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Seeing color in nebulae October 28, 2007

Posted by dorigo in astronomy, internet, personal, physics, science.

An interesting debate arose recently on an italian forum I sometimes browse. Can we see color tones in extended objects with telescopes ? Which objects, with which scopes ? Which colors do we see ?

The matter is intriguing, because it encompasses several issues, from the physics of light emission in excited gases to the optics of light collection by astronomical instruments, to the physiology of light detection by our retina, to the decoding of the information by our brain. But it is even more intriguing because opinions differ wildly, with even the most experienced observers sometimes retreating into defending undefensible positions, as they claim to see color in objects whose surface brightness is hundreds of times too faint to allow any color detection. Or groups of amateurs convinced that they were seeing blue and yellow tints in the following object, visiting a 80cm observatory instrument [here the nebula, NGC2392 -also known as the “eskimo nebula”, a planetary in Gemini, is reproduced from a picture whose authors took extreme care in making color reproduction as faithful as possible]:

Blue, fine. But yellow ? And yet, there were a dozen amateurs, and all agreed on the yellow in the outer regions… A mass hallucination ? Or rather, had somebody seen previously the following, more famous, picture taken by the Hubble (in FALSE colors!) ?:

Nice yellow in the outer shell, indeed. I imagine one observer with this image in his or her mind getting to put his or her eye close to the eyepiece of the 80cm instrument with big expectations, and then seeing just what he or she expected, fooling self: blue in the inner, easier-to-see globe, and yellow in the fainter outer halo – a feature which small instruments do not even show. And I imagine the suggestion being passed to the other observers gathering around the instrument, as a quick virus: a second weak mind says “Well, I’ll be damned, It does look yellow“. And then a third, and a fourth, and the others feel encircled already, and they yield after being granted a second look, under threat that their eyes be deemed “untrained”. Building consensus with the bandwagon effect. Am I too much of a skeptic ?

Well, the thing is, our brain’s color perception is due to cones in our retina, whose signals are then interpreted by a very complex system. See some interesting reading material on the issue

Then, as far as the main suspect is concerned, we do know our brain plays real dirty tricks. Check examples 1 and 2 in this very nice set of examples, for instance. In the examples, the dominance of a tone of colour in the background completely fools our mind into assigning color to what is uncolored. Or check this other example on the right: do you see a different tint in the purplish squares of the two swaths ? Surely, the purple squares bordered by green squares are a different tone. Or not ?

Well, they actually are of the very same tint, as is clear by looking at the squares enlarged (check picture below). Our brain plays tricks because it is trained to interpret messages that are sometimes contradictory, incomplete, or faint. Actually, the faintest a stimulus is on the cones, the easier it is for our brain to assign a color incorrectly, as is easy to check by opening a drawer to pick socks of any given color in a insufficiently lit room.

The critical measure for color perception is indeed, unsurprisingly, surface brightness. Cones tend to perceive light and attempt a color assignment to objects until these have a brightness above magnitude 16 per squared arcsecond; from then on, our vision starts relying solely on rods, it becomes scotopic, and grey is all we see.

Fine“, I can hear some say, “then the trick is just to get a larger instrument: if all what matters is surface brightness, then there is a minimum telescope diameter that allows to go below that threshold for any given object“. That is common wisdom – people imagine that a galaxy looked through the Keck telescope looks as bright as a street lamp – but it is, surprisingly, FALSE. Surface brightness does not increase with the light gathering ability of optical instruments used. For an optimal exit pupil (that is, one matching the width of your dark-adapted eye: 7 millimeters, if your eye is young, or rather 5 if you are middle-aged) the required magnification of larger diameter optics is higher, so the total light is spread over a larger apparent diameter. And if you lower the magnification, then the exit pupil becomes larger than 7 millimeters, and you are to all effects diaphragming your optics!

