The only problem is people tend to over edit their stacked star trails to get thicker trails and it is a little too unrealistic. A big draw back of stacking is your shutter life, every camera has a pre-determined shutter life, this is how many shots your shutter can take before breaking, the cheaper the camera the shorter the life, but if you are stacking 30 second shots, this will mount of very quickly. Each have pros and cons again it is all personal preference on which one you prefer to use.If you don't know what a star trail is it is where you are in an exposure and the earth rotates on it's axis while the stars stay still in the sky, this draws lines in yours pictures. The earth moves so fast that you can get a star trail within 15 seconds of exposure, a very small trail but it gives you an idea of how much of a trail you can get within 5 or 10 minutes. The trail will come out differently depending where your camera is pointed in the sky and even your location in the world. All stars rotate in a circle around the north star so if you are composed with the north star in your shot you will get a circular trail around the north star so pick wisely where the north star is positioned in your composition. The further away from the north star you shoot the less curved the trail is. if you are on the equator your trail will be more or less straight but you will get slight curvature just above and below.
How to find the North Star
Firstly you need to find the big dipper (The Plough) which looks like this.
It is quite easy to spot in the sky as long as you know what it looks like. As we can see is consists of 7 stars. You will need to locate the big dipper because no matter what time of year it is it will be always be in the same relation with the north star. What you need to do once located the big dipper is follow the line on the bucket straight and the star directly in it's path is the north star as shown below. If you have any doubt it doesn't harm to carry a compass on you just so you know which direction is north.
The subject matter in a star trail is always important. Finding the perfect subject to compose into your star trail will make the picture so much more interesting and eye catching. Even though you may be tempted to just aim your camera directly upwards and capture a few minutes of star trails, taking the time to compose an interesting subject with the sky always pays off better. There are two ways to achieve star trails and both work as well as each other, so it's all about personal preference.
Long Exposure
The first is to do 1 single long exposure. In this picture below I left the shutter open for about 1 hour at F/5.6. The only problem with doing star trails this way is that no matter how long your exposure is, it will take that same length of time to do the in camera noise reduction if using it. So this photo below took a total of 2 hours.
I feel this method of star trail is more of an art form because it is like shooting film, so much effort goes in to one picture, then after all that time it could still come out wrong. It really teaches you about exposure, aperture and it's relation to light.
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Church Tower. 1 Hour Long Exposure
The problem with doing single exposures is that any light pollution near by will burn in quite badly and may cause the image to come out more yellow or orange, this will reflect in the sky as well as we can see in the image above. Also if the moon is out this will some what wash out the stars and they will appear fainter than they should be.
The second way to star trail is to do a series of short exposures with out in camera noise reduction and blending them in photoshop to create 1 star trail.
The only problem is people tend to over edit their stacked star trails to get thicker trails and it is a little too unrealistic. A big draw back of stacking is your shutter life, every camera has a pre-determined shutter life, this is how many shots your shutter can take before breaking, the cheaper the camera the shorter the life, but if you are stacking 30 second shots, this will mount of very quickly. Each have pros and cons again it is all personal preference on which one you prefer to use.If you don't know what a star trail is it is where you are in an exposure and the earth rotates on it's axis while the stars stay still in the sky, this draws the star lines in yours pictures. You will often have to do a lighting shot to bring out the foreground because of the shortened exposures. The advantage of this method is you don't have to use in camera noise reduction and the stars seem to come out a lot brighter. Also if you do happen to fog up or frost over you can use the stack up until that point so the whole trail is not ruined. If there are any bad frames you can just edit out the bad bits from the star trail. In this picture below I did 120 30 second shots. This method is better for battery power and saving time. I Feel this way of star trailing is not a real picture because the final outcome is not one exposure and it is a digital representation of what would have happened in the single exposure.
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Dead Tree. 1 Hour Long Stack
A third option is to do a mixture of single exposure and stacking, sounds weird I know but what I have started doing is, stacking 5/10 minute exposures, so I could do 3 10 minute exposures and stack them, they looks more like a single exposure but is actually stacked.
Portaloo - 6 - 5 Minute Exposures Stacked.
This is now a mixture of single exposures and stacking, making a more natural trail but easier on your batteries and shutter life, the best of both worlds, but once again if you prefer one of the above methods or even if you have a different method do what is best for you.
Software
The only problem is people tend to over edit their stacked star trails to get thicker trails and it is a little too unrealistic. A big draw back of stacking is your shutter life, every camera has a pre-determined shutter life, this is how many shots your shutter can take before breaking, the cheaper the camera the shorter the life, but if you are stacking 30 second shots, this will mount up very quickly.
Now if you are stacking, the long winded process of putting it together by hand would involve you dragging and dropping layer upon layer on top of one another and lightening each layer. So some great chaps have made some software to make the whole process easier, You download the software, install it on your Mac/PC and then click open, once opened you select the files you wish to stack, and click process, save and your done, if you don't like the final image, go back and tweak individual images then re-stack it. Simple as that, now the two main software packages are Star Trails which is a PC only programme, and StarStax which was first only available on Mac but is now available on all platforms.
StarStax can be found here - www.starstax.net
Star Trails can be found here - www.startrails.de
Before we finished I shall quickly run through how to edit your stack well, there is nothing worse than a badly edited stack, some times you can give away your stack by bad editing. So here is the quickest and easiest way to edit a stack together. I use Star Stax and Photoshop to edit my stacks together and there is a good reason why. A bad stack usually consists of the static stars from the lighting shot mixed in with the star trail, so you would get a dot then a line in your stack, this gives away the stack and makes a bad trail.
Open up Star Stax, and then click open, and select all of the images that your stack will consist of, except the lighting shot, so when you click process it will stack them all together and it will look a little something like this.
So this is what our stacked image will look like, a lot of people make the mistake when stacking, not to do a lighting shot, if you don't you star trail will look like this, a silhouette with a star trail. Now if there is any street lights or walking through the shot with the torch on you can use a black feathered paint brush to get rid of them, it may look messy but it will be ok once we blend the lighting shot in. Now we import this image and our lighting shot in to photoshop, your lighting shot will look a little something like this.
You will have to paint out the static stars in the sky to avoid getting the dot and line image I spoke about earlier. Once you have done this both images are ready for blending, drag them both on to the same file, so they are two individual layers, then you need to go in to the blending options, which can be found above the layers, it will say Normal at first but you need to switch to to Lighten, this then takes all of the lightened areas on the lighting shot and blends them in to the darker areas on the background shot. The end product will look like this.
Now you have mastered standard exposures and star trails you can start adding light painting in to them as well.