Color Images

Abstract

We’ve been working with images in “white” light. It is possible to create color images as well. The process is a little bit more complicated than just white light, where shades or gray are determined simply from the number of photons counted on each pixel. “Pretty colors” require images of your object in at least three filters (green, red, and blue). At Mt. Cuba, we use the Johnson filter system, so our typical filters are u (ultraviolet), v (visual), b (blue) and r (red). The U filter only allows ultraviolet light (around 3500 Angstroms) through, the B (blue) filter lets light around 4500 Angstroms through, the V (visible, also called green) filter lets light around 5500 A through, and the R (red) filter lets light around 6500 A through. Image file names will contain the appropriate filter letter in their names. An image taken with no filter (usually includes “nf” in the file name) helps improve the detail in the final image. Calibrated color images also require flat fields taken in each filter. The three different images are calibrated, using the corresponding flat taken with the same filter, and then combined to create a final color image. The steps below outline the reduction process. This information is also covered in the book. I don’t know what page, but make sure you read the relevant pages before attempting this exercise.