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What colors are visible through an amateur telescope?

When one looks at a nebula through an amateur telescope are colors visible at all or is the image just black and white? What size aperture is required to see color?

The colours that you see in astronomical images are sometimes misleading - they are not actually the colours that you would see with your eye, but represent different wavelengths of light that have been combined to produce the image. When you think about it, that's exactly what the colours we see with our eyes actually represent: each colour in the rainbow is light at a slightly different wavelength, with blue light having the shortest wavelength (or highest energy) and red light having the longest wavelength (or lowest energy).

Let's illustrate with an example. Have a look at this beautiful image taken at the Cerro Tololo Observatory of the Carina Nebula (credit: Nathan Smith, University of Minnesota/NOAO/AURA/NSF). If you look hard, you can see three basic colours in the picture: red, blue and green (all of the other colours are combinations of these). These three colours are not what you would see with a powerful optical telescope, but they represent the emission from three different molecules: oxygen, hydrogen and sulphur.

A powerful optical telescope "sees" the same nebula as in this photograph (credit: NOAO/AURA/NSF). Here the red is as you would see with your eyes, and comes from the oxygen in the nebula.

So don't let the colour in an astronomical picture deceive you! More often than not it is there to help you better understand the detail of the picture, but it doesn't represent the true appearance of the object.

Luckily, the ability to detect colours with an amateur telescope does not depend on the aperture size; if you can resolve the object, you will see its colour (you have to take the atmosphere into account, though...).

June 2002, Kristine Spekkens (more by Kristine Spekkens) (Like this Answer)

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