Unconvinced ? Let’s make an example. Take a 100mm – diameter refractor – a very common size for premium apochromats in the market. Let us say a f/5 instrument, that is 500mm of focal length, although that is irrelevant (it only affects the optimal eyepiece). To get the optimal 7mm exit pupil, you have to then magnify by 100/7=14.3x, so you use an eyepiece with a focal length of 500/14.3=35 mm. Let’s then take a nebula with a apparent magnitude of 9, spread over an area of 1000 squared arcseconds – numbers roughly corresponding to the planetary shown in the pictures above. The mean surface brightness equals about 16.5, let’s say from 16 to 17 depending on the spot you are looking at. Now enlarge the area by 14.3 times, and increase the light output by a factor of (100/7)^2, the ratio between the area of the telescope lens and the dark-adapted eye. Surprise, the surface brightness has not changed, since it got multiplied by the light collection factor (x200) and divided by the increase in apparent area (/200). Worse than that, in fact, since the telescope’s lenses have absorbed a small but not insignificant fraction of the light…

Now, take a 10 meters instrument. To converge the huge number of photons into a 7mm pupil you need the not trivial magnification of x1430. The nebula still fits an eyepiece field of view (30″x1430 means about 12 degrees across), but the area has the same brightness per squared arcsecond!  

Then why is it that larger instruments allow you to see fainter fuzzies ? It is because the fuzzies become larger at optimal exit pupil, and the eye recognizes much more easily low surface brightness objects if their apparent size is larger – a fact that was well studied with data collected by the military during WWII!

The above is all theory. Now, there are things called sky quality meters that allow to measure the surface brightness of extended swaths of sky at night. Take one of those and aim them at clouds dimly lit by street sodium lights: you see them white-grayish if the surface brightness is 16 or lower. But if you take a picture you will notice they are, in fact, orange! Our color perception below the 15-16 threshold is totally messed up. And still, there are respected amateur astronomers, who even write on magazines, swearing they saw pinkish hues on objects with surface brightness of 20-22 (such as the Veil nebula)!

Now, I had the luck of seeing the Veil nebula through the Lick 90cm refractor (see picture). A wonderful instrument! And the Veil nebula was a glorious sight. But it was grey, grey! Not pink! A friend of mine summarized the claim of seeing color in objects as faint as the Veil by saying it is like claiming one can jump a kilometer, or run 100 meters in 0.1 seconds. In fact, between 16 and 21, there are two orders of magnitude in surface brightness, or a x100 decrease. Not something the physiological differences between human beings can explain!

The bad thing when people diffuse incorrect claims on the observability of details such as photographic color on faint nebulae in large instruments is that they lead their readers to believe they have the wrong instrument in their hands: bigger is better, so surely, if I buy that large apochromat, I will have to pay a loan, but I will get to see as in this picture ?

People who love astronomy and who are amazed by the night sky’s wonders should start with a pair of good binoculars. No need to break the bank!



1. Louise - October 29, 2007

Many Space photos published in magazines have the colours enhanced. Sometimes that is very useful, as when infrared, UV or even radio data translated into visible wavelengths.

Good that you take the trouble to look in the telescope. It is amazing how many of today’s astronomers never look through an eyepiece. I have fond memories of seeing the Ring Nebula through that 36-inch at Lick. That crazy moving floor is fun too!

2. Guess Who - October 29, 2007

Re. “Building consensus with the bandwagon effect.”, remember Asch:


3. dorigo - October 29, 2007

Hi Louise,

You are right to point that out – putting false color in pictures has a scientific reason, and it is not just to make them fancier. It is a point I had not made in the post.
So you’ve been to the Lick yourself! Fantastic place, really.


4. changcho - October 29, 2007

Hey, you were viewing the 36″ refractor at Lick! Good for you. Did you get visit Lick’s tomb? I did, in a dark and stormy night several years ago as a student. I should mention you were very close to where I live (well, whithin ~50 km).

I should also mention I’ve never seen color in any deep sky object when viewing through the telescope. And, the biggest telescope I’ve seen through is the refractor at Lowell Observatory a few years ago; the Ring Nebula looked gray; nice but gray.

5. Quasar9 - October 29, 2007

Hi Dorigo, interesting post.
Some earlier images here and here.

Hubblesite tells us that “The nebula’s glowing gases produce the colours in this image: nitrogen (red), hydrogen (green), oxygen (blue), and helium (violet).”
Of course what they mean is that these colours have been rendered or applied to the ‘spectometry’ of those gases.

APoD tells us “The outer disk contains unusual light-year long orange filaments. (not yellow).

