…continuedHybrid Composite Imaging
Step by Step
Once I’ve collected all my data, I calibrate each image with the camera’s software, applying dark frames and flat fields. I usually work on each of the images separately until I’m ready to combine them, balancing the color and stretching the data to display both the bright and the dim areas until I have two finished color (LRGB) images. I always keep my data in 16-bit mode until the very last step — once converted to 8-bit, information is lost that cannot be recovered.
I then bring the images into RegiStar to precisely align the individual frames. It’s important to register the widefield image to the narrow-field one, selecting the bicubic interpolation scheme. This will resample the lower-resolution, wide-field image to match the high-resolution version. In this way all the detail in the higher-resolution image is preserved and is just waiting to be displayed either in a full-resolution digital display or a large-format print. When I click the Register command in the Operations menu, the program prompts me to select the source image (which will be my wide-field image) and the reference image. The program will find common stars in each view regardless of image scale or rotation. Once the images are registered, the next step involves using the Crop/Pad Image control. There are several methods for cropping the image. Under Target dimensions, select "Trim to image data." This will create a resized and rotated wide-field image. I save this as a new file, since I don’t want to combine each image yet but rather will do additional work on the individual frames to remove gradients and contrast differences with Photoshop.
Merging the Two Images
Which version of Photoshop you use will determine your next step; Photoshop CS fully utilizes 16-bit data for layering files, while earlier versions of the program require you to convert images to 8-bit data before enabling the Copy and Paste options.
Open both images in Photoshop, select your narrow-field image, and copy it using the Select All and Copy commands. You can now paste it onto the newly registered and resampled wide-field version. To line up both images, open your Layers palette and change the top layer (Layer 1) to "Difference." This will generate what appears to be a solarized version of the top layer with the bottom layer showing through in places where the images aren’t aligned. I use the Move tool with the image scale set to 100 percent and manually shift the top layer until it cancels out the bottom layer. The image will then display areas that differ only between the background image and the top layer, the most obvious difference being the star sizes. I now change the top layer to display as "Normal."
The next step is the most challenging: making the images match in contrast, brightness, color, and star sizes. I frequently need to do some selective processing of certain regions to blend the data. There are many ways to do this, and I find the strategy varies with each project. Experiment with different layer-blending modes, such as Normal, Lighten, Darken, Color, and Luminance. In a given composite, usually one of these modes works best. It may take several layers of blending, sometimes even using different blending modes on each layer.