Why do we need filters?
Different gases emit or absorb different wavelengths (colours) of light. Astronomers use a variety of filters to study features of the objects they observe at different wavelengths.
Most astronomical CCD detectors are colour blind – they cannot distinguish different colours. To create colour images, at least three images, made through red, green and blue filters, need to be combined. In practice, astronomers use dedicated filters that only transmit certain wavelengths of light, chosen for their scientific relevance. For instance, glowing hydrogen stands out when observed at a wavelength of around 656 nanometres. Using filter wheels, it’s easy to change observing wavelength, selecting one filter after another, even at infrared or ultraviolet wavelengths.
Have you ever looked at the world through a piece of red cellophane? Everything looks very different! In the same way, astronomers see different things when they look through coloured filters.