the effect is basically what a tilt/shift lens would do when you tilt the front of the lens. Rotating/tilting the plane of focus so that the film/chip and the focus plane aren't parallel anymore, thus the top and bottom of the image goes out of focus. A gradient defocus like yours in photoshop achieves very much a similar effect.
If you want this technique to work/look even better, try shooting from higher up. Like from the roof of your house, or the balcony of a building.
Here are some of my lens/technique tests using an odd technique, one that is harder to describe. It involves many images (40+), and no blur in photoshop, the blur is all optical Basically, it's a very fast lens (85mm/1.2) shot at 1 meter focus distance (where the headlight almost fills the frame), and shot a grid of images to capture the entire car. The longer and very faster lens gives the thin DOF of a long/fast lens, but the perspective of a wide angle lens (18mm or 28mm, there abouts) Similar to what the shot would look like with an 85mm 1.2 on a large format camera (if such a lens existed)
very cool I was never able to enjoy astro photography (tried it a few times) but I can enjoy the results of others Thanks for posting them! Are you using any tracking bases? multiple exposures on the base, or just one long one? (I assume you are using camera lenses, not telescopes or mirror lenses?)