Part 2: Making Composite Images of the Moon

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In the first article in this series I recounted my initial experiences of using a webcam to take pictures of the Moon. The procedure was surprisingly simple, and the results quite pleasing. By removing the lens from the webcam and replacing it with the telescope you convert the webcam into a very long-focus (high magnification) camera. This setup is ideal for photographing small but bright objects like planets as well as capturing the surface detail of the Moon. One shortcoming with this system is that the very high magnification results in a restricted field of view. Large objects, like the Moon, are too large for a single frame. The easiest way to work around this problem is to take lots of frames each containing one small part of the Moon, and the put them all together to produce a composite image, or mosaic. I did a quick one of these in the previous article, and while nice enough the joins between the individual frames were obvious. This time we'll try and make a mosaic that looks like just one big image and not lots of small ones stuck together.

There are two stages to making a decent mosaic photograph of the Moon. The first is to make sure all the constituent frames share the same lighting, colour and contrast. The more alike the frames are to being with, the less manipulation will be required afterwards when you start sticking them together. Manipulation of the frames is the second stage in the process. Unfortunately for the absolute beginner, AppleWorks has only the most limited tools for manipulating photographs in its painting module. It can stick images together, resize them and even export them as a variety of web-friendly formats, but it can do very little to the actual images themselves beyond (somewhat arbitrary) lightening and darkening. Proper bitmap image editing applications are probably essential, though a low-cost shareware program like GraphicConverter could be just as suitable as a high-end graphics applications like Adobe Photoshop.

Let's look at the first stage first. Attached to my Meade LX-90 8" SCT, the Philips ToUCam was able to photograph only a small part of the entire disk of the Moon at once. I used a Celestron f6.3 reducer/corrector to decrease the magnification and increase the field of view, but even so this set up allowed me to record only about one-twentieth of the surface of the Moon per shot. The tracking of this telescope is good enough to get a few seconds worth of video of a certain spot of the Moon without any problems. I found that three seconds of video was plenty, after which I would stop recording and by using the handheld controller move the telescope around to another spot to record some more video. Although a handheld controller isn't necessary, a motorized equatorial mount for the telescope probably is, otherwise the Moon drifts off the field of view quickly.

Because a webcam can record a series of frames taken in rapid succession (i.e., movies) you avoid the problem of bad seeing. With luck, at least one frame from the video should be perfect. Obviously movies take up more space than stills, but two or three seconds of video should be enough, after all by default most webcams record at a frame rate of about 30 frames per second. It is best not to use any compression at this stage so that you minimize the loss of detail that comes from using video compression (such as QuickTime or AVI). Also record the video at as great a screen resolution as you can, ideally 640 by 480, using the deepest colour depth that you can (8-bit grayscale is okay, but 16-bit colour is better). Since the pixels record the detail, the more pixels there are the better the final picture will look. Of course this has a downside: three seconds of true colour, 640 by 480 video is going to take up 10 MB or more, and because of the limited field of view you may need to record twenty or more videos to cover the entire surface of the Moon.

Try and record all the videos in one session. I found that this meant that all the videos were pretty similar in terms of brightness and contrast. The exception to this seems to be frames where only a small piece of the Moon is featured and most of the rest of the frame is black sky; in these shots the webcam seems to boost the brightness of the frame automatically and the colours come out a little different. Obviously the best thing to do is make sure that every frame contains as much of the Moon's surface as you can fit.

After half an hour I had 200 MB of video. Reviewing these in the QuickTime Player, the atmospheric turbulence is very obvious, as the picture continually seems to boil. Copying a single frame is easy. Once you have a frame you like, use Command-C to copy that frame to the clipboard, and then paste it into a graphics program document. Don't flatten the image but keep each pasted frame as a separate layer. This allows you to move them around to get perfect alignment, and equally importantly, it allows you to change things like brightness or colour balance of one frame without affecting all the other frames. Repeat this until you have built up the complete mosaic, as shown here:

This is where the second stage of the operation, image manipulation, begins. There are two tasks here: to make sure all the frames are aligned perfectly, and then to check that each has the same brightness, contrast and colouration. Aligning frames turned out to be especially easy in Photoshop where the opacity of layers can be adjusted. Set the upper layer to 50 percent opacity and details like craters in underlying frames can be seen clearly. Since there is usually a good deal of overlap between adjoining frames, it becomes a simple matter to overlay matching craters and suchlike until you get a perfect fit. Then readjust the opacity of the upper frame back to 100 percent. Even without this feature, most graphics packages allow you to toggle a given layer between being visible or invisible. By doing this as you use the arrow keys to nudge a frame up and down or side to side matching the overlapping regions is easy and thus adjacent frames can be properly aligned.

Dealing with differences in brightness and contrast can be remedied using those sliders in the graphics package you use, but I found the hue/saturation slider much more useful. Occasional frames were a bit more yellow than most of the others, and by reducing the yellow very slightly and boosting the blue and cyan, any differences were removed easily. You can see one such region in the frame just above the Mare Crisium.

With all this done the next thing to do is go to the background layer and fill in the blank (i.e., white) spaces. The non-illuminated half of the Moon doesn't look right filled in with black; instead use the eyedropper tool too select some of the very dark grey from some part of that region that has been photographed and then use the paint bucket tool to fill in the rest of that region. For the night sky part repeat this process, but this time selecting some of the dark sky. Finally, flatten the image and if necessary use the brightness and contrast sliders to darken the image. If you need to, resize the image downwards to make it smaller and quicker to load on a web page. My finished image is here:

Half Moon - click to enlarge

Compared with simple shots of lunar features, producing whole Moon images is a much more involved process, but I think you'll agree the results are worthwhile. This same basic technique can be used on images of the Sun as well, but naturally you would need to take appropriate safety precautions to protect your eyes, telescope and camera.

In the next article in this series (Part 3) I'll take a look at planetary imaging.

- Neale Monks

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