Advanced Techniques

Rarely is blending multiple images together as easy as adjusting Levels as we saw in the last section.  If the individual images are taken in different conditions, such as on different nights, there will often be greater variance from image to image.  Varying levels of background skyglow, changes in seeing conditions, and differing amounts of atmospheric extinction all play a factor in the difference between individual mosaic images.  Getting the images to match is very much an art, but Photoshop offers many tools to make it possible to make flawless mosaics.

Advanced Alignment Techniques

By far the easiest way to align mosaic images is using an automated alignment routine in a program like MaxIm DL.  Although a bit more work is involved, aligning images is also possible in Photoshop.

Below are two frames of the nebula IC1848 in Perseus.  Each image is a 6x3 minute exposure through an H-alpha filter.

Above:  Top and bottom halves of IC1848

Overlaying the images using the techniques from the previous section, it can be seen that not only are the images misaligned in the x and y axes, but there is also a rotation factor.  One image is rotated with respect to the other.  This can be due to a number of factors, including field rotation from a less-than-optimum polar alignment or from camera rotation, especially if the images are taken on separate nights.

Above:  Overlapped sections of the two frames showing rotation

Select Edit > Transform > Rotate or press Ctrl-T to bring up the Transform menu.

Above:  The Rotate field in the Transform menu

Use the Rotate field to adjust the angle of one layer to match the other.  Positive angles rotate the image clockwise, negative angles counterclockwise.  It usually doesn't take much adjustment to cancel out rotation.  In the above example, the image was rotated 1.1° clockwise.

The best procedure for determining rotation is as follows:

• Align a reference star using the Move tool (the bright star in the lower left of the image above was used as a reference)

• Select a rotation amount, say 1.0°

• Click on the image to activate the Move tool

• Use the arrow keys on the keyboard to shift the image to realign the reference star

• See if the rotation was too little or too much

• Rotate again if necessary

• Click on image again to shift position

• Continue until rotation is eliminated

Other adjustments can be made using the Transform menu, including Scale adjustments which are not normally needed but may be useful under some circumstances.

Using Layer Masks to Blend Mosaics

Layer masks are ideal for blending images.  They allow a smooth transition from one image to the next, eliminating the abrupt change from one frame to the next.  You also have a lot of control over the blending of the images.

Above:  Obvious seam between two frames

Select Layer > Add Layer Mask or click on the Add Layer Mask button in the Layers window (second button on the bottom).

Above:  New Layer Mask on top layer

The next step is to create a gradient in the layer mask.  This determines which parts of the image show through the mask and which are blocked.  Areas of the layer mask that are white allow the image to be visible, and black areas of the mask hide the image.  Grey areas will allow varying amounts of the image to show through.

Press D to select the default colors (white foreground and black background).  Select the Gradient tool (keyboard shortcut G).

Note the position of the top of the background image (Layer 2 in this example).  Click and drag from the top of the background layer to the bottom of the active layer.  This creates a gradient from white to black across the overlapping areas of the two images.

Above:  Vertical line shows the Gradient tool being dragged from top to bottom.  The length of the line corresponds to the overlapping area of the images.

Above:  Layer mask with gradient

Above:  Overlapping areas.  Notice the seam has disappeared.

Often you will need to combine Layer Mask techniques with the previous technique of adjusting Levels to get perfect results.  Every mosaic will be different, hence this is as much art as science, so experiment to find the best results for each image.

Above:  Final mosaic of IC1848.  Click for larger image.

Next, Color Mosaics....

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