Tag Archives: average

Advanced Star Trail Tricks

Published: Oct 11, 2012
Last Update: February 14, 2018 (remove Flash)

I have been playing with Star Trail processing for quite a while.  Ever since I wrote the StarCircleAcademy Stacking Action I’ve been tweaking processing to try different things. Sometimes failure is inevitable, sometimes… well, you’ll see.

First, you may want to look back through my earlier columns on shooting and processing star trails because this is not a primer on star trails – it builds on what I’ve previously written and this is not a good place to try to understand what stacking is.

Second, please understand that I use a variety of tools but almost all of my more successful endeavors end up as layers that are combined in Photoshop (CS5 at the moment).  You could combine your layers in GIMP if you don’t have Photoshop, but you’ll be out of luck if you try to use Lightroom.

Here are my star trail effects:

  1. Smoothee – Averaged sky and/or foreground to reduce the grittiness that sometimes results from brighten stacks. I’ve been espousing this for quite a while. See the Simple Astrophotography Processing Technique.
  2. Blobulous – stars at the beginning (or end) of a trail are made to stand out from the rest of the trail.
  3. Comets – star trails appear to grow brighter and the end of the trail looks like the nucleus of a comet.
  4. Streakers – Like comet only the trails are longer
  5. Blackened – A clever trick removes sky glow from light pollution, the moon, or twilight.

And of course you can make “Blobulous Comets” and “Blobulous Streakers” and “Blackened Smoothee Comets” and more.

Building Blocks

To creatively combine exposures, I usually create the following stacked frames.

  • Dark (Darken in Image Stacker/StarStax)
    The darkest elements emerge – especially the hot pixels
  • Brighten (aka lighten) stack
    The Brightest of everything is present, including hot pixel and more noticeable noise
  • Average
    Contrast is reduced, smoothness increased.
  • Additive (called “Stack” in Image Stacker)
    Hot pixels become really bright.
  • Scaled (called Stack/Average in Image Stacker)
    Allows some increase in brightness but more smoothness, too. Experiment with different divisors.

Normally I create all of these combinations using Image Stacker against my JPG files because it is really easy to do.  I end up with a set of frames something like these although I’ve significantly brightened them so that they are easier to see.


In a Nutshell: Combine the Average stack over the Brighten stack using Normal mode at 45% opacity.

I’ll start with the Smoothee technique since it’s probably the easiest to do and perhaps the easiest to understand.  The problem with “Brightness” (or lighten as it’s called in Photoshop) is that it will also pick up all the hot pixels, and the brightest bits of noise.  Averaging on the other hand tends to smooth out everything except for truly hot pixels since most noise is random. By putting an averaged stack as a layer over the brighten stack and then adjusting the blending modes and opacity you get a smoother sky and foreground.  Exactly what settings to use depend on the images, but surprisingly many of the blending modes for the Average layer work here including Darken, Multiply, Overlay, and Normal. The starting place for Opacity is about 45%.

Hint: You can also use an Additive stack instead of the average stack but usually only the Normal blend mode will work.  For even more fun combine the Additive stack and the Average stack.

For additional smoothness you can also subtract the “Darken Stack” while adjusting the opacity to prevent halos and weirdness.


In a Nutshell: Add one of the single frames more than once.

What do “Blobs” look like? Like this…

“Fat Star” processing.

There are two ways to produce “Blobs”. One way is to add “Comets” to a smoothed star trail. The other is to simply pick an image (usually the last one in the set) and add it in using “Add” or “Screen” mode. To make the blob more pronounced duplicate the last frame so it’s added twice. BUT remember when you add in any single image the hot pixels are going to come out… and even more so if you add an image twice.

Comets and Streakers

These two techniques require some fancy stacking techniques. Fortunately I’ve created an action to do all the fancy stuff.  I’ll be rolling out the action and the explanation to my Photo Manipulation Webinar participants first <NOTE: The Advanced Stacker PLUS action has been released and is available for purchase in our store>.

Oh, here is a peak at what the Comet action looks like:

What's The Point?

And here is what an animation of comets might look like:

Star Rise



I know you’re going to ask so let me save you some typing. Except for the “Comet” image above, all images used in these illustrations were taken during the Ancient Bristlecone Pine Workshop in the Patriarch Grove on White Mountain, East of Bishop, California.

The 34 or so images that I’ve combined in the examples above were all taken with the following settings: Canon 50D, ISO 400, f/3.5, 79 seconds, 10-22mm lens at 15mm.

Dark Frames And Your Night Photography

In an upcoming webinar (Down with the Noise) I explain a lot about noise: causes, contravention and cures. This is a bit of a prelude and addresses the questions:

  1. What is a dark frame?
  2. What do I do with my dark frame?
  3. What do I do if I don’t have dark frame(s)?

What Is a Dark Frame?

