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(meteorobs) RE: meteorobs-digest V1 #119
Hi George
>From: GeoZay@aoldot com
>Date: Sun, 25 Aug 1996 14:47:54 -0400
>Subject: Re: (meteorobs) Re: rotating shutters
>Hello! We had a discussion about rotating shutters awhile back...somewhere in
>the discussion I mentioned that my shutters RPM's varied greatly according to
>my stroboscope. I reported that I usually have RPM's between 2100 and 2600
>and that recently I had a rate of about 2500 in the early evening and during
>the early morning hours I had an RPM measurement of about 1300.
> To my surprise, I found that my rotating
>shutter also appeared stationary at 1300 as well as near 2500 with no hint of
>extra "ghost-like" images of the two blades. The blinking of the stroboscope
>were obviously of different rates...The only thing I can conclude was that
>one of these was a "harmonic" of the other of sorts. Is this normal? I personally
>feel that the 1300 RPM is half that of the actual 2500(or could be near 2600
>actually...or even that the 1300 is actually 1250). But I was never aware
>that a stroboscope could do this.
Yes, this is normal. Just try to imagine the actual position of the shutter at
each light flash of the stroboscope.
Actually, I think the actual rotation speed of your shutter might be 625 instead of
1250 or 2500. 625 RMP gives about 20 breaks per second with a two-blade shutter
which gives reasonable meteor pictures. 1250 rpm already gives a lot of wind noise
(but better meteor pictures) and 2500 rpm sounds incredibly fast. I do not have a
stroboscope at hand here, but if your true speed is 625, I would expect to see a
single (two blade) shutter image with stroboscope speeds of 625 and 1250, and a
double image (cross) of the shutter when the stroboscope speed is 2500.
The discrepancy of 100 out of 2500 (4% error) is not unusual for analogue
measuring instruments. If your instrument has a digital display something could be
wrong.
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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