Snod35 V1 - Vibration Pattern
Getting the motor to move the focusing element in an optimum way is tricky business. The key is to design your adapter in such a way that the motor causes the focusing element to make a nice circular patter. The image below to the left is a close up of a focusing element with a very rough surface and a small chip near the center (ground with 25 micron aluminum oxide). In this image the motor is off. The image at center shows what happens when the motor is spinning the focusing element properly. The small chip turns into a translucent circle/oval. This disperses the grain over a large area and does an excellent job of hiding the grain of the ground surface. With the Snod35 V1 the approximate size of the circular pattern that a given instance of grain will make is 1 - 1.5mm.
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There are several factors that will determine how the focusing element will vibrate. The obvious one is the motor. Typically, when the motor spins to fast or when the motor's offset weight does not have enough mass, the vibration pattern will be very small and not uniform across the surface of the focusing element. In these cases, the bits of grain may become only slightly blurry or may appear as a translucent line or arc. When the focusing element moves in this way, the grain will still be hidden in well-lit settings and at lower f/stops. However, at higher f/stops and in low light, the pattern that each pit of grain is making will become more and more apparent.
Other factors that might prevent the motor from making the nice circular pattern include the type of extensions used (syringe sticks, threaded rod, wire, etc), how tightly the extensions are secured to the base, the length of the extensions, and the weight of the vibrating assembly. The image above at right shows one alternative method that will yield decent results with a 12mm pancake motor (solarbotics.com).
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