Defocused Star Trails -- Show the color (temperature) of stars

[macnmotion]

I'm sure most of you have seen Star Trail shots -- long exposures showing the paths of stars as the Earth rotates.

A number of years ago I attended a lecture by renowned astrophotographer David Malin and he introduced a method of defocusing throughout the course of a long star trail exposure to elicit some scientific data about the star.

In short, every star has a temperature, and based on that temperature it has a color. We see most stars as near white because the brightness of the point light overwhelms our senses (there are exceptions of some beautiful colorful stars you can see with binoculars or a telescope).

Star trail shots normally show stars as white trails across the sky, as the point light source fills each light bucket on the chip (or on the film). By defocusing your exposure, you can spread out the light so that the chip (film) is not overwhelmed. The result is a very colorful image that correctly indicates the temperature of the stars.

I attempted this technique on the constellation Cassiopeia in the Fall of 2002. I was using Film at the time. I simply put my Olympus OM-1 on a tripod and initiated a 30-minute exposure to create a star trail image. I started the exposure in perfect focus. However every 3 minutes I very carefully turned the lens out of focus, in equal amounts every step. The result is a growing cone of diffused light representing each star, and as the light spreads enough the star's true color and temperature are displayed.

I have a full tutorial on this technique at Untitled Document

With digital cameras, you can try taking a long exposure however you will likely be overwhelmed with digital noise. Many astrophotographers now take a large number of shorter exposures and use software to combine them (stack them) into one exposure. For example you can use Photoshop CS5 "Statistics (Maximum)" command to stack star trail photos. There is a lot of software (both free and commercial) to handle this processing. So you might take 10 3-minute exposures in Bulb mode, quickly altering focus between exposures. Note that if you do this, you must keep the time between exposures as short as possible to avoid gaps in the trails.

Here is an example of this technique:

[OsmosisStudios]

Very interesting technique and result. Might be worth a shot.

[JohnnieBoy]

I normally don't care for star trail photography but your technique gives it a different twist.
I like it. Good work.

The star temperature chart is pretty neat as well.

I've got to spend some time looking up again