6. Quasar9 - October 29, 2007

I like the way Chandra breaks down “composite” images into its constituent parts.

7. Quasar9 - October 29, 2007

If they’d called it The Clown Nebula from the beginning
do you think people would have realised hubble had painted the Eskimo’s face with loud colours.

8. dorigo - October 29, 2007

Hi Changcho,

remind me, the Lowell is what, 80cm or so ? Then we have another testimony of gray nebulae staying gray in larger instruments 🙂

I observe M57 almost any time I am out with the 16″ dob during summer months, because I know perfectly well the magnitude of faint 15th and 16th magnitude stars around, and it allows me to check the visual limit (I usually stop at 16.1). Never have I seen hues in it… Although maybe I am using too much power. But really, M57 is on the wrong side of the edge of brightness for color detection.

Hi Quasar9,
thanks for the image links. Yes, orange is more appropriate. I wonder if the dominant figure in the gathering I mentioned looking at NGC2392 through the 80cm reflector had colored glasses on 🙂


9. carlbrannen - October 30, 2007

I hope that you keep up these sorts of posts. These are things I did not know about deep sky objects. I’d always assumed that the bigger the scope, the brighter the image, but I guess this is only true for point sources.

This makes me recalibrate myself in terms of understanding just how bright these objects have to be. They have to be brighter than amount of light that I need to see color at night. That’s a lot brighter than I intuited.

10. Peter Vercauteren - October 30, 2007

Hmmm… Some things are true in what you say, like how easy it is to make people “think” they see something. But I have to strongly disagree to others. It is perfectly possible to see colour in the brighter nebulae (Orion, Lagoon, Swan, Dumbell etc) as from an aperture of about 8″. In my 18″ scope I clearly see the (unfiltered) Orion nebula as blue-green with even a faint reddish tint near the brightest edge. In fact, until I bought my 18″ I had never seen this red hue before so wasn’t expecting it at all. This means that my observation was totally unbiased. I have been observing for 26 years with all kinds of telescopes so I know very well that I shouldn’t compare a visual observation with a photograph.

The issue about the exit pupil is also correct. But nevertheless there is a huge difference between an 8″ and 18″ telescope, not to mention a 25″. So your comparison with a 10m instrument was a bit over the edge. In fact, by following your reasoning you could argue that you could just as well buy a pair of binos because you won’t see anything more in a big instrument, apart from what the higher magnification allows you to regarding small detail. This is also over the edge, I know. But it just shows how careful you have to be with numbers.

And BTW, I have never seen any colour in M57 either. The Blue Snowball, however…



11. dorigo - October 30, 2007

Dear Peter,

the conclusions we have drawn from experimental observations of objects in dim lighting have convinced me and a few other visual observers that at magnitudes fainter than 16/squared arcsecond the color one may see is totally due to an illusion by the brain: it is inaccurate to the limit of totally random.

Some of the experiments are easy to perform if you have a sky quality meter: for instance, point it to clouds at night from an average polluted sky, determine the surface brightness of the clouds, and see if you can detect the orange hue they have due to sodium lights. Then take a picture, which will show them in orange.

Other experiments are also easy. Take out of a drawer a few socks at night, after being dark-adapted. Have somebody increase gradually the light filtering from outside so that the reading of the SQM is in the range 18 to 14. Take note of the color of eack sock at 0.5 magnitude increments. You will see that the original colors you attributed to the socks were random, and that they become correct only when the lighting becomes high enough.

Seeing green-blue in low surface brightness nebulae is not uncommon because green is the light our eye is most sensitive to, and our brain thus tends to interpret insufficiently lit surfaces as having that tone. Seeing red is very, very hard because red is not a color our eye responds to easily.

I think of the list you provide, only the snowball and the inner part of M42 and the Lagoon allow to see color. Test for yourself with socks and a SQM… The math of surface brightness is easy once you remember that the scale is logarithmic.

