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Re: (meteorobs) Re: rotating shutters
Hi Lew
>Marc, I wondered if perhaps the fact that my design is using a tape-recorder
>motor (presumably very speed-sensitive) might imply a more reliable r.p.m.? Of
>course, this original application (hand-held recording) is not as critical as
>for a floppy disk drive! Thoughts appreciated,
The required accuracy depends on your type of observation. For single station observations a very
accurate rotating shutter is not necessary. To use the angular velocity information for such
observations, you have to assume the height in the atmosphere and the associated shower
anyway; then you can plot the radiant probability distribution on the backwards prolonged trail, as
done in Rainer Arlt's Radiant software. As the height estimates only have a 5% accuracy, the
accuracy of the rotating shutter need not be much better, let's say 3%. For George's shutter you
would allow then 2500 +/- 75 RPM (BTW, the quoted value of 1400 RPM is problably an alias of
2800 RPM!).
I have no experience with measuring the speed fluctuations of a 12V DC motor, but I would
expect air pressure variations and variations in the shape of the shutter blade to have a significant
influence. So, with an unregulated motor you cannot avoid to measure the speed at least once
during the night. I see no reason for large fluctuations during the night, apart from the wind,
unless something is really wrong (bad electrical contacts, fluctuating power supply, lousy
stroboscope).
For double-station observations you want a better accuracy of your rotating shutter, typically
0.1%.
Success with photographing!
Marc
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Marc de Lignie
Prins Hendrikplein 42 Tel. +31 (70) 320 5673
NL-2264 SN Leidschendam Fax. +31 (70) 332 6477
The Netherlands E-mail: mcdelign@pidot net
Predicted progress always fails. Real progress is all the small
steps you realize to have made when looking back in time.
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