Sandcurves

I've been playing around with photography a lot lately - I really want to learn how to take better pictures and I just generally enjoy photography.  I always enjoy learning new things and going through tutorials.  I also have trouble sleeping - I think I just need a lot less sleep than most people. 

So, I've been doing star trails.  I've done star trails before, but always holding the shutter open.  I don't have a tripod (that isn't broken, anyway) and so I started looking at stacking star trails. 

Over the last few days it has been uncharacteristically cloudy in Windhoek, but last night it cleared up completely.  This morning I was up at 4h30 - for no good reason, and thought "clear sky, no moon..." and so decided to kill a little time taking a star trail. 

Since I don't have a tripod, I set up on a small blanket on the ground, folded to angle the camera.  Without the southern cross visible, I had to take my best guess at where south was, and did a good enough job at that.  Unlike the northern hemisphere, it is a little trickier in the south to figure out the precise south point. 

I took 82 images until morning started to show.  I guess I could have continued a little, but it was also a bit cold this morning.  I took each image for 30 seconds (simply because that was the max that my camera gives me for free).  You can do the math 82 images at 30 seconds, 41 minutes.

The previous time I did it, I had set it at an ISO of 1600, which gave me to much noise.  I had read in another star trail tutorial where the author (Harold) did his using 100 ISO, so tried that this time.  It certainly seemed to work better. 

I set the Aperture at F5.0, and somewhat focused to infinity (I didn't really worry to much about that).  Of course, the auto-focus needs to be disabled. 

It is kind of a boring image, as far as star trails go - being just outside my back door, but I am just practicing, and trying first of all to get it right in Gimp. 

The results:

How to create a stacked star trail in Gimp

It's not a great star trail, but I thought that I would share how I went about doing it in Gimp.  This is sort of tutorial, but I'm not writing it because I know, but because this is what I did.  If you know more, please share in the comments or link to your own tutorial.

When you take the images it is simple - get some nice foreground (unlike mine), make sure your battery is charged, and be sure you have enough space for some 100 images.  Also be sure you have the time and patience to do it - it takes a while.  If you have some way to do off camera shuttering (I have a little radio transmitter to fire my shutter - which is nice for trying to get birds at a feeder or something like that.)  Take the 30 second exposure, and then you can wait say 10 to 20 seconds - on this image I used the noise of our electrical fence, which has a snapping electrical noise in its box, to count off roughly ten seconds each time. Make sure your camera doesn't move, of course. 

Now download your images to your computer - simple.  Make sure you have a dedicated file created for it - in my case it is "/home/vernon/res/Pictures/2010_09_21/". 

You can probably create a script to do all this, but I did it all manually.  One of the bing things to consider is if you took 100 images or so, if you open them all up, you are going to kill your RAM and your computer will come to a standstill.  So, what you want to do is open each one, one at a time and process it and flatten the image. 

Decide on your order, which I guess doesn't matter that much, and open the first image as a new image in Gimp. 

Now go file>Open as Layers... and open the second image.  With 80 or so images you don't want to get confused, so make a note on a peace of paper which image you are opening. 

Now, in the Layers dialog, change the Mode from Normal to Lighten Only.  This is where the magic will start to happen after a few images. 

To save ourselves having 82 layers (which would slow your computer down a lot, even if you have a bunch of RAM) we are going to flatten the image after every new layer.  So go to Image>Flatten Image. 

And repeat - for all the images you took - open as a layer, set to "lighten only", and flatten.  Remember to write them all down, so that you remember where you were if you get distracted. 

Once all your images are in, you should already see a reasonable star trail.  If it isn't that clear, just go to Colors>Curves and drag the curves line towards the top left and you should start seeing a more clear star trail. 

Play around a bit and you should end up with something fun.

Here is a nice one I found done by someone else. 

[Image by Rotia | license]

Some resources

• The Gimp - The Gimp - GNU Image Manipulation Program - website
• Discussion - A discussion on Flickr's Gimp group about stacking star images
• Tutorial - Stacking Star Trails: Tips and Techniques

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