About larger instruments providing a better view, besides magnification it is due to the fact that our eye is more sensitive to larger areas (which stimulate our rods more) in conditions of low contrast. The surface brightness of an object does not increase with aperture!
Please check the link above to the research with military data. It is “enlightening” 🙂


12. Peter Vercauteren - October 30, 2007

I’m still not entirely convinced, although I agree that the kind of colour you see can be deceiving. But this does not do away with the fact that you can indeed see colour on objects with a high surface brightness. And I have seen clear blue-green colour in all of the objects I mentioned and I still forgot e.g. the Cat’s Eye. What I’ve said about the reddish hue in the Orion Nebula, this has been confirmed by many independent observers with similarly-sized or larger telescopes.

Furthermore, how do you explain that many bright nebulae turn definitely blue-green when seen through an UHC or OIII filter? This rules out the argument that under total darkness colour perception of the human eye is non-existent. If this were true, you would still only see these nebulae as grey, even though the filter will only allow blue-green frequencies to pass. And when I used the filter for the first time, I wasn’t prepared for the contrast increase regarding colour so I was also unbiased there.

Cheers and… interesting discussion anyway, 😉


13. dorigo - October 30, 2007


don’t forget the bandwagon effect when discussing barely detectable details among visual observers…

The fact that you may interpret as green something seen through a filter only says that the eye+brain system tends to attribute the green tone to colors it can’t decode. This is because green is the frequency to which our detection system is most sensitive.


14. dorigo - October 30, 2007

In any case, see the interesting article here for a discussion of color perception. It is in fact unsurprising that there are different opinions on the matter of perception of color in dimly lit objects… What IS surprising is that we can in fact agree on what is yellow and what is red!


15. changcho - October 30, 2007

Tomasso – We observed through the big refractor, which I think is 24″ (60 cm) aperture. This was for a family “pilgrimage” to Meteor Crater, in Arizona, since impacts are an interest of mine. Since Lowell Observatory was really close in Flagstaff, we decided to visit it as well.

16. dorigo - October 31, 2007

Hi Changcho,

thanks! My name is Tommaso btw…
Is it straightforward to get to put your eye on the instrument or was it organized somehow ? Is there some waiting list ? I once went to Yerkes but in the end they did not allow even to look at the instrument.


17. changcho - November 3, 2007

Ah, sorry Tommaso, I knew one of the letters in your name was doubled, it’s just that I got the wrong letter!

Yes, the viewing trough the Clark 60 cm refractor was organized, as it was part of a somewhat late-night tour of Lowell. If you’re in the Arizona area, and near Flagstaff, I strongly recommend you visit; Flagstaff is a very nice town as well, and the geology of Arizona is fascinating. And of course, make sure to visit nearby Meteor Crater (which is the real reason I went to the area in the 1st place).


18. dorigo - November 4, 2007

Ok, I will keep that in mind… I was in Arizona 15 years ago, and it is unlikely I will be there again any time soon… But one never knows!


19. mike clemens - March 8, 2008

This is a narrowband image of the eskimo nebula, those colors dont reflect reality at all. I think the true blue presentation is what your eyes might pick up on, if you’re lucky.

20. dorigo - March 8, 2008

Dear Mike,

which one are you referring to ? The first one, according to the authors, was done with care in order to reproduce the spectral response of the human eye. And it is not a question of being lucky: the nebula has a surface brightness insufficient to pick up color.


21. mike clemens - March 22, 2008

Yes the first one shows the colors a single shot DSLR would record for example. Do you know the size of this object? I have not seen it yet in my telescope but I want to try before the Sun comes back to Alaska.

22. dorigo - March 22, 2008

Hi Mike,

as far as can I recall it is of the order of 30-40 arcseconds across. A internet search for NGC2392 will solve any doubt, but I am blogging from a very low-band site and so you’ve better do it yourself…

In any case, the object shows well at medium to high power. I advise at least 150x.


23. mike clemens - April 4, 2008

This page inspired me to find this object. I did go after it with my telescope, and I will says it is a beautiful object in an 8″ refractor. I shot about 1000 seconds of the object. Unfortunately my camera cant help with your color topic!

24. mike clemens - April 4, 2008

This object is 20″ across by the way. My name links to the picture.

25. dorigo - April 4, 2008

Hi Mike,

great shot! Your picture shows the full outer ring with lots of detail. An 8″ TEC refractor is a real beauty of an instrument, contgratulations!


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