On Wed, Feb 19, 2014 at 7:14 PM, Chuck Norcutt <
chucknorcutt@xxxxxxxxxxxxxxxx> wrote:
> ps: There are some great gallery shots there but the one I found most
> intriguing was this one of the rocket sled.
> <
> http://www.meggaflash.com/gallery?AG_MK=0&AG_form_paginInitPages_1=1&AG_form_albumInitFolders_1=/gallery2&AG_form_scrollTop=0&AG_form_scrollLeft=0&AG_MK=0
> ?
>
> The slowest burn time bulb is the PF200 with burn time in excess of 40
> ms. The rocket sled is traveling at 4800 ft/sec. or 4.8 ft per ms.
> During the 40 ms burn time of the flash bulb the sled travels 192 feet.
> So, unlike more conventional electronic flash scenarios we are used
> to, even the fastest meggflash bulb can't freeze the action. Yet the
> rocket sled does seem to be frozen... except for the background which is
> very blurred horizontally. Conclusion: The camera was panning with the
> sled, or more correctly, moving along with the sled. Somehow the camera
> and flash had to be moved parallel with the sled at nearly the same
> speed as the sled in order to freeze the motion. I wonder how that was
> done!
>
Not the camera, but the image. It's a special high-speed camera called a
smear camera. There's a little about it on Wikipedia:
http://en.wikipedia.org/wiki/High_speed_photography
We've still got some rotating-mirror cameras here at work, but they've been
almost completely replaced by digital high-speed cameras. Many of our film
cameras use 70-mm film, which given our Byzantine purchasing rules is
effectively impossible to get. The last time I used a film camera was
nearly 10 years ago. Our rocket sled track hasn't been used in probably
close to 20 years.
As for flash, back in the day argon candles were commonly used:
http://en.wikipedia.org/wiki/Argon_flash
Experiments often had more explosives in the lighting system than in the
object being studied. Now we have a couple of Megasun flash systems:
http://prismscience.com/system_components.php
We're listed in the sample uses page:
http://prismscience.com/sample_uses_lanl.php
I mostly use the 15 kJ 700 us system. While not as fun as a big argon
candle, they're much more convenient. We've also been known to destroy a
few Alien Bees. They're cheap enough to be considered disposable, and while
not nearly as bright as the Megasun sometimes they're enough.
Mark
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