The Hybrid Image Why Attempt It

There are certain challenges unique to astrophotography. Astronomical objects have both large- and small-scale detail. They often have an extraordinary dynamic range with very dim and very bright areas in the same field. Many objects have emission properties not captured by conventional filters. Images have much greater impact if the object is viewed in its environment. Therefore a large field size is highly desirable. The last point is an important one as the most powerful astro-images evoke the vastness of our universe and possess a three-dimensional quality. This can only be achieved with highly resolved detail spread over a large field showing the object within its setting.

The technique of creating hybrid images answers these challenges in the following way. Small- and large-scale details are recorded using appropriate focal lengths - long focal length for small-scale detail, intermediate and short focal length for large-scale detail. We want to achieve large field size without sacrificing resolution, so mosaics play a key role. We capture both dim and bright areas with equal detail by using appropriate f-ratios and narrowband (hydrogen alpha) filters if needed. We then have the difficult task of composing all the above into a single unique image.

When I started astrophotography, one of the most difficult obstacles in creating these types of images was the inability to incorporate multiple frames, taken at different focal lengths, into the same image. In 2000 Auriga Imaging brought out the software program RegiStar. RegiStar essentially revolutionized amateur astro-imaging and provided the tools to go forward with creative and complex imaging projects. RegiStar allows the imager to register two or more images taken at different image scales so they can be used to create a single unique image (see Figure 10.1).

Figure 10.1. The working space of RegiStar allows for easy registration of frames taken at different focal lengths, for later compositing.

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