Stacking images in Composition

Using six images of an Iridium flare as an example, this tutorial shows how to combine and process them in Composition
Resources needed: Compo and the six images (630K zip file, images scaled to half-size)
There is also a page on achieving the same effect in Photodesk.
Updated 2 Aug 2003: Composition author Rob Davison has written a CompoScript program to do this. Details here.

Our new camera is a Canon 10D digital SLR. We chose this model partly because of its superb low-light characteristics, which make it very suitable for astronomical work.

Most consumer digital cameras suffer from 'hot pixels' when the exposure goes over a few seconds. These show up as star-like dots in an image ... obviously something to be avoided when you're trying to take pictures of stars. For this reason, many cameras will limit the exposure time to some degree -- some models will let you take 30 second exposures, but most constrain it to a couple of seconds; we can see many hot pixels in 2-second (the maximum) exposures from our Fuji cameras.

However, the 10D uses a different type of sensor which suffers much less from this problem. It's currently the digital camera of choice for astrophotography, unless you move into dedicated (and expensive) CCD cameras.

One of our first experiments was a sequence of shots of an Iridium flare, taken from our heavily light-polluted back garden in Leeds just after midnight on 14 July; there was also a full moon.

A couple of test shots established that we couldn't push the exposure much over 15 seconds without the sky washing out in an orange glow. We ended up with a sequence of five images exposed for 6 seconds each, taken consecutively; the camera was on a tripod. These caught the entire sequence of the flare. (In retrospect, what we should have done was stop down the aperture so we could increase the exposure time and avoid the inevitable gaps. Next time.)

Here's one of the shots, unprocessed apart from scaling to 25%. The inset shows the brightest part of the flare at full size, with a few stars visible:

Combining the images in Composition is fairly straightforward. But please bear in mind I'm not an expert with the program, so there may well be better ways to do the various steps! Let us know if you have any improvements or suggestions.

1. Drag the first image (0173) onto Compo's iconbar icon. This will create and open a canvas the same size as the image. Scale the view to 50% so you can see everything. You may find it easier to run all six images through ChangeFSI or Paint first to convert them to sprites.
2. The original images are all 3072x2048 2.5MB JPEGs, and combining all six in Compo requires about 150MB of RAM. This works very well on an Iyonix with 512MB of RAM, but as Compo doesn't have virtual memory you'd need a different strategy on a lower-spec machine (combine two at a time, saving the intermediate stages). The images in the archive are half-size, and you'll need about 40MB to comfortably process them.
3. Now drag the second image (0174) on to the canvas you've just created.
4. It won't be aligned, so select the image by clicking on it and open the Layout dialogue (Ctrl-L). Set the options up like this:
   

   

   Aligning the first pair of images

5. Click on Move selection. The second image should now completely cover the first one.
6. Next is blending. Compo and Photodesk have different ways of blending images -- Screen gave the best results in Photodesk, but Compo doesn't offer this. Instead we'll use the Lighter option.
7. With the top image still selected, click on the Opacity icon in the toolbar (green circle over black square). Click on the dialogue's size icon if you can't see it all.
   

   

   Setting the blend mode

8. Using the bump arrows, choose Lighter; leave the opacity level at 100%. Click on OK, and you should be able to see the Iridium trails from both images.
9. Repeat steps 3-8 for each of the remaining images. You should be left with this:
   

   

   All five images overlaid

10. If you want to adjust brightness and colour values, it's easiest to 'flatten' the image -- merge them all into one. Compo doesn't have a direct flatten facility, but simply exporting as a sprite will have the same effect.
11. Use the standard Compo tools on the resultant sprite for adjusting the brightness etc; here's one method. First I adjusted the red channel in the Curves dialogue to reduce the orange glow. Notice how, unlike Photodesk, you can drag the curve's handles outside the box, which gives greater flexibility (although if you drag a handle right outside the entire dialogue box, you can't access it later without clicking on a Reset button).
    The 'instant update' effect -- drag the handles with Adjust -- also works properly on individual channels, unlike in Photodesk.
    

    

    Adjusting the red channel

12. Then I added a 15% blue Tint:
    

    

    Creating a blue tint

13. And here's the result:
    

    

    Iridium flare

Notes

• An Iridium flare is caused by one of the many Iridium satellites reflecting sunlight off its slowly tumbling body. They can be very bright indeed (the one here was magnitude -8), and are very common all over the world. They can be precisely predicted for any location, and the Heavens Above web site will show you upcoming flares for your site.
• See the accompanying Photodesk page for image details.
• If you look closely and compare the final picture here with the one on the above page, you can see an extra trail at the top. After doing the Photodesk page, I discovered that we'd actually taken six images of the flare, not five. The archive has been updated.
• Compo coped extremely well with six very large 24MB images. Saving in Compo format resulted in a 144MB file (!), and although screen updates naturally suffered slightly with this much data to throw around, there were no problems at all.
  Loading this 144MB file on an Iyonix takes 8 seconds.

Update (2 Aug 2003)

Composition's author, Rob Davison, has written a short CompoScript program which totally automates the laborious stacking part of the above procedure. This demonstrates very neatly the power of scripting in a graphics program.

The program will cope with any number of identically sized images, of any type that Compo can handle. Here's what to do:

• Download the script (1K zip file)
• Download the images (630K zip file, images scaled to half-size)
• Unzip the images to a new directory
• Unzip the script and drop it onto Compo's iconbar icon
• Drag the image directory onto the window that appears

The script will run and you'll be left with the result. It's fascinating watching the picture 'develop' in front of your eyes.