Mechanics of KAP
Posted by Tom Benedict on 28/05/2009
On the off-chance that my previous post sparked some interest in doing kite aerial photography, I’ll go into the mechanics of actually hanging a camera from a kite line, and convincing it to do what the photographer wants. The approach used can be anything from quite simple to otherworldly complex. A lot depends on what the photographer actually needs, but even more depends on what the photographer is convinced they want. It’s important to keep these two ideas separate when you look at hanging a camera from a kite line.
By far the simplest KAP rig I use is one I designed spedifically for doing mapping. It was built for one specific trip, but it’s such a versatile rig I’ve used it numerous times since. The requierments of the trip drove the design: For starters, there is very little wind at the site, so lifting a heavy KAP rig was out of the question. The bulk of the weight in this rig is the camera itself. The rig’s weight is almost negligible. For the purpose of mapping, all the pictures needed to be as orthogonal as possible, so there is no provision to tilt or pan the camera. And we would need to take pictures over the course of several hours with no opportunities to change batteries during the flight, so battery life was of paramount importance.
There is no provision for the photographer to tell the camera when to take a picture. Instead the camera is running CHDK, a toolkit that runs on top of the camera’s native firmware. In this case CHDK was running an intervalometer script that let the camera take pictures every five seconds without any user intervention. No mechanical linkages, no servos, no fancy electronics, just the camera itself running a pre-canned program.
In the end the rig performed marvellously. Complete with camera and the four AA batteries the camera requires, the rig came in just over 450g. It flew on one set of batteries for two straight days, and took more than 2,000 photographs. The photos from that trip are being used to map an archaeological site, and will be used in papers that should be published in the next year or so.
At the other extreme is my radio controlled KAP rig. It’s built almost entirely out of off-the-shelf parts from Brooxes, the main supplier of commercial KAP equipment. The rig allows the photographer to pan and tilt the camera, and to operate the shutter. Brooxes sells other components that would allow the camera to rotate from horizontal to vertical, and other accessories exist that allow the photographer additional control over the camera itself.
The rig started off as a Brooxes BBKK, but over the course of a few years I added a carbon fiber leg kit, gear reduction for the pan axis, a set of PeKaBe blocks for the suspension, and a number of other improvements. Two changes have been made since this photograph was taken: The first was to replace the aging 72MHz AM radio with a 2.4GHz Turborix. The second was to remove the shutter servo and replace it with a GentLED-CHDK. Unfortunately I haven’t photographed this rig since the changes were made.
The GentLED-CHDK is one of the cooler pieces of hardware I’ve bought for my rig. It is a smart cable that plugs into the RC receiver on one end, and the camera’s USB port on the other. When the photographer flips the shutter control on the RC transmitter (in my case I set it up as a switch rather than a joystick), the GentLED-CHDK sends +5V down the USB cable to the camera. One of the features of CHDK is that you can monitor the +5V line on the USB port and take action when the camera sees it. In my case I run a script on my camera that says as long as +5V is being applied, behave as if the shutter button is being held down. In single-shot mode, this means each time I flip the switch the camera will take a picture. In continuous shutter mode this means that I can hold down the switch on the transmitter and the camera will keep taking pictures as fast as it can, roughly every 1.1 second.
The two rigs are as different as they could be and still get the job done. One was the product of a few hours of thinking and about an hour’s worth of time in the shop. Total expenditure was maybe $50 for the Picavet suspension, most of that being the PeKaBe blocks. The bulk of the rig came out of the scrap box. The other was the product of a few years of tinkering with a commercial rig. Total expenditure is probably several hundred dollars by now, but still probably less than a good carbon fiber tripod and professional ball head.
What is important to take away from this is that there is no one right way to hang a camera from a kite line. And depending on what you actually need, an extremely simple rig like the ortho mapping rig may get the job done just as well as a more complicated, heavier rig like the BBKK. Before becoming discouraged at the prospect of designing and building a rig from scratch, consider the commercial options. And before becoming discouraged by the cost of the commercial kits, consider the possibility of simplifying your requirements and making one yourself.
It’s possible for anyone to do aerial photography. If you want to do it, do it.