Lunar mosaics made easy.
THE MOON IS arguably the night sky's most enticing target. It's big, bright, and chock-a-block with breathtaking detail. But in spite of these attributes (and to some extent, because of them), it can be a challenge to image. Why? Because for the most part if you want to photograph the Moon, you have to choose between two conflicting goals: high resolution and a large field of view.
A quick snapshot through a telescope will capture the whole of the Moon, but it will not have enough resolution to record the kinds of details that probably made you want to take the picture in the first place. Conversely, you can zoom in and record fabulous detail of a single crater or rille, but you'll lose the majesty of a big-picture perspective.
Fortunately, there are ways of having the best of both worlds through the process of mosaicking. In a nutshell, mosaicking is simply a way of stitching together a bunch of small images into a single big one. If each piece is rich with detail, the resulting big picture will be too.
The procedure involves three main steps: acquiring the individual images, processing them, and assembling them into a mosaic. Although you can build a mosaic out of photographs from a digital camera or scans of film negatives or prints, the technique is especially useful for images acquired with computer webcams.
Inexpensive webcams have the ability to record fantastic amounts of detail on the Moon, but they produce images that are relatively small (S&T: June 2003, page 117). For example, the near-ubiquitous Philips ToUcam Pro produces images that are 640 by 480 pixels. Compare that with common 4-megapixel digital cameras, which typically spit out 2,272-by-1,704-pixel pictures. To put it another way, with a webcam you would be hard-pressed to get a decent-quality print that was bigger than 4 1/4 x 3 1/4 inches--not exactly a picture suitable for framing. To match the output of a 4-megapixel camera you would need to use at least a 4-by-4 array of webcam images--16 individual images.
Getting the Pieces
Regardless of whether you plan on using a digital camera, astronomical CCD camera, or a webcam to acquire your images, the mosaicking process imposes a few extra demands over the usual ones of choosing your image scale and making sure you have achieved accurate focus. The most important consideration is to ensure that when you capture your images, you do so with the final results in mind and plan accordingly. For example, say that the Moon is a waxing crescent and you want to get a nice portrait of Mare Nectaris and the trio of magnificent craters, Theophilus, Cyrillus, and Catharina, that lie near its western shore. The first thing you will want to do is get some sense of how many images it will take to cover the region and how they will have to be arrayed. While you can figure this out with software and lots of head scratching, I find it is much easier just to hook up the camera and see what the resulting field coverage looks like. This allows me to approximate how many images I'll need to cover this region of the Moon and plan a shooting sequence.
One strategy to avoid is following the curve of the lunar terminator. Trust me--it gets very confusing very fast, and after only a few images you will likely lose track of which parts of the Moon you've shot and which you haven't. I usually work by moving the scope north to south in one-picture increments. I begin at the terminator and work toward the limb, making successive north-to-south passes until I have completely imaged the area I'm interested in. This way, most of the time I'm only moving the scope in declination. But the exact details on how you proceed are less important than actually having a plan and sticking to it to avoid confusion. To paraphrase a scuba diving saying, "Plan your image and image your plan."
The second important consideration is to make sure to build in enough overlap between images so that your final mosaic doesn't have any annoying gaps. How much overlap is enough? The answer depends to some extent on how you go about assembling your mosaic, but in general, too much is better than not enough since the only penalty for overdoing it is that you will need more frames to cover the same piece of the Moon. I usually make sure there is about 25 percent overlap between adjoining frames. Others use less, and experience will eventually lead you to your own preferences.
Finally, try to keep exposure differences between adjacent images to a minimum. While it's probably unavoidable that you will need different exposures at the terminator than at the limb, if you make the changes gradually (say, change the exposure one setting per north-south column instead of two settings every other column), your chances of producing a seamless mosaic later on are improved.
Once you've acquired your images, it's time to process them in preparation for assembling the mosaic. Some imagers leave the majority of their image processing until after the final mosaic is assembled, but I find that I am able to get better results when I do the bulk of my image manipulation up front and leave only the minor adjustments for after the mosaic is assembled. Treating the puzzle pieces individually gives you the chance to even out the brightness differences and tweak the sharpness--both adjustments that make assembling the mosaic easier.
If you are working with webcam images, you will have to perform the extra step of stacking frames with a program such as RegiStax (http://aberrator.astronomy.net/registax; see the April issue, page 130) or similar software. To minimize image-to-image differences, it's a good idea to use approximately the same number of frames per stack.
Regardless of whether I'm using stacked webcam frames or pictures from a digital camera, I do the same basic processing steps. (I use Adobe Photoshop, though there are lots of other programs that can make these adjustments.) First, unless I'm specifically after a color image, I convert to grayscale. It's amazing how slight variations in tint become glaringly obvious when two pictures are seen side by side. Second, I adjust the brightness and contrast of all images until they match reasonably well. In Photoshop I use the Levels command to perform this task. There is no need to get an exact match at this stage since, as I will explain later, the software I use to assemble the mosaic can compensate for differences in brightness. And third, I tweak the sharpness with the unsharp-mask filter. This final step is the one that is most important to perform at this stage instead of later because when you sharpen an assembled mosaic, you enhance the seams between mosaic segments, making them easier to spot.
