One of the only drawbacks to CCD imaging compared to film astrophotography is that film is generally much larger than a CCD chip, meaning more of the sky is covered with a given telescope.  There are new CCDs that have 35mm format chips, but the cost is still high, so most imagers find themselves using smaller cameras.  Also, even with a large-format CCD, some objects are so huge that they require mosaic techniques.  Making a mosaic involves taking multiple images with a CCD camera, capturing a large object in smaller segments, and then piecing the individual images together to create one large picture.

See the Advanced Imaging section for information on how to take images for mosaics.

Start Simple

Begin using a black and white image for your first mosaic attempt.  Color balance can be the hardest part of getting individual images to match, so start by eliminating that step.  Learn the basics first and then move on.  (The Color Techniques section follows.)

Below are the two halves (top and bottom) of the Rosette Nebula.  Even through a short-focal-length refractor (4" f/6) and using a fairly large CCD camera (ST-10XME) was used, it was not possible to capture this extremely large object in one image (the Rosette covers the area of seven full moons).

Image Preparations

Begin by cropping out any rows or columns along the edges of the images which may appear dark due to any overlapping that occurred while combining the individual CCD exposures.  In the image above of the top half of the Rosette, there is a black line along the bottom of the picture that must be removed before making the mosaic.  If left in, dark lines along the overlapping areas of the picture will appear in the final mosaic, prohibiting a smooth transition between pictures.

Tip:  Sometimes it is best to just crop a few rows and columns from all around the images to avoid any problems.

Create New Canvas

Select File > New from the main menu or press Ctrl-N.  In the new window give the image a name and select an appropriate size for the canvas.  Make sure the new image is larger than the combined size of the individual files, enough so that there is plenty of room to spare on top, bottom, and sides.  You can always crop the image later, but give yourself enough room to work. 

The example files above are each 2200 pixels wide and 1500 pixels tall, so a canvas about 2500 x 3000 should be sufficient.  For a black and white image, choose Grayscale as the Mode (a color image would have a Mode of RGB Color).  A transparent background is okay, but white is probably the easiest to work with, so set Background Contents to White.

Place the Images onto the Canvas

Select one of the image files as the active window, then choose Select > All or press Ctrl-A to select the entire image.  Select Edit > Copy or press Ctrl-C to copy the image.

Now choose the blank canvas as the active window and select Edit > Paste or press Ctrl-V.  This will place the copied image into the center of the mosaic window, which is probably not at all where it needs to be.  Select the Move tool from the toolbar to shift the image.

For now just drag the image to its approximate location in the final image.  In this example, the top image is dragged to the top part of the blank mosaic image.

Note:  A new layer is created each time you paste an image into the mosaic.  Do not merge the layers until the image is finished as you need to be able to manipulate the files independently to put the mosaic together.

Select the second image, copy as above, then paste into the mosaic image.  Choose the Move tool and drag the image to its approximate position in the mosaic.  Continue for each image until all the parts of the mosaic are in place.

Tip:  For a convenient shortcut, just select the Move tool and click and drag each individual image into the new mosaic file.  A new layer will be created each time.

Aligning the Images

To align the images, choose the Move tool.  Select one of the layers containing a portion of the image in the Layers window.  (If the Layers window is not visible, select Window > Show Layers or press F7)  In the Layers window, run the Opacity slider down to about 50%.

You should now be able to see through one picture to the other in order to align them.  Using the Move tool, drag the images into approximately the right position.

If it is not already, zoom in to 100%, or even 200%, using the Magnify tool (keyboard shortcut Z).

Above:  A close up at 100% of the unaligned mosaic segments.  Notice the matching stars which need to be aligned.

With the Move tool selected, use the arrow keys on the keyboard to shift one image into registration with the other.

Tip:  Holding the Shift key while using the arrows will move the image in 10-pixel steps for larger movements.

In the Layers window, move the Opacity slider back up to 100%.

Zoom back in (if necessary) to see that there is probably a distinct line where one image overlaps the other.  Removing this is the key to the art of making mosaics.  It is pretty straightforward for a black and white image, and making a grayscale mosaic first will teach you the basic techniques necessary to go the next step and make color mosaics (covered in a later section).

Above:  Close-up showing the line visible in the mosaic.

Either layer can be selected for matching, but if it is probably best to choose the one which appears lighter or less contrasty (in this case the bottom layer). 

Select Image > Adjust > Levels or press Ctrl-L to open the Levels window.  Generally, simply adjusting the middle slider will match one image to the other.  Occasionally, tweaking the right or left slider will be necessary.  Remember that moving the middle slider left will brighten the image and moving it right will darken the image.  In this example the lighter-shaded layer was selected so the middle slider was moved slightly to the right to darken the image to match the other layer.

Above:  The Layers window adjusted to match the bottom half of the Rosette mosaic to the top half.

Above:  A close-up showing a seamless mosaic after adjusting Layers.

Cleaning Up the Mosaic

Zoom back out to see the entire mosaic image.  Select Layer > Flatten Image or choose Flatten Image from the pull-down menu in the Layers window.  This will create a single-layer image. 

Select the Crop tool and cut out the extraneous parts of the image that overlap along with the white background canvas.

Final Touches

Once you have pieced together the mosaic you can use the techniques outlined in the Aesthetic Image Adjustment section to enhance the image as you like.

Above:  The final Rosette Nebula mosaic.  Click on the image to view a higher resolution image (still about 3 times smaller than the original mosaic file).

Next, Advanced Mosaic Techniques...

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