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Data for LMC N 180B were obtained from WFPC2 observations taken in 1998 for the purpose of science. Filters that isolated light emitted by hydrogen and oxygen gas were used. Since only two filters were available from the dataset, it was a challenge to gain a large range of colors provides difficulty. This example of image processing is solely for artistic purposes, and obviously diverges from the arguments that surround the validity of the color in most HST images.

Creating a Two-Filter Composite Image

By Amit Kapadia (American University/Summer 2006 Heritage Intern)

Note to readers: This example of image processing is solely for artistic purposes, and obviously diverges from the arguments that surround the validity of the color in most HST images.

These are images created from the original data taken from the HST archive. On the left is N 180B taken with the F502N filter (oxygen) and the F656N filter (hydrogen). From these images there is not much definition in the structure, and the tonal range is limited. Using Photoshop FITS Liberator (Photoshop plug-in), we are able to see differentiations in these grayscale images.

The HST data are stretched using FITS Liberator. When working with only two filters it can be quite difficult to maximize colo and a direct composite using just the two filters may appear washed out and dull. One technique we use is to create a third filter by combining the two. A composite is put together using the two filters to make a third layer, which is the average of the two. Adjustments are made to achieve the best color/tonal ranges possible.

Here examples of three filters (oxygen, oxygen+hydrogen, and hydrogen)
after they have been stretched for maximum contrast.

Once we have three layers to work with, we can then add color to each layer. In Photoshop we add a Hue/Saturation adjustment layer, and assign blue, green, red to the oxygen, oxygen+hydrogen, and hydrogen filter images.

We then change the blending mode of each layer to "screen." We now have a rough color composite. Using other Photoshop techniques, we can maximize color and contrast differentiations. Adjustments are made to maximize the texture and detail in the image. Most of the adjustments are made to the entire image, but minor tweaks may be made to individual layers. Adjustments can be important when there exists exposure differences between filters.

Final adjustments are made to individual color layers or color selections to minimize noise, and bring out as much contrast as possible. Complementary colors serve to make the image appear more 3-dimensional. Final cosmetic cleaning on the flattened Photoshop file remove artifacts caused by the camera and CCD chips.