Hybrid Composite Imaging
Combing images from different cameras and telescopes results in stunning celestial vistas.
CCD technology is progressing exponentially. In the past decade chips have become larger and more sensitive. Thus, many astrophotographers (including me) find themselves turning new cameras and telescopes on familiar targets. On these return visits, I often find myself shooting images of the same object with instruments of different focal lengths than I used in the past. Longer focal lengths resolve smaller-scale features than shots made with widefield instruments. But my new images don’t always supersede my earlier work many times my earlier shots have either higher resolution or a wider field than is achievable with my current setup. I’ve developed a technique that takes advantage of both types of images to create pictures showing large swaths of the sky filled with intricate, small-scale detail.
In my article "Creating High- Resolution Color CCD Mosaics with Photoshop" (S&T: August 2003, page 130), I detailed my method of stitching together multiple frames to create a large, high-resolution image. This technique is well suited for expansive galaxies and nebulae too large for a single image to capture. Since my purchase of an SBIG STL-11000M camera, I can now cover much larger areas of the sky. By adding a few steps to my mosaic technique, I can also combine an earlier high-resolution shot with the wider-field image and retain the aesthetic appeal of both.
When I started in astrophotography, one of the most difficult obstacles I came up against in creating these types of images was melding multiple frames taken at different focal lengths into a single picture. In 2000 Auriga Imaging introduced the computer program RegiStar (S&T: October 2000, page 80), which revolutionized amateur astro imaging and provided me with a crucial tool for my complex imaging projects. The software allows me to register two or more images taken at different image scales so they can be used to create a single view, which I call a "hybrid composite."
This is an image made by layering separate frames taken of the same field using techniques that optimize various aspects of the scene, such as resolution, color, and field of view. I align the images using RegiStar and blend or layer them together in Adobe Photoshop. The classic example of a subject for a composite would be M42, the Orion Nebula. A short exposure of the Trapezium area would be layered over a deeper image of the nebula to retain the detail usually overexposed and thus obliterated in the deep exposure alone.
Probably the element most critical to success with complex imaging projects is proper planning. I often use charting programs such as Software Bisque’s TheSky to help me plan the number and overlap area of frames needed for a particular composite. TheSky even has a customizable template showing the field of view of my various imaging systems. This preparation is an essential exercise before I go out with the telescope. RegiStar allows me to align two frames with an overlap of as little as 3 percent, so I plot my mosaic coverage with this in mind.
There are also choices I can make that shorten the amount of work needed later. I sometimes have a great wide-field color image, so I just need to shoot a high-resolution monochrome image to add luminance to an area of particular interest. Alternatively, I may have an excellent highresolution shot and may need to shoot a wide-field RGB or hydrogen-alpha image to expand my coverage of the field. This greatly reduces time needed at the telescope.