Cluster camera focus

Cluster camera sits in the middle of our range of fields of view at 4 degrees. The lens is a 180mm Nikkor which we have in the past manually focused. As the reliability of the system has improved we have been visiting the site less often to do maintenance and refocus the cameras; the result of this is that we have found the change in operating temperature between winter and summer is enough to effect the focus of the cluster camera. The solution to this.. find a way to automatically focus the camera. As with everything we do there is a vast difference between the simplicity of the statement, and the reality of the task.

The Cluster imaging set up consists of a FLI Microline camera with an E2V CCD47-10 chip, attached to this is a FLI filter wheel, then finally the lens at the very front.

The lens has a Nikon AF mount, but driving the focus system on the lens requires mechanically turning what looks like a screw head on the lens mount. With the back focus of the lens and the filter wheel hardware sitting directly behind it, there is no where to fit a mechanism to drive the lens.

We briefly considered switching to using a Canon lens. The Canon lenses use an electrical interface to control focus and aperture. However a number of reasons deterred us from this approach. Amongst these were the lack of availability of adapters to mount the Canon lens on the FLI gear, the problems of then wiring in to the lens control connector, and finally we had some concerns about the back focus on the lenses.

The final solution that we decided to use was to mechanically turn the focus ring of the lens. This has the advantage that it can easily be used with new lenses and requires no modification of the lens or any of the other existing imaging hardware.

For speed of development we went with a PIC18f2550 module which already had the oscillator and USB hardware on board. We coupled this to a stepper motor drive module, again to speed up the development.
Our firmware for the PIC module allows the focuser to appear as a USB serial device on a Linux or Windows machine which allows us to work with it easily from minicom.

On the hardware side we mounted the stepper motor to the side of the lens; the motor drives a worm via a flexible coupling. The worm drives a pivoting nut which is attached to an arm mounted on the lens. As the worm is rotated the nut travels up or down the worm, rotating the lens focus adjustment as it goes.

The cluster camera focussing system will now enable us to keep the cluster camera focussed in the periods between the maintenance trips. As the focus only requires changing seasonally, we have no automated software to drive the focuser... so if you see Cluster camera getting out of focus let us know.

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