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Someone asked me this question:

To see the color of the Great Nebula in Orion, how large the aperture of the telescope should be.

I don’t even know whether this is possible. I understand that the colorful patterns of the nebulae are from the cameras. Besides, my own experience shows that when I target the telescope at them, I see nothing but some white blur.

So is this possible when we watch the deep sky objects with a really huge telescope, under the finest conditions?

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Very large (for amateur) scopes in very dark places, with healthy eyes on a good night have credibly reported seeing color in nebula. The Orion is about as good as it gets, lots of photons, my eyes are not particularly good, but have been out with others who claim noticing color in the Trapezium in a thirty-ish inch reflector

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YES and NO.

YES, you will get "color" of nebulae. Reason of this is that because of very low luminosity for small diameter telescope, your eye is using "grey vision". When luminosity will reach certain level, your eye will start adopt color receptors. Good luminosity levels begin from diameters above 200mm.

NO, that "color" which you may observe does NOT have anything in common with colors which you see everywhere. Because most nebulae emitting in near-infrared and ultra-blue regions, which when mixing, give "greenish" light. But this color is result of your brain mixing different wavelengths, not actual green wave. Some people see it green, others see "pink" etc...

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I have been able to see color in the Orion nebula (M42) with my 16 inch dobsonian from a heavily light polluted city. This is only in the center (trapezium area), where I can see a reddish color. Interestingly, it's much more difficult to see this red color from a dark location. Several other people in my astronomy club have also noticed this, and one explanation that I got was that the cone cells are still active in a light polluted city, so I'm able to notice the red color ( I don't know whether it's true). From a good dark location, I've seen color on many planetary nebulae such as the Ghost of Jupiter. You usually see these are greenish in color, if you have a suitably large scope. With my 16 inch, I've seen color in a small number of bright objects (almost always limited to greens), but most objects don't show color. Having observed in instruments ranging from 2 inch to 18 inches, it certainly seems like a larger aperture allows you to see color in a larger number of objects. In my personal experience, I've only been able to notice color in the brighter objects in scopes 16 inches and above.

Disclaimer: I don't have any mathematical relation for aperture v/s color visibility; it appears to depend heavily on the observers' eye.

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Yes, it is, and that is the reason why telescope has been getting larger and larger. You need a good telescope to be able to see a distant object bigger in an image. When the object is bigger in the image, the light will spread out following the real shape of the object, and instead of seeing a white blur spot you will see detailed colour. Think of a car approaching you from a far.

To be precise, what you need is a telescope with good "angular resolution," which is equivalent to the "aperture" size in this context.

So, if you know the physical size of an object and its distant, you can work on some math to estimate the minimum aperture that your object will be spatially resolved.

Check https://en.wikipedia.org/wiki/Angular_resolution.

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    $\begingroup$ Sorry, but I am still a little bit confused. How does this explain the color in the photographs? As far as I know, the angular resolution of a telescope is $\theta \approx 1.22\frac {\lambda} D$. It is a constant value regardless of what is behind the telescope, either an eye or a CCD camera. $\endgroup$ – fantasie Dec 22 '18 at 12:11
  • $\begingroup$ So, the case of a car approaching, you start from seeing an object as a point without clear details. The color you see is the accumulation of every part of the object. Therefore, you see the dominant colour. For the case of your nebular, the accumulation of all colors results in white color when your brain processes it. If you see other object that has different combination of wavelength, let's say Mars, you see red-ish. When the object gets larger, you can see each part clearly of what color is dominant. Like, you see dark spots on the sun because you can make it large enough. $\endgroup$ – Kornpob Bhirombhakdi Dec 22 '18 at 14:31
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    $\begingroup$ Note that you also need color filters to include only certain colors when you observe. Then, when you re-assign the RGB color when you re-construct the image. $\endgroup$ – Kornpob Bhirombhakdi Dec 22 '18 at 14:32
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    $\begingroup$ The main reason why you don't see colors is that your eye's rods (that see only brightness difference) are much more sensitive than the cones (that see color). Same reason why you stoip seeing color at twilight, and the physical origin of the saying that "in darkness, all cats are gray" $\endgroup$ – Thriveth Dec 24 '18 at 0:33
  • $\begingroup$ I wanted to comment as the two comments just above. Which I have "+1". $\endgroup$ – Alchimista Dec 24 '18 at 10:23

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