Seeing color in nebulae October 28, 2007Posted 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!