For a Christmas Tesla oriented project, I made a Christmas "tree" highlighted by sparks.  

Tesla Christmas tree 2007

Merry Christmas!  This is a single photo from a Nikon D70s digital camera and is a 91 second exposure of a real functioning Tesla coil and is the result of perhaps 50 hours of preparation.  It is cropped but otherwise completely untouched.  It is not, repeat not, photoshopped. It does however achieve the result using special effects which I will explain.  (Feel free to use the pics here as per the  Creative Commons License.  If you want the hi-res pic 1800 pixels for print media then cost is $10 Email me.) 
Discovery Channel Canada shot the construction and running of this as a 5 minute video segment which aired on Dec 20 2007 in Canada.

   (click for Video Windows Media Player 2 mins, 370k)

Above is the very dull video showing the sparks being patiently and slowly guided inside the confines of the frame. Otherwise there is complete darkness until the final flash that illuminates me at the end when I am in position.  There are no color filters on the video. Imagine all the sparks you see adding up to the final photo.

Early ideas
I planned to have a large tree shape made of sparks topped by a 5 pointed star to simulate a conventional Christmas tree.

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Here is an early setup in testing showing the less than full sized frame of the tree.  By making sparks between the rod that I control and the frame, I should get a tree shape. Various testing was done to work out the best techniques.

The Star first attempt
The star will be as high as possible.  To do this I have had to have a much higher support than before.  To do this I have strung a rope between two trees about 20 feet at the ends.  This won't support a large weight but may stabilize a tall structure. 

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As a test I have mounted my Nikon D70s camera in a small Faraday cage so I can take a picture looking directly down on the Tesla coil.  This "skycam" is shown suspended about 17 feet in the air ie 10 feet above the TC using a wide angle lens. It is in a custom support which is rain/sun/EMF resistant yet able to pass IR from the font to allow the long distance remote control to work. Also the flash can be used.

In testing how to best represent the star in sparks, the TC for the star photos below had 5 x 6 foot earthed vertical rods which protrude roughly level with my short rotating breakout point. The gap from breakout point to rotating rod is only about 2 feet in the top photo. I couldn't have it too wide or the camera won't fit it in. These shots are looking down on the coil from above.  A rare view for a Tesla coil.

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This star is more floral than astronomical so I didn't use it.  It hasn't been as star shaped as I would have liked. Everything except the sparks are blurred because there was wind moving the camera - even at 10 pm when I had my final shot. Hence the round toroid and the circle of the breakout points are odd shapes.  So on to further planning for a better star using the frame method.....

Construction

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The left photo shows the frame with a star pattern held there by wire and fishing line.  You can't see the star well as the line is very fine so it doesn't show up in the photo. The tree is a bit smaller than I had hoped for but the whole thing is still 15ft high. The right photo shows me holding a 3.2 m (10 ft) fishing rod with the end wired to the Tesla coil.  I use this to place sparks to the frame.  There is an earthed cable clipped on halfway.  Probably unnecessary but is a safety measure.  There is also a fine earth wire not seen here trailing between me and the TC.

Special effects
The main special effect is that of a long exposure (91 seconds) so that all sparks in a certain period get into the picture. During this time if the sparks are made to go anywhere in the tree shape then they will show up on the final photo.

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The photos above show some early results with a green filter. The different colors are achieved by using my color filter setup. This gets changed during exposure and in the final shot the tree trunk sparks are red, the tree shaped sparks are green and the star sparks are golden.

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The left photo shows the color filter setup in front of the camera that will need to be rotated manually at the correct time to change colors. The center photo shows the black screen that I am behind for almost all of the exposure except for the flash at the end.  Not really sure if it was needed. The right photo shows me in the Santa costume. Inflatable and cost $29.99. Some unkind person intimated that I was well, portly, not realizing it was inflatable.

One of my sons runs the camera and does the filter rotations at my signal, my other son does the video and my wife does the high voltage on/off, adjusting the voltage to give strong consistent sparks. After a few 90+ second runs the tungsten contacts had worn down and needed readjustment.

This photo in the media 2007
Generated 100,000 hits to my site in 1 week.
Shown in  Metro newspaper page 3 article in UK (1.7 million readers).
Has been on Discovery Channel Canada in Daily Planet segment December 20, 2007.(Session 4) Featured in 81 sites (look for Xmas TC note on the sites listed in Backwards links).
Google search for "Xmas" gets 54,000,000 hits. Mine site peaked at number 4 in May 2008. I don't really deserve that.