The Cairn Optospin IV continues our long tradition in filter wheel design, dating back to the 1980s, but taking full advantage of the continuing developments in motor and control technology that have taken place since then. It continues our tried and tested approach of mounting the motor directly within the hub of the filter wheel, in order to maintain a compact size (no motor bulge!) and to minimise the inertial and other losses associated with geared connections. Our previous design of this type used a relatively small wheel with 12.5mm diameter filters in order to further reduce the inertia - and hence the response time - but by using a newer and much more powerful type of motor, we have been able to obtain similar performance with this 25mm filter design.

This design also continues our previous tradition of producing filter wheels that can spin continuously over a wide range of speeds, as well as being able to step between filter positions. Continuous spinning allows wavelengths to be changed on a millisecond timescale, as required for high-speed imaging, and also avoids the inevitable vibration problems associated with stepwise operation. However, with this design we have given considerable thought to that form of operation as well. The larger wheel size required to accommodate a larger number of filter positions substantially increases the wheel inertia, and hence both the stepping time and the vibration problem. We have therefore standardised on a six-filter wheel, but it is offset within its housing so that two housings can be joined in an interlocking fashion, allowing both wheels to be incorporated within the same overall optical and mechanical path length of 35mm. Amongst the other obvious possibilities, it allows the controller box to treat this combination as a single ten-filter wheel (five active positions and one open position in each one), but with substantially reduced inertia - and hence stepping time - than a single ten-filter wheel could possibly achieve. Another useful mode, which is also supported, is to use the second wheel as a counterbalance, driving it so as to compensate for the reaction torque from the other wheel, and hence reduce vibration.

The controller box can drive up to four filter wheels simultaneously, and can interconnect with other equipment by either a digital or a USB interface. In spin mode, any or all wheels can be spun in both speed and phase synchronisation, either at an internally programmed speed, or locked to an external reference frequency. In step mode, wheels can be simultaneously but independently stepped between any combination of filter positions.

Key Features

• Continuous spinning  up to 10,000 rpm (1msec per filter), set by internal or external reference frequency
•  Stepping times down to 30msec between adjacent filters, 50msec between opposite ones
•  Compact size, only 100 x 100 x 35mm
•  Two filter wheels can be mounted within the same overall 35mm optical path length
•  Six standard 25mm filter positions per wheel
•  Paired wheels can simulate a single ten-position wheel, with substantial speed advantage
•  Control via USB or digital interface
•  Control box can drive up to four wheels