Background

The image sensor of a digital camera, whether in a phone or in an astronomical instrument, contains an array of light receptors that only detect levels of light, not color. In order to convert this greyscale image into a color image, filters are used to select light over a specific range of wavelengths, then colors are assigned to the image created by each filter.

Astronomical images created from broadband filters are typically constructed using the chromatic ordering technique, in which the light from each filter is assigned a color based on wavelengths. The light transmitted by the filter passing the shortest wavelengths is assigned the shortest wavelength of visible light color in the image, and the process continues with each successive filter, so that the light passing through the filter that transmits the longest wavelengths of light is assigned the visible light color with the longest wavelength.

OpenStax Astronomy textbook links:

Visible light detectors and instruments

Electromagnetic Spectrum

Teacher Notes

1. Our investigations are designed so that students cannot proceed to the next page without answering each question. If you would like to quickly preview the entire investigation, you can create a free account and use the “logged in" mode on the Start page.
2. It may be useful to have a discussion with students, after they have completed the practice image, to clarify which options are available for creating a new image. This should occur on page 25, “Putting it all Together”. When students have selected their object(s) to investigate and form a question, they should draft a plan (as described in the investigation) for which filters and images they will need and how they will construct and analyze their image(s).
3. The information about how filters pass light on p. 4 of the investigation (including the interactive filter tool) is intentionally oversimplified. Filters are often designed to pass a range of colors, not just one. For instance, an orange filter may pass red and yellow light too.

4. Not all six filters need to be used to construct a color image. Students can decide which filters to use based on the suggested guidelines illustrated in the tutorial examples, and they can also turn off a filter if it doesn’t contribute to the desired result.