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View Full Version : Where's the colour?



TheAussieAstronomer
2009-Apr-19, 07:08 AM
When I view nebulae through a telescope, I'm somewhat disappointed that all i see is a bit of white fuzz. The Great Nebula in Orion is a prime example. Is the lack of colour due to atmospheric disturbance, light pollution or is it that the colour images are simply false colour images?

cjl
2009-Apr-19, 07:15 AM
A lot of it is just that they are quite dim, even through fairly sizable scopes. The portions of your eyes that see color require a lot more light than the portions that sense brightness alone, and as a result, unless you have a very large telescope, you will not see more than the slightest hint of color on most objects.

astromark
2009-Apr-19, 08:25 AM
Not all those images in 'sky and telescope' are doctored... With the aid of an motor driven guidance system a photographic image can be taken spanning hours of light capturing clarity. That is the secrete. Time of exposure. With the arrival of the CCD imager we can now build complex stacks of a single image. Thinking four images over two hours and then added to a single infrared image of a few hours.. the end image will be an impressive full color image that you as a astronomer can never see. Our eye is just not the best tool in the box for astronomy. We do not detect color until levels of light go up. I to share your disappointment as the Orion Nebular does come up short of expectations... remembering that your eye was never meant to see it and with just 4.5 mm appiture... can't. So you use a telescope that might have 200 mm of aperture but still see only a fussy dull white smudge. If you could capture that image and add to it some hours of other images, then yes a glorious red and blue smudge will fill your computer screen... thats just the way it is. Remembering that it is 1300 Ly away.

George
2009-Apr-19, 08:48 PM
Yes, as has been stated, the objects are just not bright enough to trigger responses in your color cones within your eyes. Our eye's rods give us greater resolution of the object, but they produce no color information for our brains to "see" color, so only degrees of gray do we see.

However, also stated above, large telescopes do allow some measure of color to emerge. Using an 87 inch at McDonald, I got to see a distinct bluish-white ring in the Eskimo nebula, and my son seemed to see it as much more blue than I did. Other objects also are bright enough to allow some color view of them.

By brightness, we are refering to surface brightness. This is the amount of light for any given area. Surpsingly, telescopes really do not improve the level of surface brightness since the larger the telescope, the greater the magnification, so there is not much, if any, gain in real surface brightness. If you use a large telescope and an eyepiece that produces a very low magnification, then the light coming out of the eyepiece, will not all get into your eye, and there are no tricks around this other than using longer exposures for sensors or film.

Also, the colors you do see in many images are not necessarily the colors you would see even if they were bright enough for our eyes to see them in color. Astronomers have a color scheme they often use to assist them in seeing what elements are active in these regions, instead of worring about color accuracy as we would see them.

George
2009-Apr-19, 11:06 PM
There are many common words that have different spelling but the same meaning. The words "color" and "colour" is just one example.

Regardless, insults and name calling are very inappropriate. :mad: Your time here will likely be short, or was that your intent anyway?

Tucson_Tim
2009-Apr-19, 11:20 PM
Regardless, insults and name calling are very inappropriate. :mad: Your time here will likely be short, or was that your intent anyway?

Yes. He made several posts, each one insulting. His last insulting post was in one of my threads. He's gone.

George
2009-Apr-19, 11:43 PM
His time was shorter than I thought. :) :clap:

Jeff Root
2009-Apr-20, 04:03 AM
Did he complain about "color" or "colour"? I love them both.

-- Jeff, in Minneapolis

George
2009-Apr-20, 04:12 AM
The now missing person claimed "colour" was a misspelling, then tossed in a verbal grenade to add his own warped color, but got collared instead.

Jeff Root
2009-Apr-20, 05:19 AM
Ah! Collard greens. Very chlorophyll.

-- Jeff, in Minneapolis

NEOWatcher
2009-Apr-20, 12:56 PM
...By brightness, we are refering to surface brightness. This is the amount of light for any given area. Surpsingly, telescopes really do not improve the level of surface brightness since the larger the telescope, the greater the magnification, so there is not much, if any, gain in real surface brightness....
Let me get this straight, because I think I'm getting hung up on some of your wording.
Surface brightness seems straight forward. I would probably add that it's for any given area of the object. But; that would only be added clarification.
But; I'm thinking that the concept of the larger telescope, and magnification isn't quite covering it. If the amount of magnification and the aperture size is consistant, I agree with your statement. But; holding one of those constant, wouldn't the result be an overall increase or decrease in brightness?
A higher magnification spreads the light out making it dimmer, while a higher aperture lets more in, making it brighter?

George
2009-Apr-20, 01:58 PM
If the amount of magnification and the aperture size is consistant, I agree with your statement. But; holding one of those constant, wouldn't the result be an overall increase or decrease in brightness?
I was quite surprised when I learned that we can not truly increase the surface brightness of an object like a nebula by going to larger aperature. There is a great thread on this issue (http://www.bautforum.com/space-astronomy-questions-answers/46471-surface-brightening.html).


