Stitching together stacks of stars

- Previously, you saw me out in the cold battling the elements, try... yeah, okay, it wasn't that bad. Anyway, I was out shootin' star trail pictures. I was trying to get one of those images where, or a couple of those images where I've got streaks of stars in the sky, and I was doing it using a method whereby I was taking lots of images at shorter shutter speeds, still very long, but shorter, so that I could stack them together. So, rather than shooting a single, 4-hour shot, I was shooting a series of 30-second shots, with the idea that they could be combined later.

Now, I'm gonna show you how to combine them. And I'm actually gonna show you how to combine a couple of images because it turns out the first one I shot didn't really work, but I wanna show you that one because there's something important that we can learn from it. So, I've copied the images over from my camera. I've got a whole bunch of JPEGs. Now, I actually shot in RAW, and to speed things up, I have converted those to JPEG. I did that in Photoshop using the Image Processor, which I can get to from Bridge. Select a bunch of images, choose Tools, Photoshop, Image Processor, and then I can have it run through and batch-process all of these from RAW into JPEG.

You may say, "Well, why not just shoot JPEG "in the first place?" and that's not a bad question. I would argue that it's best to shoot RAW because in that RAW conversion process, Photoshop is going to apply some noise reduction that might turn out to be really useful given that your're shooting a high ISO in the dark. So, I still like shooting in RAW. I also have white balance control that way. I could go through and do a batch white balance change on my images before I convert them to JPEG. But ultimately, I'm working with JPEGs, and I've got them all open in Bridge here. There are two ways that I can do this operation of stacking 'em.

I can select all my images in Bridge, and go to Tools, Photoshop, Load Files into Photoshop Layers. If I'm working in Lightroom, I can do the same thing. I can select all these images, right-click, and go to Edit In, and choose Load Files into Photoshop Layers. When I do that, it launches Photoshop if it's not open already, and loads all of these images into a single document with a separate JPEG file in each layer. So, what I've got is a bunch of images that are overlapping each other. I'm going to use the Blending Mode controls so that the images get blended together in a way where I see the streaks of stars all composited together.

Blending modes are found in this pop-up menu over here in the Layers palette, and what they do is control how pixels in one layer combine with pixels in the underlying layers. When Blending Mode is set to Normal, pixels in an upper layer completely overwrite pixels in a lower layer. This menu is full of all sorts of different options, and basically, they're just different mathematic or logical operators that can be performed between pixels. So, if I select all of these layers, which I can do by clicking on the topmost one, and going all the way down here to the bottom, I have 57 of them, holding down the shift key, and clicking on the bottom, that selects all the layers, I can then go here and switch to Lighten, and when I do that, poof, I have star trails.

Now, what Lighten is doing is it's saying to keep the pixel on top if it is lighter than the pixel below. So, I'm just going through and gathering up all the brightest pixels, and making them visible in this one single image. So, this is pretty cool, I've got star trails, you can even see that I've got some airplanes flying through, which I may want to do something about, we'll get to that later. The problem is I've got stuttery star trails. I've got Morse code star trails. It's a bunch of little dots. I wasn't expecting that. And fortunately, I reviewed this in the field.

I actually did this merge in the field to find out if my star trails were working, and the conclusion I came to was this is peculiarity of the way that the intervalometer that's built into my camera works. So, in my camera, I had long-exposure noise reduction turned on. That's a process called dark frame subtraction, so, after I shoot an image, the camera then sits there and shoots another image without opening the shutter, of the exact same duration, so basically, it's taking a picture of black so that it can get a record of what the noise is in the image.

And, noise varies depending on camera temperature and things like that, so it's gonna be a map specific to the time that I'm shooting. And then internally, it does some math to scrub that noise out of my RAW file before it saves it. So, if I'm doing a 30-second exposure after each shot, there's a 30-second delay while it does that dark-frame subtraction. It turns out that my intervalometer doesn't start its count until after that long-exposure noise reduction is done. The practical upshot is that it was a minute and a half between opening the shutter, and that was enough time that the stars were moving, and so my trails are overlapping.