A dark frame is one or more images taken at the same exposure length, ISO and ambient temperature as the light (normal) frames but with the lens or body cap on the camera to prevent any light from reaching the sensor. When doing many kinds of night and low light photography dark frames can be quite helpful. And when doing star trails or other night imagery dark frames may save your bacon. A dark frame is what your camera does after a long exposure when long exposure noise reduction is turned on. But you’ll be far more efficient if you take those frames yourself. If you’re taking 100 light frames, e.g. for a star trail, you can take 3 or four dark frames and waste 50% less time (and not have gaps!)

Contrary to popular belief dark frames and long exposure noise reduction do little to reduce the random noise that is present in every exposure. That random noise is most pernicious in dark photos and shadow areas.  Dark frames, however are good for the following things:

  1. Reducing or eliminating hot pixels and amp glow
  2. Removing any “bias” in your image – that is bringing the black back.
    Want me to translate that: an unexposed area on your sensor should read as “0,0,0”  for Red, Green and Blue but I will bet you you don’t get zero!

This would probably be a good place to show you what a dark frame looks like. But you’ll be disappointed. Dark frames are usually quite black.  So instead of showing you JUST the dark frame, here is the dark frame boosted to show the speckles from hell – though they may not be obvious. Here I have made the speckles more obvious by boosting the darks using Curves in Photoshop. At this level of detail there are not any obvious hot pixels.

Dark Frame Overview – Boosted to show details. Note where the markers are – they are shown in the next frames.

And next is the same dark frame zoomed to 3200% unaltered. Hover your cursor over the image to see the same area boosted using curves.

Single Dark Frame (Linear Mode) Unmodified – or cursor over to see boosted version.

When inspected carefully, and with the dark level significantly increased it is possible to notice the hot pixels and possibly banding in a dark frame.  While there were plenty of red speckles and obviously green and blue as well in my stable of dark frames, the “hot pixels” didn’t leap out at me.  If you look carefully at the image you’ll notice I also used the color sampler tool to provide RGB values for 3 different locations on the image.  Of note is location 1 where the R (red) value is 14.  What is particularly worrisome about that value is that even after the entire frame has been boosted to the equivalent of 1.6 stops, you’ll notice that a value of 14 is still larger than all three colors at spot 3. After boosting, that red pixel really stands out with a value of 44.  Our first take away is that boosting the brightness boosts the noise.  The second thing to notice is that the red spot is NOT a hot pixel.  How do we know? Compare 4 dark frames (all boosted)

Boosted Dark Frame                             Click these–>   Frame 1 ~ Frame 2 ~ Frame 3 ~ Frame 4

Takeaway 2: There really is randomness!

Now that we have noticed the randomness, we realize that if we average enough of these frames together we can get the average “bias” – that is the amount of offset above zero in the image.  And if there are hot pixels, the good news is they will be in there too.

But How Do I Use a Dark Frame?

The simplest answer is to feed your dark frame(s) to a program that already knows what to do with them like StarStax, StarTrails or Image Stacker.  But you can do it yourself, and perhaps more elegantly using Photoshop.  How?  Place the dark frame as a layer over the image you want to correct and change the blend mode to Subtract (or difference). Adjust the opacity of the blend until it looks just right.

But I Did Not Take a Dark Frame, Now What?

All is not lost. If you have enough frames you can create a unique kind of dark frame. I took over 300 28-second exposure for a star trail along Lake Gaston in North Carolina.

In the image below I created the top frame using the Brighten mode in StarStax. I could just as easily have created the top frame using the StarCircleAcademy Stacking Action.

I used Darken mode to create the middle frame by feeding it my 100 darkest images. Using Darken mode as the stacking option means that hot or stuck pixels that are in every image as well as the lowest value of sky glow will be collected into a single result.

I then loaded the light (Brighten Mode) and the dark (Darken mode) frames into Photoshop. I placed the dark image over the brighten stack and changed the blend mode to Subtract.

What Happened Here?
Dark Frame Substitute process

Several interesting things happened:

  1. The hot pixels were almost completely annihilated
  2. The sky gradient caused by lights glowing in the distance was also almost eliminated.
  3. The contrast in the sky and elsewhere was improved
  4. The red bias on the railing was mostly removed.

A few less desirable things happened, too. The bright red glow on the railing once subtracted caused some of the railing to turn green. And the subtraction created some “holes” and “halos” in the image – especially where the brightest lights are found.  With some minor touch up, most of those issues can easily be fixed.

Is this the end? By no means! There are a LOT more interesting techniques to follow. Stay tuned.

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