Saturday, May 12, 2007
The submariner's solution
Ever tried to aim a camera trap on the end of a pole at a hole in a tree 12 feet above your head?
What is that nut talking about, you ask?
I'm talking about the trials and tribulations of photographing cavity-nesting birds. You are looking up into the foliage, the sun usually in your eyes, and from one angle the camera looks dead on target. Then you move for a different take, and it looks off target. Photographing the comings and goings of a small bird up close with a camera trap takes the patience of Job. It's a bear adjusting the camera so that the passive infra-red sensor (or PIR) detects the critical area of activity.
If you're not dozing yet, you may be wondering "what's the problem?"
Well, the fresnel lens helps the PIR detect moving heat in a cone-shaped volume that expands with distance from the sensor. At short distances the sensor's area of reception is quite small. I measured it in one of my cameras and found that when my hand was moving only 3 feet away the sensor detected it only within a 5 inch circle.
This means that it takes a lot of trial and error eye-balling to get the PIR 'spot-on' the nest hole.
The other day I bought a cheap laser pointer with the idea of solving the problem. I was going to make a mount for the laser on top of the camera case, so I could aim the PIR quickly and with more certainty.
Then I thought I'd better run the idea by my neighbor Richard.
Richard is a submarine veteran (remember the mechanic Johann in "Das Boot?"), and he's a wizard at all things mechanical.
Well, Richard suggested going with a periscope. At first I was skeptical, but he rummaged around his shop, and in no time assembled a miniature periscope from a piece of square metal tubing, a piece of aluminum bent at 45 degrees, and a small mirror. We velcroed the device to the back of the camera, and aimed the laser up the tube. It worked like a charm.
Next I had to measure the distance between the laser spot and the PIR sensor. To correct for parallax the reflected laser spot should be several inches above the target (depending on the distance between the reflected beam and the center of the PIR). Then you adjust the camera's position on the pole till you get it right. This is pretty easy when using a telescoping fiberglass pole. It's a more tedious process when using square tubing with connecting sleeves.
The next day I hauled the gear down the hill to a cavity located in the end of a broken limb. The periscope indicated the camera was too low. I raised the camera on the pole, but I couldn't tell if I had it right. You see, the trunk wasn't vertical, and after I raised the camera I couldn't see the laser beam from the ground.
The joke was on me. I could only conclude that while technology can solve a lot of problems, nature doesn't always cooperate.