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Photography of Small Animals using a Crossed-Beam Sensor

 

 

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A few nights ago, I heard my wife say, "Eek! There's a mouse!" (Ok, I just made up the eek part, but it sounded appropriate.) Well, we put out a live trap, caught our mouse overnight, and released it outdoors by the woodpile in the morning. That night, I again heard, "Eek! There's a mouse!" We weren't sure if the mouse had made its way back into the house or if there had been more than one mouse. In any case, I had the idea to try photographing the mouse using the Crossed-Beam Sensor (CBS). While the CBS was designed for photographing birds in flight, I realized it could also be used as a photo trap for small animals on the ground.

I built a ramp and platform to raise the ground level to the beam level of the CBS photoframe. My set up is shown in photos to the left and right. The level of the platform was about an inch below the horizontal beam of the photogate frame. I drilled a hole in the platform for the vertical beam of the frame to pass. I positioned the Nikon D700 camera with 105mm micro lens and two wireless flash units on a short tripod at platform level. I set up everything on the floor of the laundry room. I used a trail of sunflower seeds and bits of cheese to lure the mouse up the ramp and onto the platform.

I set the camera in manual mode and focused on the center of the photogate frame where the mouse would trip the trigger circuit. I used a small aperture, f/22 for good depth-of-field, shutter speed of 60, flash units set on 1/4th power, and ISO of 800. I adjusted the CBS control box to a mid-range sensitivity which would be appropriate for operation in dark conditions. I also connected the box to AC power with a 9V AC/DC adapter since the unit would be on all night. I hoped that the battery in the camera and the batteries in the flash units would hold out. (They did.) I turned out the room lights and went to bed.
From an examination of the photo metadata the next morning, I saw that the mouse had shown up within a few hours of when I had turned the lights out. In a period of 10 minutes, the mouse had eaten all the food and taken 11 selfies. Two of them are shown to left and right. In the left photo, the mouse is at the intersection of the two photogate beams. In the right photo, the mouse is several inches to the right of center. If the mouse had entered the horizontal beam at this point, that wouldn't have tripped the trigger circuit. What apparently happened is that the mouse tripped the sensor at the beam intersection and jumped to the right as the shutter of the camera was opening. I think that the lag time of the camera could account for the position of the mouse.
The next night, I set up the live trap to capture the mouse again. I set the trap on the platform so that I could get photos of the mouse entering. The photo to the left shows the mouse investigating, and the one to the right shows the mouse entering the trap. As it turns out, the 29 photos that were taken show the mouse entering and leaving the trap repeatedly. The mouse was smart enough to avoid tripping the trap door. He did eventually trip the door, however. The next morning, I released the mouse in a field far from the house, expecting that it wouldn't find its way back. We did, however, have a mouse in the house the next night, so I assume there were several mice in the house to start with. More drastic measures will be needed to get rid of them. Field mice have been know to transmit hantavirus in their urine and droppings. We've been careful in handling the trap and cleaning the area with a bleach solution, but continuing to have mice in the house isn't a good idea.
That didn't end my experimentation. I moved the equipment outdoors to capture photos of chipmunks. My outdoor set up is shown to left and right. I used the same ramp and platform as for the mouse. I had to adjust the sensitivity of the CBS control box for operation in ambient light. I continued to used flash to highlight the subject. I also scattered dirt and pine needles on the platform for a more natural look. Like with the mouse, it didn't take long for the chipmunk to find the trail of seeds that I left. One photo is shown below.
   

 

 


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