A brief look at Stacking

by: Simon Tang

As with many people that go into astrophotography, it is littered with obstacles that just seem to get you frustrated. Not only does it seem complicated, it's also boring, tedious and labor intensive. The reward is an extremely disappointing image or just a screen with error after error.

Before I begin, this is not a "how to" guide on using stacking software but more an insight into using it and understanding it.

The principle behind it is very simple. You take multiple images of the same subject and put them on top of each other. Since we all know that the earth moves, each image you take seems to show the stars moving in a clockwise direction. Most, if not all, stacking software helps you align these images so you dont have to manually place each image and rotate them.

Not only does stacking software align your images, it also helps with noise reduction which I will get into in a moment.

One big advantage to stacking is that it is like having a tracking mount that allows you to do long exposures. 10 1 second images stacked together is like having 1 picture that was exposed for 10 seconds.

So why stack?

Not everyone can afford expensive tracking mounts to achieve the all important long exposures and for those that have tried to take pictures with long exposures, you will be familiar with the image below.

Yes, that's right! Look at those streaks! So what can we do to fix this issue? Let's take a quick look at your camera. If you own a Canon, Nikon or Sony Alpha DSLR camera, you are already ahead of the game. Cellphones simply will not work as you cannot adjust the settings of the camera to achieve the results that you want.

This image shows how all the DSLR cameras have the same basic functions that are important to astrophotography.

The 3 most important functions that you will learn about when taking pictures of the night sky is ISO, Shutter Speed and F Stop. You may already be familiar with these terms but lets break them down.

ISO - The measurement of the sensitivity to light. Your DSLR has a sensor that can be adjusted to make it more sensitive to light. The higher the ISO setting, the more light it can see. This does come with a draw back however as the higher you go, the more noise on the image.

Shutter Speed - The length of time the sensor is exposed to light. The longer you leave it open, the more light the sensor can gather. Again, their is a problem. Since the earth is moving, the stars will appear to streak across the image leaving trails or stars that look more like comets than anything else.

F Stop - This is how much light is let in onto the sensor. The bigger the F Stop the more light that can come in. Their is also disadvantages to having a large F Stop. Without being too technical, the part that is in focus, known as the focal plane, becomes very narrow. What this means is that achieving focus on an object can be quite difficult as the smallest nudge to the focus ring can cause an image to go out of focus. Even worse, if the camera moves, you will also go out of focus.

Since I want to keep this as simple as possible and you want to start snapping the skies, we will go through a basic setup.

Setup your camera and place it on a tripod and point it at the sky. It doesn't matter where as sometimes part of the fun is exploring the image you took.

Using whatever lens you have, zoom out so you can see as much of the sky as possible. Every lens is different so it will vary depending on the type you have.

Set your ISO setting to a value of 3200, your shutter speed to 2 seconds and your F Stop to as low as it can go. (sually this around F4.0)

At this point, if your camera has a timer mode, I strongly recommend that you use it to avoid any movements that might blur the image.

Take at least 20 pictures of the sky on the high quality setting of your camera. I strongly recommend setting your camera to take RAW photos. More info can be found in your manual or simply searching for it online.

You should have an image similar to the one below. (Image has been reduced in size to fit this article)

You will notice several things. The most important is what appears to be fuzz. This is noise from having such a high ISO sensitivity. Some cameras handle noise better than others. But dont panic.

If your images appear to have star trailing, change the shutter speed to something faster like 1 second. This should not be a problem if you are zoomed all the way out.

Another thing you might notice is that you may not see much or you can just about make out something but not sure what it is.

In the above example, what you are looking at is actually the Andromeda Galaxy near the center. Lets take a closer look at the image.

As you can see from the image, it's incredibly grainy and not a lot of detail.

The next step is to download DeepSkyStacker (FOR FREE!!!) from this website:http://deepskystacker.free.fr/english/index.html

Once installed, you are ready to start the stacking process.

This software can be quite complex and hard to use, especially if you have never used it before and not familiar with the concepts behind it. The endless technical menus can scare any novice away and leave you with a sense of frustration. Even the video tutorials out there on YouTube and other sites make the entire process look like you are trying to control the Hubble Space Telescope!

So before you freak out, we are going to do a very simple stack where you do not need to know anything about the program. Every step is based from the default setting from when you installed it. This wont give you the best result, but it will give you an idea of what you can expect.

Remember, this is a very basic guide into seeing what stacking can do. I understand that everyone has a different camera as well as a different computer. Some of you have Macs and some have PCs. Deep Sky Stacker is ONLY available for PC, so for those of you who are Mac users, just follow along and pretend, or you can find a friend with a PC and use theirs. (I will do a Mac version of this article at a later date)

So lets get stuck in!

Download the photos from your camera to your computer.

Open the program and click on Open Picture Files...

Go to the folder where you downloaded the files and select Open

In the box at the bottom where you see the list of files, select the top file, scroll to the bottom, hold shift and click the last file. This will select all the files. Next click on the Check All button on the left hand menu. This will place a tick on the images selected.

Next, click on Register checked pictures... which is highlighted red on the left hand menu.

Since we are going to keep this really simple, we are simply going to ignore everything and click OK.

Your computer will burst into life and start crunching numbers. Depending on how old your computer is, this can take a long time so go make a cup of tea or coffee.

Once it's done, you will see that your image will start to appear. Since we are simply going through this, you might end up with a screen similar to what is below where all you see if a white screen.

Dont panic! Their actually is data in the file. You will now notice a new box appear at the bottom that shows a 3 bars and a line graph. Tick the checkbox that says "Linked settings". This will make it easier to play with.

Move the middle arrow button of the slider to the left as highlighted by the red circle and click "Apply". You can actually drag any of the slides and they will all move. The idea is to slide it along until you get the image to appear how you want. Their is no right or wrong way.

Once you are happy, save the picture by clicking of "Save picture to file..."

Open the image and compare it with one of the images that you took earlier.

If done correctly, you will notice big differences between the two images. Stars that were not there before will be visable. 

Here is the example of the stacked image compared with the original. 

As you can see, a lot of the noise is gone and the Andromeda galaxy is clearer. More stars can be seen.

This may not seem like much but this is how it all starts. Spending the time playing with you camera can give you great results and using stacking can give you even greater results using what you currently have.

Once you get an idea of what you can achieve with the basics, start experimenting. Zoom in closer. Change the shutter speed. Increase the ISO. Don't be afraid to play with the settings. Remember their is no right or wrong way to do it. A lot of what i've achieved is through trial and error. It has been more satisfying to obtain results from the learning process than it is to simply expect a magical one button does it all.

Most importantly, have fun doing this!