scene might as well be a search sequence for aircraft trails.
Dozens of them show up in this long cumulative exposure, composed
of 100 frames of 30 seconds each. The earths rotation in only
50 minutes is readily seen here.
Taking the image
In the old film
days, we simply put the camera on Bulb, and used a cable release
and left it for hours on end exposing the night skies movements.
Today, it is done quite differently as I will explain here. Because
the camera requires a dark frame for each exposure made over
a few seconds with a DSLR, shooting star trails becomes not one
long exposure, but a series of shorter exposures combined to
form the equivalent image time. While I could have easily gone
a 50 minute exposure with my 10d with no problems, another 50
minute dark frame would have to follow to be later subtracted
to form the final image. But here I tried something very different.
I learned from
some of the DSLR forums that if you put your camera in burst
mode, such that it takes photos continuously as you hold the
button down for say sports images, or birds taking off - and
then put the exposure in manual mode and the exposure set for
30 seconds (the maximum for the dial on the camera), it would
"burst" the 30 second exposures and have enough time
to save them to the card between shots. Thus the camera shoots
for hours on end all by itself by simply locking the shutter
button down with an (electronic) cable release. The applications
here are such that I can point my camera north, open up the lens
and start bursting 30 second frames to make hundreds of snap
shots of the polar rotation unattended.
A few nights ago
I tried this for the first time, and it worked great. I could
have even thrown in a few 30s dark frames at the end if I wished.
The RAW images
were converted to TIffs with Pixmantec Raw converter, and by
setting the middle shots color balance for all of them automatically,
the images were nearly identical. But that was the easy part.
Since I can only open about 4 or 5 frames of this 36Mb size up
at once in Photoshop to stack them. We had a problem! I had 100
shots, each 36mb each to assemble. After a bit, I put together
a Photoshop "Action", which opened up each image, copied
them, closed them and stacked and flattened the pasted copy onto
another image that stayed on the screen the whole time. By batch
processing, the action ran on all 100 images and stacked them
one at a time on to the black blank master, combined them with
lighten on the layers and then flattened.
The result was
hours in coming, a very slow process for so many huge files!
By combining with lighten, the above image was made, creating
a stunning star trails sequence with no gaps in the stars seen.
The final image
was normally processed with a gamma stretch and curves black
point. Here is a single 30s frame to compare:
Polaris is just
to the left of center, and the head of Draco upper right.