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Re: (meteorobs) Rotating shutter RPM's
Hi everbody
There has been a lot of Email back and forth regarding the rotational
speed of George's shutter and strobe and I am a little confused as to
whether he has sorted it out yet.
The method to use is to mark one of the blades clearly with a felt tip
pen, or something like that. Then start it going and turn on the strobe
starting at zero RPM and gradually speeding up. You will pass through a
number of harmonics at which time you will see the marked blade
stationary in one place. Then as you increase speed you will suddenly
see two marked blades at 180 degrees to each other. The strobe is now
firing at exactly twice the rotational speed of the blade, which has only
had time to make half a revolution. If you doubled this frequency of the
strobe again you will see four marked blades at 90 degrees to each other.
The highest speed at which you see a single stationary marked blade is
the actual rotational speed of the shutter blades.
The explanation for the harmonics below the actual rotational speed is
that the marked blade has had time enough to make two, four, eight, etc.,
rotations before the strobe fires.
The fifty RPM error George is experiencing is, no doubt, due to lack of
precision of the strobe, or operator (heaven forbid ;-) ) Or,
experimental error as we always used to excuse it in Lab Class.
I hope this helps to sort it out.
Ron Rennie
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