colored green by the filter. All the hot areas look green. In the case of the razor blade the refracted light is just blocked and the image in that hot area area is dark like in the video above.

It took some time to get the hole in the colored filter in exactly the right place but once I had it there I plugged in a soldering iron to make some heat and made a video!!
 

In the above video you can see that the density changes are creating a pleasing green color. Also some of the area that I expected to be green are just darker. This could be because the hole that I made in the acetate has rough edges despite my best cutting efforts and are blocking the light. It could also be because I don't know what I am doing and why I think this works is wrong... either is equally probable... If anyone has any ideas about how to do this please let me know by emailing me at ottobelden@yahoo.com or leave a comment.  If you have any questions about this do the same!
  
I'd like to hear from anyone who knows more about this than I do or anyone who has dome this sort of thing themselves. Messing around with this stuff is a lot of fun, I wish I had more time to do it.