You may have seen me and others writing about stacking shots, so what's this stacking thing all about? Well, it's the process of taking multiple shots of the same subject such as a galaxy or nebula and combining them together in such as way that the camera 'noise' is reduced and the detail and colours are enhanced. There's no fakery here and nearly every good astro shot you've ever seen, including those from NASA and the Hubble Telescope, have been processed in similar ways. It's important to have a camera that can take RAW images, as these contain every bit of information that the sensor has captured, including unwanted noise. Images are really just sets of numerical data stored on a memory card and made to look like photos by the camera software. They contain values which represent the colour and brightness of each tiny pixel that makes up each image.
What is Noise?
All digital cameras generate noise which shows up as random coloured dots on the screen or photo. Generally you don't see this noise in normal photos, but at night or in dark situations it becomes a lot more evident. It's a bit like the 'snow' effect you get when watching a TV channel with no signal. The noise is generated by the camera's sensor and it's dependent on the temperature and the lighting conditions. Each photo you take at night has a slightly different noise pattern on it and this needs to be removed - fortunately it can be to a large extent.
What Are Calibration Frames?
Calibration frames are extra shots you take to improve the final image. There are three main types:
- Darks - a set of shots taken at the same time as your target but with the lens covered
- Bias Frames - a set of shots taken at the highest shutter speed your camera can manage
- Flats - a set of shots taken with the lens pointed at an evenly lit bright subject
I'll be adding a little more detail on these later. There are other types of calibration frames too, but in my experience they are less commonly used. The main shots you take of the subject are known as Lights or Subs.
It's usual to take as many calibration frames as time permits, but a minimum of 20 of each is usually recommended by most people. Some cameras can also process shots in-camera to avoid the need for Darks and Flats.
How Do You Stack Shots?
Stacking can be done manually in certain image processing programs like Adobe Photoshop and GIMP, but most people use a program that partially automates the process. Perhaps the two most popular ones (because they're free and good) are called Deep Sky Stacker (DSS) and AutoStakkert3 (AS!3) bling with PIPP which prepares a video to be stacked as separate frames. There are other dedicated astrophotography programs such as PixInsight, MaximXL and Astro Pixel Processer but these can be quite expensive. I'm currently investigating a similar free program call SiriL.
With these programs you load in your shots (Subs or Lights), add any calibration frames you have if desired (Darks, Bias and Flats) and DSS will stack them together to produce a single image which contains a lot less noise and more detail than any one of the single shots. Before starting the process there are many settings than can be altered which you need to learn about to improve the final image, but the default settings will do when you are starting out.
What is Image Processing?
Stacking isn't the end of the process and the next bit is probably the hardest. The image produced by stacking usually needs to be processed in image processing software such as GIMP, Affinity, PaintShopPro or Photoshop. These packages range from being completely free to fairly cheap and affordable and on to being pretty expensive, although you can sometimes pay monthly for them.
The stacked image is loaded into some image processing software and various techniques are used to bring out the details stored in the file. There's a lot more than you can actually see and it's amazing to watch it appear as you process the file. The main aspects of the software that need to be mastered are:
- Layers
- Levels
- Curves
- Masks
- Blending Modes
- Filters
- Noise Reduction
Image processing is a very complex subject about which I am still learning. I'm probably only on the first rung of the tall ladder and so I'm not going to attempt any detailed explanations here. Instead, I will provide links to the YouTube tutorials I use to learn about the subject and they will appear here shortly. Or. if you can't wait, just go to YouTube and search for 'Astrophotography Tutorials' - that's what I did.
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