Some Assembly Required
With the individual images processed, it's time to assemble the mosaic. There are two ways to proceed. Some amateurs use Photoshop and painstakingly piece together their final images. (For a detailed description of this method, pay a visit to Frank Barrett's Web page, http://celestialwonders .com.) Others assemble their mosaics with software designed for astronomical image processing, like MaxIm DL (www.cyanogen.com). The results are often excellent, though the procedure can be time consuming, especially if you have many segments to assemble.
But if you want to avoid spending a lot of time creating your masterpiece, there's good news. Thanks to the popularity of digital photography, now there is user-friendly software available that's designed specifically for assembling panoramas from images made with a digital camera. It turns out that this software can also be used to make seamless lunar mosaics almost automatically. The best part is that if you own a digital camera, it likely came with the software needed to stitch together a stunning picture of the Moon.
There are other software options too, as a quick Google search will demonstrate. Software like ImageAssembler (www.panavue.com), Stitcher (www.realviz.com), and Panorama Factory (www.panoramafactory.com) will do the job. I mainly use Panorama Maker, which comes bundled with Nikon digital cameras. Even if you don't have a Nikon camera, you can get the software directly from the manufacturer, ArcSoft (www.arcsoft.com), for $39.95.
Although the details vary depending on the software package, the essential procedure is the same. You first select the images you want to mosaic, arrange them on a grid so that they are positioned correctly relative to one another, and then press the Start button. A few seconds later, if all goes well, your mosaic is all done. Sometimes it really does work out that way, but more often than not, some degree of user intervention will be required.
If you get Moon pictures that don't join together properly, the first thing to check is that you have selected the right images and have organized them correctly. Next, make sure that you have entered the focal length of the lens used to capture your pictures. The software uses this information to calculate how much distortion must be corrected in order to seamlessly combine the images. Since most telescopes have focal lengths far greater than normal telephoto lenses, you simply choose the longest focal length listed by the software.
The mosaicking program works by looking for features common to adjacent images--which is why you want to have sufficient overlap between images. Once it has done this, it will warp, rotate, and blend the images so that they fit together --most software will even compensate for variations in brightness and tint. This last step is where the real magic occurs. When everything works, the seams are utterly invisible. Indeed (as the picture above shows), it can even join pictures that have no business being together. Sometimes the mismatched results are so good that it's only when you notice a major crater missing that you become aware that something has gone wrong!
But when the computer joins images together at some crazy angle, or fails altogether, the problem usually stems from the software failing to automatically identify and match reference points. This is where you jump in. Most stitching software allows the user to manually select two or three reference points that appear on both images. Prominent mountain peaks or small craters are perfect--the more distinct the feature, the better. This step can take more than one try, and you will have to work with one image pair at a time. If your first attempt doesn't work, choose different features and try again. Work your way through the various seams until all the elements in your array have been joined together.
Obviously, the more pictures you are using, the longer this will take, but in my experience the software usually manages to join half the elements together on its own automatically, so I end up fussing with only some of them. Finally, remember to save your completed mosaic as a TIFF or in some other noncompressed format, especially if you plan on doing additional touchups later.
The last stage in the procedure is to open the mosaic in your favorite image-editing program. Carefully rotate the image so that north or south is up (depending on your preferences), and crop the inevitable jagged edges from partial overlaps on the edges of the picture. You might choose not to do this if you find you're trimming out a good portion of the image (usually the result of poor planning at the telescope). And if you don't find a jagged image profile unattractive, then there's no need to sacrifice good data for aesthetics.
Next, give the brightness and contrast adjustments a final tweak to make sure your blacks are really black and your whites pure white. Last, apply additional sharpening only if it is really necessary. I've found that if there's one fault common to a lot of Moon pictures it is over-zealous use of the sharpening tool. Find the level of sharpening that looks good, and then back off a bit--too little is better than too much. Once you're done, you should be able to stand back and admire a nice, big lunar portrait that is rich in detail.
My experience with building lunar mosaics has shown me that the process is largely iterative. You acquire some images, assemble them, and then use the experience to modify how you go about acquiring the images the next time. While some imaging processes lend themselves to cookbook, step-by-step procedures, mosaicking tends to be more experimental. The methods I've outlined here should be regarded only as guidelines--don't be afraid to experiment with your equipment and software.
Associate editor GARY SERONIK has been moonstruck since childhood. He images the lunar surface with a variety of telescopes from his home in suburban Boston, Massachusetts.
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|Publication:||Sky & Telescope|
|Date:||Sep 1, 2004|
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