Astrophotography with a DSLR and Tripod (Part Two)

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I meant to get this up a bit sooner than this, but we've had about four clear nights on the bounce here, so I've been otherwise engaged, as you can imagine (if you live in the UK). You have to take advantage of every clear night you get here, 'cos you don't know if you're ever going to see another one :) You'll find Part One of this blog here by the way.

OK. The last blog covered simple astro shots using a camera and tripod. This one covers a slightly different technique known as "stacking", which many of you will know about. You'll probably also know why we do it, but I'll go into a little detail about that just to be sure. To demonstrate, I'll be using an image that was taken using longer exposures than you can achieve with just a camera and tripod, but as it's just for demonstration purposes hopefully you'll forgive me for this :)

 
The above image is a comparison of a single sub-frame - "sub" or "light" using the jargon (which means a single exposure) - on the left, and a stacked image on the right. The single sub is a 60 second exposure and the stacked image is a stack of 88 sixty second subs. This is obviously zoomed right in to reveal the individual pixels. The difference is kinda obvious! :) Let's clear up one thing right away: the difference in background colour is not a direct result of stacking as such - there is an option in Deep Sky Stacker (the software I use to stack) to use "RGB Channels Background Calibration". That means turn the background a neutral grey, a process that the software performs before spitting out the final result. I select this option by default, so the background is always a neutral grey - hence the difference in the colour of the background in the two images above.

Now what you can see in the image on the left is a star (obviously), and an awful lot of noise, which accounts for the blotchy background. Noise is caused by the electrical circuitry in the camera (among other things) and is affected by the iso setting (higher settings create more noise - much like film), and the exposure length. As we generally use high iso and long exposures in astrophotography, we can't win! The one useful thing about noise is that it is randomly generated i.e. a pixel that appears dark in one sub may appear lighter (or darker) in the next sub (the "tonal value" varies). We can use that characteristic to our advantage: If we stack multiple images eventually the noise will disappear (almost), simply because the tonal value of the noise pixels vary in each image. Imagine six columns of 100 randomly generated numbers between 0 and 255. If we average each column of 100 numbers we should find that the averages will be much the same, as the numbers were randomly generated. The more numbers we use, the closer the match will be when the columns are averaged. The same goes for the tonal values of the noise pixels - the more subs we stack, the closer the tonal values of the noise pixels will become, and the noise will lessen.  I hope that's clear :). As regards the "signal" (jargon for the bit of the image that we want to keep), in this case the star, the value of each pixel will be more or less the same in each sub, so the signal will remain in the final image. It's important to understand that stacking doesn't "boost" the signal as such, it just makes the signal clearer - less swamped by the noise.

The reason we want to remove the noise (apart from the fact that it looks horrendous), is that, in most cases, we need to "stretch" the image (improve the contrast) to bring out faint detail. If we stretch the image, we also stretch the noise, which is bad!

In the above image, both sides have been stretched by an identical amount. On the left, bad. On the right, good - not perfect, but an awful lot better. :) Now a single star is reasonably bright - if we were looking at faint details here, the signal in the image on the left would be lost in the noise. It would be much clearer in the image on the right.

"So how do I stack images?" I hear you ask. I'm glad you asked me that! You could do it manually in whatever software package you use to edit your photos - I use Photoshop. The software needs to support layers (most do I believe) and you need to know how to use them. Unfortunately I can't do a tutorial on each of the software packages out there, so you'll need to do a bit of research on the interent if you don't know about layers. Or you could use software that is dedicated to this stuff, like Deep Sky Stacker (DSS) - other software packages are available!! ;)

Manual Stacking

Pick your best sub and make it the background layer - not sure how important this is, but do it anyway :) Set the opacity of this layer to 100% (research your software if you don't know what this means). Then stack each sub in a new layer, setting the opacity as follows:

2 subs: Background 100%, 2nd sub 50%
3 subs: Background 100%, 2nd sub 50%, 3rd sub 33%
4 subs: Background 100%, 2nd sub 50%, 3rd sub 33%, 4th sub 25%
5 subs: Background 100%, 2nd sub 50%, 3rd sub 33%, 4th sub 25%, 5th sub 20%
and so on.... Get the idea?

The only thing you need to be careful of here is to make sure each of your subs are exactly aligned, otherwise you'll get rather fuzzy results.

Now if that all sounds a bit much, try this:

Stacking in DSS

You'll find Deep Sky Stacker, which is FREE, here:

Deep Sky Stacker

Luckily I've done a video tutorial on DSS, which you'll find on my YouTube channel here:

Deep Sky Stacker Tutorial

I tried embedding this, but it came out a bit small :) This video goes into the intricacies of calibration frames such as darks, flats and bias. Don't worry about that stuff at this stage - just stack your "lights" - I'll explain about calibration frames in a later blog.

You'll also find tutorials on my YouTube channel about processing images, starting with the basics. I use Photoshop in these tutorials, but the principles are the same for most photo editing applications (as far as I'm aware). If you don't know about this stuff, download GIMP (GNU Image Manipulation Program) from here - it's FREE again! :

GIMP

There are plenty of resources on the internet for GIMP. The more comfortable you are with your software, the better you'll be at this stuff - trust me :)

OK guys. Hope this is useful. If you use this process with some success, feel free to post your results on my facebook page :)

Speak again soon no doubt, but in the meantime may your skies be forever cloudless :)

Doug