Star Trails from Multiple Exposures

20 min read

Deviation Actions

pesterle's avatar
By
Published:
1.4K Views
:iconprojecteducate:
:iconprojecteducate:


Animal, Plants, and Nature Photography


Now that digital cameras have much better light sensitivity, and lower noise at higher ISOs, a lot of people have started to try out night photography.  One of the types of images that fall into this category are images of star trails.  But how do you do this?  Is it simply a matter of putting the camera into bulb mode and leaving the shutter open for an extended period of time?  Well, you can do this, but there are problems with this method.

If you leave the shutter open for a long time, the sensor will get hot.  And when it gets hot, it produces more noise.  Also, if a car drives by, or someone shines light on your foreground that you don't want, it's not very easy to remove without damaging the foreground image.  If an airplane flies over, and you don't want that to appear in your image, it's difficult to remove it without affecting the star trails as well.

My preferred method of taking images of star trails is to take a series of exposures, and then combine them together later in Photoshop.  If you get any stray light while taking any of your exposures, it's a simple matter of either removing those lights from the individual frames, or just masking them out.

TurretTrails by pesterle
This image contains airplanes in addition to the star trails.  Also, a lot of the foreground light was provided by the full moon.

If you want some light on your foreground, you may have heard of the technique known as light painting.  This would be when someone would shine a flashlight on the foreground during part or all of the exposure.  But getting a consistent illumination on your foreground from one exposure to the next is rather difficult.  And tedious.

Instead, I find that it's much easier to set up some low-output lights to illuminate the scene.  Having dim lights is necessary, because you don't want to make the foreground much brighter than the sky.  I've worked with a couple of other photographers in the past who use a couple of Z96 LED panels, but there are cheaper options available.

So once the lights are set up and have chosen my composition, I put my camera into continuous shooting mode, set the aperture for the widest aperture that my lens will render sharp images in, set the ISO to 1600 (or whatever provides the most sensitivity with acceptable noise), and set the exposure to 30 seconds.  Then I attach a locking cable release, click the button, and lock it.  Then I can walk away, take a nap, find some coffee, or whatever, and the camera will take care of the rest.

MesaTrails by pesterle
Here we didn't have nearly enough light on the foreground Arch.  Also some clouds broke up the star trails on the left side of this image.


How long you let the camera take exposures for really depends on how long you want your star trails to be.  And that will depend on which part of the sky you are pointing at.  It takes 24 hours for a star to complete a full circle around the celestial poles.  Stars near the poles will have shorter trails during a given amount of time than stars near the celestial equator.

In the example I'm showing here, I've captured about 40 minutes of images, which should give you an idea of how much stars will move during that time.

Once you've imported your images to your computer, it's time to start the merging process.

Before I merge the images into Photoshop, I want to make sure that the sky is sufficiently dark throughout all of the images.  If it isn't, the images with brighter skies will overwhelm the images with dark skies, and you may not see any of the fainter stars.
Screen Shot 2015-09-20 at 10.21.39 PM by pesterle


Once I've decided which images I'm going to use, I want to make sure that they have the same raw settings applied.  I usually use auto white balance, and so the camera may have chosen a different white balance for each image.  I want to make sure the colors are consistent between all of the images, so I'll set the white balance on one of them (I typically use 4000K for the temperature when dealing with star photography), and they copy these settings to all of the other images.

Then I'll take a handful of images to merge in photoshop.  If you have 80 images, you won't want to load all of those images into photoshop at the same time.  Your computer will hate you.  I usually do about 10 to 15 at a time.  If I have 80 images, I can combine 10 at a time, and do this 8 times.  Then I'll take the 8 resulting images, and combine those together.

For the selection of images, I'll load them into Photoshop as layers (Lightroom -- right click -> Edit In -> Load as Layers in Photoshop).  When the images are loaded, I select all of the layers in the image except for the bottom one, and then change the blend mode to "Lighten".  Now you have an image with some short star trails.  Pretty simple.
Screen Shot 2015-09-20 at 10.26.57 PM by pesterle
Screen Shot 2015-09-20 at 10.27.19 PM by pesterle
Voila!

Screen Shot 2015-09-20 at 10.27.31 PM by pesterle






But there may be a few things that you need to fix.  Stray lights, airplanes or satellites, and maybe hot pixels.  I'll carefully examine the combined image, looking for things that I'll need to remove, and make a mental note of them.  Once I've identified things that I need to remove, I'll go through each individual image looking for the parts that I need to remove.  In the case of airplanes, satellites, and hot pixels, I remove them using the Spot Healing Brush Tool.  For stray lights, I'll use a regular brush tool (black, 30% opacity or so, normal blend mode), and darken those areas.  You can also use masks to hide these areas.

Once I've removed the elements that I don't want in the combined image, I'll flatten all of the layers, and then save the image.  Then I repeat this process for the other individual images until I've combined all of my exposures.

Now I just have a handful of combined images.  Again, I load these into Photoshop layers, and use the lighten blend mode on all of the layers except for the bottom one.  Since I removed the stray lights, etc. in creating the intermediate images, I don't need to do that at this point.  All I need to do is flatten the layers, and then save the image.

If I want to do some additional processing to the image, I can do that at this point.  But that's for another time.

Moab.11.14--11113-Edit-Edit by pesterle


Here's a video showing how I go through these steps:



Comments4
Join the community to add your comment. Already a deviant? Log In
Halcyon1990's avatar
Excellent. Thanks pesterle. :)