Visual Astronomy and Seeing Colours

Photo of Messier 101 vs what you see in the eyepiece in a small telescope.

“I’m looking at such-and-such an object with my telescope for the first time. I can see it, but I don’t see any colour. What’s wrong with my telescope? Do I need a bigger telescope, or a filter?”

This is probably the most common surprise for newcomers to Astronomy, or for visitors to the eyepiece. The beautiful colours you see in astrophotographs are only visible in astrophotographs. When you are observing “live” at the eyepiece of an amateur telescope, you see in black and white.

Comparison of colourful Hubble image of M42 with the gray wisps seen in an amateur telescope

There is nothing wrong with your equipment. The problem is with your eyes, and you can’t fix it.

It’s physiological. Your eye is simply incapable of detecting colour in very dim light. The eye contains two different kinds of cells for detecting light, called Rods and Cones. Cone cells can detect colour, but they are not sensitive to dim light. Rod cells are far more numerous, and can detect very dim light, but they do not detect colour.

Bright objects, like Saturn and Jupiter, are bright enough to be seen in colour. But galaxies and nebulae are far too dim. The colour you can see in the dimmest light is green, so some very bright nebulae such as M42 (the Orion Nebula) may show a slight greenish tint to some observers in good conditions.

So what about those beautiful photographs you’ve seen? The film or CCDs in cameras are very sensitive to light and, more important, are exposed for long periods, often many minutes or hours, allowing enough light to accumulate to detect the colour wavelengths. In fact, even most astrophotographs are not actually photographed in colour, in the sense of a single exposure producing a colour image like in your snapshot camera. Instead, multiple black and white exposures are made through different coloured filters, and the results are then combined in a computer to produce the coloured image you see published.

Typical amateur astrophotographs involved hours of exposure time, often over several nights, and many hours of computer processing. Publication-quality photos like the spectacular Hubble image of M101 at the top of this page, or M42 to the right, may have involved years of data acquisition and computer processing.

Summary: There is nothing wrong with your equipment. Visual astronomy is done in black and white. Don’t let this spoil your enjoyment — looking for these objects, finding them, and observing their structure is a wonderful pass time, even without the colour.

Some after-thoughts

  1. Did you buy a telescope with beautiful colour photographs of galaxies and nebulae on the box, implying strongly that you would see things like that when looking through the telescope? It’s misleading advertising, and reputable manufacturers don’t do it. But many disreputable manufacturers do. It’s probably the biggest single disappointment faced by newcomers, and the biggest reason people leave the hobby. This is why most texts and articles recommend not buying a telescope right away, visiting star parties, and getting the help of an experienced observer.
  2. While the two types of cells are distributed throughout the retina (the back of the eye, where the light lands), they are not evenly distributed. The less-sensitive Cone cells are clustered in the centre. That’s why “averted vision” works: for very dim objects, if you look slightly off to the side so the light falls on the more sensitive Rod cells, away from the centre of your retina, you will see more detail.
  3. “Oh, to see colour I need to do photography? Ok, I’ll get right to work on photography then.”

    Don’t, not yet! Be warned, astrophotography, of the type that produced those photographs you’re thinking about, is extremely difficult, often disappointing beginners enough to drive them away from a rewarding hobby. Enjoy your equipment with visual observation, read a lot, and build up to photography slowly. If you seriously want to produce good astrophotographs, be prepared to take years to develop the skills, and be prepared to spend more money — many thousands of dollars. Your first telescope may be up to the job of photography, but your tripod and mount, and possibly your camera, almost certainly aren’t.

One Comment

  1. An EXCELLENT post – and I’m glad I found it. I was searching the skies last night for M1, had a great deal of difficulty due to poor seeing conditions (Light pollution be damned!) and this really hit home.

    I was fortunate to view M42 the other night, and did sense some color (blue/blue-green/grey) but it was far more muted than photographs I saw taken the same night. I assumed it was optical, but hadn’t truly considered the ‘why’. Your write up makes it crystal clear! Thanks!

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