A higher magnification spreads the light out making it dimmer, while a higher aperture lets more in, making it brighter?
Yes, and this is why the surface brightness of any region of a nebula remains the same if the aperture is increased. Since magnification (angular) is simply the ratio of the main and eyepiece focal lengths, one would think that a larger mirror with the same focal length as the smaller one would gather more light, and it does, but without the dimming effect of greater magnification. Thus, the object would appear brighter.

Unfortunately, this assumes one can get that light into the eye, and that is where the problem hits us. Using an eyepiece with a very large focal length will decrease the magnification but this also changes the exit pupil, which can easily get larger than the aperture of the eye. When it does exceed the eye's opening, then less light actually enters the eye, and the object would appear dimmer.

grant hutchison
2009-Apr-20, 04:54 PM
Yes. For extended objects, what you gain at the aperture you lose at the eyepiece, and there's no way to get around it without some sort of light amplification. You need to generate photons (in a photomultiplier), or collect photons over a time-period longer than the human eye uses (by photography).

However, we can see extended objects through a telescope which we can't see with the naked eye. Which is surprising, given that we can't increase the surface brightness of the object itself. This turns out to be a trick of retinal physiology: the threshold contrast for the detection of small objects is much higher than it is for larger objects. So if you make a distant galaxy appear bigger, you give yourself a better chance of picking it out though the foreground sky brightness, even though the surface brightness of sky and object are unchanged.

Grant Hutchison

George
2009-Apr-20, 05:11 PM
However, we can see extended objects through a telescope which we can't see with the naked eye. Which is surprising, given that we can't increase the surface brightness of the object itself. This turns out to be a trick of retinal physiology: the threshold contrast for the detection of small objects is much higher than it is for larger objects. So if you make a distant galaxy appear bigger, you give yourself a better chance of picking it out though the foreground sky brightness, even though the surface brightness of sky and object are unchanged.
Shooting from the hip, as usual :), I would guess that several factors come into play to explain this. One is that the high resolutioin central area (ie fovea) is mainly blind to most dim objects (scotopic levels), so the net effective photon flux for small extended objects suffer compared to these same objects when magnified. Also, using larger aperture will increase the total photon flux upon the retina, which seems to give at least a psychological advanatage in that it becomes more dominant to what is being observed.

With greater aperture comes greater resolution, so I wonder if this too does not improve what our mind thinks in terms of brightness?

Tim Thompson
2009-Apr-20, 06:12 PM
The Great Nebula in Orion is a prime example.

Yes, as has been stated, the objects are just not bright enough to trigger responses in your color cones within your eyes.
Indeed the lack of color is due to the natural constraint of human eyes. Your eye only looks at something for about 0.1 seconds before it reports the signal to the brain, and starts over. So you never get more than about 0.1 second's worth of light to work with. So even with the aid of the larger aperture of a telescope, you still have limited light to work with. But put a camera on that same telescope and you can hold the shutter open for as long as you like, the limit being when your camera film or CCD detector saturate. The 2006 HST panorama of the Orion Nebula (http://hubblesite.org/newscenter/archive/releases/nebula/emission/2006/01/) combines 105 orbits of HST time, which certainly amounts to at least a few tens of hours of exposure time, and additional imagery from the 2.2 meter telescope at La Silla. So the telescope cameras can add up light for tens or even hundreds of hours, compared to a mere 0.1 seconds for your eyes. Our eyes just can't compete with that kind of light collecting (and the Spitzer Space Telescope added a few hours of its own exposure time to the HST & La Silla images: Spitzer 2006 press release (http://www.spitzer.caltech.edu/Media/releases/ssc2006-21/release.shtml)).


However, also stated above, large telescopes do allow some measure of color to emerge. ...
The largest telescope I have looked through is the 60-inch on Mt. Wilson (which is available for rent (http://www.mtwilson.edu/60in.php), by the way). The Orion Nebula shows some color, though still not a lot. But the blue color of the Snowball, as mentioned by George, shows up well in the 60-inch too. So it seems clear, the way to see color by eye is to go for the really big apertures.

George
2009-Apr-20, 08:41 PM
The largest telescope I have looked through is the 60-inch on Mt. Wilson (which is available for rent (http://www.mtwilson.edu/60in.php), by the way). That's looks like a great program for any club. Get 20 people paying only $45 for half-night viewing through a 60 inch is a nice deal.

The McDonald Observatory also has viewing through the 107 inch, too, though driving out there is a chore.


But the blue color of the Snowball, as mentioned by George, shows up well in the 60-inch too. So it seems clear, the way to see color by eye is to go for the really big apertures. [Mine was the Eskimo nebula (NGC 2392).]