Hi Mark,
Congratulations of seeing your first nebula. M42 is a stunning object -- certainly in the top three nebulae in the sky.
Quote:
Originally Posted by icytailmark
well i finally viewd the Orion Nebula in my new telescope!!! It was pretty easy to find. I used a UHC filter that i bought from DGM. However i was a little disappointed because i could not see any colour in the nebula.
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What size 'scope are you using Mark?
I can understand your disappointment Mark but you seem to be labouring under a very, very common misconception among newcomers to amateur astronomy. Probably a majority, if not a large majority of first time viewers expect to see an image like the ones in the books, on the P.C screen or on the box the telescope came in.
These images are made with cameras that are in
some ways better than the human eye
1.they can have their shutter open for long periods and collect light making the final image brighter and
2. They have different sensitivities to other wavelengths the eye does poorly with.
3. The image can be processed to enhance contrast and detail that would otherwise be buried in the noise.
However, the human eye is still, at least on a moment-for-moment basis, more sensitive than a CCD but unfortunately it is stuck in "real-time" -- it can't integrate an image. And the brain can't do the "tricks" done in photoshop to extract data out of the noise.
Most if not all the images you see in books and on the P.C have been taken with expensive and sophisticated equipment and frequently amount to several minutes -- if not several hours of exposure time over multiple exposures that are stacked over each other. The CCD that takes the image is very sensitive to the pink and red colours that emission nebulae shine at so strongly (the H-Alpha emission line) and that is why the images are so strongly colour oriented that way. Our eyes are pretty lousy at those wavelengths in low light-levels.
The eye has two different cells that do the work to make an image -- the rods and the cones.
In basic terms, rods only see black and white but are much more sensitive than the cones that see colour. Hence a lot of what you see through a telescope aimed at the night sky seems black-and-white. A lot of light is needed to make those cone cells work which is a prime reason we do not see colour well in the dark -- there isn't enough light present to get those lazy cones up of their bums and doing what they do. The most easily seen colour in low light levels is a greenish/cyan.
Our cones are conspicuously poor at the red stuff in low-light levels and we perceive none of the red-light as red. When the light levels are just barely high enough to get the cones going we generally see a greenish tinge first and brightest. This colour also happens to be one of the other wavelengths that the emission nebulae shine most strongly at -- the OIII and Hydrogen-Beta lines are smack in the middle of that green/cyan zone. Your nebula filter is isolating those wavelengths for you and blocking out the others.
With the broadband, narrowband and line filters like a Deep Sky filter, UHC filter, OIII and H-Beta filter it is these wavelengths that are isolated and
apparently enhanced because the other colours and light is blocked -- increasing contrast. You cannot see any other colour when using these filters -- for all intents and purposes relevant here, they only pass green/cyan.
Returning to the unfiltered view, obviously the larger the 'scope you use, the more light is available at the focal plane and the image is brighter. The brighter the image, the more likely it is that it will turn on some other colour receptors in your eye. But every human set of eyes is different -- either by genetics or environment we all seem to see colour a little (at least) differently.
I was lucky enough once to discuss this issue with David Malin when he did a talk at my club (SASI) and he claims (and has a very fixed view on it) that he could (and indeed we) see (should see) no colour at all looking at emission nebulae through the telescope -- no matter what size the telescope is. He claims there simply isn't enough light at the right wavelength to get the cones up off their bums and doing their thing. He has only ever seen M42 as shades of grey.
But some people (many people) plainly do see some colour (mostly greenish tinges) in emission nebulae.
For what it is worth, without filters, my old 8" showed a plain lime-green tint in most of M42 -- more weakly in M8 and M17.
The 10" showed it more strongly but no other colours. But, the 12" just started to show subtle hints of other colours in M42 -- in the "wings" under a truly dark sky I was beginning to see bluish overtones and occasional dirty pinks.
With the 18" many nebulae are greenish M42 is an intense emerald green overall but distinctly shows blues and a pinky russet in the wings under a dark sky. One thing I see (using my 18") often and pretty clearly in M43 (adjacent to M42) is a very deep, dark madder lake colour over fainter green.
I hope that helps to explain what you saw and perhaps lessens your disappointment -- we all have trouble seeing those spritzy colours in the astro-images. Even when the colour (other than green/cyan) is there it is always subtle and you have to be patient. Experience (longer the better) at the eyepiece also helps quite a lot to properly adjust the "expectations" ... If you have the right
expectations you are more likely to see it that with under/over expectations.
As for the other comments -- The Merope nebula (NGC 1432, -35, IC349) is the brightest part of the nebulosity around the Pleiades. It is a reflection nebula (not an emission nebula) and therefore has a
continuous spectrum -- like a star's light has a continuous spectrum. It isn't really hard to see but you need squeaky clean optics (to take out the scattered-light factor from bright Merope itself) and a really dark sky. I've seen it in big binos but
very faintly. It shouldn't appear to have a any real colour -- (whitish/grey/ashy?)
The Tarantula nebula (NGC 2070) has always looked a more bluish shade of cyan to me (tending towards prussian blue or cobalt) than other nebulae. I almost wet myself regularly there too ...
. In large 'scopes, this is the best emission nebula in the sky -- incredible detail.
Eta Carinae (NGC 3372) is a close runner up to the Tarantula in large apertures I think. M42 is third -- in large apertures. In 8" say, that order is probably reversed.
Best,
Les D
