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Re:High speed Tesla spark photographs
Original poster: Vardan <vardan01@xxxxxxxxxxxxxxxxxxxxxxx>
Hi Russell,
The coil I have fires are roughly 300 times every second. For each
one of those firings the coil oscillates up the high voltage at the
rate of 100000 pulses every second for about 5 pulses. The actual
spark event only lasts 1/ 20000th of a second. The coil is actually
totally dark 98.5% of the time.
The mirror spins to "spread out" the very fast 100000 pulse rate
event so we can see each pulse (otherwise, they are just all on top
of each other and we can't see anything). But we have a fairly long
shutter opening and four mirrors to improve the chances of actually
catching that 1.5% of the time the spark is going. Only about 1/3 of
those pictures show the spark well centered in the sweet spot of the
camera and all and maybe 1 in 50 shows something cool. So we are
trying to catch a very fast but fairly rare event.
Fortunately, with digital cameras and rechargable batteries, we can
take all the pictures we want for free. A film camera could be used
once the digital camera has worked out all the bugs and setup though
if one could develop just the good ones with lots of "push"...
Today I forgot to turn the mirror motor on at first, so this shows
what it looks like with the mirror standing still.
The room in normal light with the camera looking at the coil looks like this:
http://drsstc.com/~piranha/PIRANHA/pictures/still-01.JPG
The toroid with a copper point is against a black background in the
mirror. The outer edge is the room around the mirror.
When the coil is turned on, the camera caught about 10 firings at the
1/30sec shutter speed. The first picture looks like this:
http://drsstc.com/~piranha/PIRANHA/pictures/still-02.JPG
In order to see it well, I push up the brightness and contrast hard:
http://drsstc.com/~piranha/PIRANHA/pictures/still-03.JPG
But here we see ten separate firings all on top of each other. No
chance of seeing the five 1/100000th second pulses that makeup each
of those firings. With the mirror spinning and some luck, we can
"catch" a single firing and its individual pulses like this:
http://drsstc.com/~piranha/PIRANHA/pictures/still-04.JPG
It is sort of like finding a needle in a haystack, but we have lots
of needles to work with ;-) As far as I know, until Peter figured
this method out, these super high speed arc events have never been
actually visible at all!!
So basically, the spinning mirror spreads the rare very fast event
out over the camera sensor's surface so we can see each single one.
I am getting too much stray light right now and the florescent light
in the room is flashing too so that is the next thing to work
on. The mounting system I have now is not really optimal so I will
redesign it once I figure out what Peter's new improvement is ;-)) I
really need another larger mirror in there to improve all the
angles. Then I will have to get a better camera...
Cheers,
Terry
At 06:05 AM 9/19/2006, you wrote:
Thanks Gary and Terry. I am a little more informed but just
barely. Gary, what is a FRES? I could not find a def that fit on the web.
Also, I can see a little better why the streaking is minimal. I
think you are assuming that you are catching just one spark strike
since more than one in a single pass of the mirror at 5000 RPM ( =
1/83 second shutter speed) would show parallel sparks along the
smear. I have seen photos of lightning where the camera is
rotated in azimuth during a single exposure and the film captures
multiple strikes. That being the case now that you have convinced
yourself that at a single mirror RPM of 5000 delivers you a single
strike the shutter speed is now basically somewhere between 1/60 and
1/125. Why don't you just shoot direct and eliminate the mirror and
smearing? Or am I still confused?
Russ