So, I turned off the dark-frame subtraction, the long-exposure noise reduction and did it again so that I was truly getting an exposure every 30 seconds, and I ended up with another batch of images. I'm not gonna save this one 'cause my second one is better. And instead of doing this one in Photoshop, I'm going to do this one with a piece of freeware called StarStaX. This is available for the Mac and for Windows and for Linux, and it really streamlines this process. So, I'm gonna go over here to StarStaX.

It says, "Drop Images Here." On the Mac in the current version, you can't do that, it'll crash and there's a very big warning and a Readme file about that. This is apparently something that Apple needs to fix before it's gonna work, so I'm gonna use this open dialogue box instead, and just go here and select all of my images. And when I do that, they'll appear over here. Here's a Blending Mode menu. All I have to do is choose the mode I want. I'm gonna start by doing Lighten. I have a number of options here, and it's cool, 'cause if I mouse over it, it gives me a little Tool Tip that shows me what each different mode does, and this button is the Process button, Start Processing.

I click that, and I actually see the images start to blur together. The first thing you saw there was a tree pop into view, and that's because for one single frame, I painted the tree with a flashlight to illuminate it, 'cause I decided I wanted that in the foreground. What's cool about this technique is if I was just shooting a single long-exposure image, I would be stuck with that tree there. With this stacking procedure, if I decide that I don't like the tree lit up, I can just take that image out of the stack, and then I'm back to just a silhouette of a tree.

I could do the same thing with these pictures of planes. I could just take those out of the stack, just remove those images, and they won't go into my composite, and I will end up not having those streaks there. Or, I can go into those individual frames and clone the planes out in Photoshop, which is much easier to do on an individual image that doesn't have trails. So, this is the Lighten mode, and it's working pretty well. It may look like there's a lot of aliasing and stuff in here, but there's really not. That's just a function of my zoom level. If I zoom all the way in, you can see that I've got nice, clear star trails here that are nicely joined together.

I was right to change my interval. These gaps here are actually tree branches that I didn't get lit up with my flashlight, so they're occluding that part of that star trail. I do have another option that I may wanna play with. Sometimes these trails can have strange gaps kind of in the middle of them, so, I'm going to switch from Lighten mode to Gap Filling, and you can see in the Tool Tip here: Create star trail images with minimal gaps. Sounds great, I'm gonna actually just hit the processing button again, it throws out my old conversion, and starts a new one.

This one takes longer to process, so before you do this, you might just do a quick Lighten to see how it looks to see if you like your composition and everything and see if you think it's worthwhile. For all of these reasons, lower noise when shooting, a little more control when shooting because you're not going to be stuck with this very, very long exposure, and for the control that you get in post-production, the ability to take out frames with airplanes in them, or to experiment with painting different things. For all of those reasons, I'm just really sold on stacking as opposed to really long exposures as a way to do this.

And, the main thing, of course, is I'm minimizing noise. So, this is motoring along here, and here's my Gap Filled result. I don't see a huge difference between it and Lighten, but I like them both. I think I will go in and play with the planes. I'm gonna leave the tree lit up; I like it, but it's too cold, I think, in color temperature. So, I'm gonna warm that frame up a little bit. I don't think it'll warm the stars. If it does, I'll just mask those out. So, I've really got a fine level of control. Another cool thing I get out of this, for free, as a bonus, is a time-lapse.

It's not a really long time-lapse, but here you can see, I got a nice shot of the sky animating itself. Because I've got all these frames, I just throw those into another piece of freeware that I use on the Mac is Time Lapse Assembler. Really nice way of building time lapses. So, I get that for free. So, I really like this. I'm curious to see how it's gonna compare to the long-exposure star trails pictures that I did, and we're gonna take a look at that in a future Practicing Photographer.