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Re: (meteorobs) Re: rotating shutters
Yes, George, harmonics can be a problem with strobes. Imagine a two vaned
shutter (or look at yours:-)
At the proper sped, the marked blade will be in the same position every
2400 th of a minute (I'm using 2400 and 1200...nice nimbers to work
with)[2400 rpm/60 sec= 40 per second), or every 1/40th of a second. Now
another 40th of a second it will be back at the same spot again. If the
strobe rate is 1200 rpm: 1200/60= 1/20th of a second. But for each strobe
flash, the blade will still be back in the same spot, it's just that you're
only looking at every OTHER time the blade gets to that position. The
difference is subtle, but the flicker rate is noticably slower at 20/sec.
Your american TV picture displays the picture at 60/sec- - no flicker is
visible. European (most) PAL TV is it 50/per second...at this rate the
flicker is just visible.
The proper technique should be to have the strobe rate higher than you
know it is, say 3000, then turn it down (lower rpm) until you see the
strobe stop the motion. If you continue down and your speeds are accurate
on the dial, the second time will be EXACTLY one half of the highest speed.
As a note for others, make sure one of the blades has some kind of a
timing mark, or you could be looking at each blade 1 at a time, which would
measure 4800 for a 2 bladed shutter. We won't even go into this for a three
bladed.....just make sure one is marked, and that's what you time with!
Mr TV guy, Wayne
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Original Text
From: GeoZay@aoldot com, on 8/25/96 2:47 PM:
To: <meteorobs@latradedot com>
Marc,
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. This
seemed
like a very big fluctuation even to me although I took the measurements
carefully etc. I just want to say that during my Perseid campaign, I was
having stroboscopic determined rates of about 2500 RPM's again...but out of
curiosity, I turned down the stroboscopes flashing lights to match that of
1300 RPM's I mentioned in the past. 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. I now tend to think
that
the 1300 is the harmonic based on nebulous reasons...have you ever
encountered this problem with a stroboscope? 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.
George Z.
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