Image colors

[quantum64]

I'm sure this has been asked before, but I couldn't find any posts related to it.

Many of the deep space images I have seen on this forum are full of color, which I realize is probably a result of filters being used to view light outside of the visible spectrum. Looking at DSOs through a telescope, I've heard there is little to no color. Is that correct?

My real question is, suppose we were much closer to these objects, would they still be just as colorless (in our spectrum of light).

[FraserClarke]

No, most of the filters used are in the visible range (H-alpha, which is a very common line used in these images, is right at the very red end of what we can see with our eyes -- some people can see it, some can't).

The reason you can't see any colour visually is because your eyes are not sensitive enough. If you look through a big enough telescope (or are closer as you suggest), there would be enough light to activate the colour sensitive cones in your eyes.

[SteveP]

Quantum

Usually the colours you see depicted in conventional DSO images are 'true colours'. You might sometimes see false colour images, eg the gold/blue/turquoise combos, which might be referred to as Hubble palette or narrowband which are designed to bring out the gaseous composition particularly of nebulae.

So why can't you see these objects in true colours?

First of all, these objects are either very distant or very dim. Even the relatively close objects are still a vast distant away

Secondly, at night, your eyes use the more sensitive part of the eye, known as 'rods' to see in the dark. Although these rods are more sensitive than cones which are better designed for daylight seeing, they are not very good with colours.

Finally, the colour images you might have seen could well have total exposures of several hours. Unfortunately human eyes in effect have a refresh rate of say 30-50 frames a second (which is why we can see movement so clearly) and the brain cannot combine several frames the way camera technology can .. even when staring at the object!

Even with very large amateur telescopes and great viewing conditions, colours are barely discernible to the human eye. But then IMO the joy of observing these objects is much greater than just seeing multi coloured objects. To answer your particular hypothetical question, if we were much, much, much, much closer to some of these objects, then yes, you would see them in colour .. and possibly in daylight too.

HTH

Steve

[simon hicks]

Quantum64,

Its just that these objects are very faint. If your eyes were 1m in diameter your would see lots of colours....the night sky would be full of colour just by looking up.

That would be cool. But we'd need extremely strong sunglasses during the day.

[Sparrow]

An excellent explaination by Steve.

As an ardent imager I have to agree that there is still a real thrill to putting your eye to the eyepiece and seeing these DSO's for real - even if they are only a grey fuzzy blob.

[ollypenrice]

There is a third kind of colour image widely seen; many of us take a natural colour image and try hard to get the colour accurate. But then we use a Hydrogen alpha (and sometimes other) filter to bring out the structural details and light that nebulae reveal as they shine in their own light. We then add this to the image, trying (with more or less success!) not to let it change the colour balance too much.

Such images will be presented as HaRGB or HaO111RGB to make clear what the image is showing.

If you could make a very large scope with an incredibly fast f ratio you could expect to see colour but the fastest visual scopes are only around f4, sad to say.

Olly

[quantum64]

Great explanation everyone. Appreciate it!