# Star Trail Advice



## Cfitz (Apr 22, 2011)

Hey guys. So its gonna be a clear night here so I've decided to do some star trail photos. I've never done anything like this before so I could use some helpful advice, like how long to keep the shutter open? Best aperture settings? How long should I paint light for? etc.

Thanks in advance.


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## dxqcanada (Apr 22, 2011)

You will need to experiment with both.

Aperture will control how bright the light trails will be.
The shutter speed will control how long the light trails will be.

I would say .. set the lens wide open. Shoot a 30s exposure ... preview the shot.
If the stars appear too bright then close down the aperture.


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## KmH (Apr 22, 2011)

How much ambient light is there where you will be shooting? The more ambient light, the shorter your exposure time will need to be. Use the lowest native ISO your camera has.

How long do you want the star trails to be? A 30 *second* shutter speed with a wide open lens, will produce what look like points of light. A 20 *minute* exposure will produce very noticable trails. Many people make star trails by making a sucession of 30 second exposures and then combine 40 or so of of them to render trails. Doing a series of shorter exposures keeps the image sensor cooler, so there is less thermal noise in the final image.

This was a 20 minute exposure (1186 seconds to be more precise) @ ISO 200 and f/11. Notice the ambient light from lights inside the house, and ambient light from a business behind the house lighting up the tree. The sky is partly lit by ambient light from a town of 30,000 whose edge is 6 miles away. It helps to have a really dark, away from lights, sky.

The brighest point there in the upper left part of the sky is Polaris, the north star. Notice the trails get longer the further from the center of rotation (Polaris) the star is.


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## Patrice (Apr 22, 2011)

The stars have apparent movement but not very fast. The arc described by the stars will have 15 degrees of arc in one hour (1/24th of a circle in one hour). 1/8 degree in 30 seconds, not a very long star trail. The longer your exposure, measured in 10's of minutes, the longer your star trails. Some camera sensors start showing significant amp glow after just a few minutes. You'll have to experiment to find out how your camera behaves. 

You'll have to focus to infinity. Careful here, some lenses focus past infinity. 

Some folks here have done trails by combining a large number of short exposures separated by a very small time interval and then 'stacking' the exposures. There are a few free stacking programs available on the web. Look to astronomy forums for many fine examples and advice.


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## LittleMike (Apr 22, 2011)

KmH's advice is right on. Lowest native ISO your camera will allow, aperture (and focal length, believe it or not) depends on how bright you want the stars to be, and shutter speed is determined by ambient light. Don't underestimate the surrounding light sources either. This shot was taken during a 3/4 moon, which provided the only light.





f/5
38 min exposure
ISO 100


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## KmH (Apr 23, 2011)

&#8593; Not counting the light from the stars themselves.


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## deeperDATA (May 1, 2011)

I tried my first set of star trails tonight. Having no experience in the area, I read a few how-to articles but find my biggest stumbling block to be the ISO speed. So a lower ISO speed is best even when doing exposures between 10 & 30 seconds? Also, aperture is crucial as well? Thanks in advance.


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