by Ann » Thu Mar 07, 2013 1:28 pm
You are mostly but not completely right, I think, starsurfer.
Certainly almost any nebula in space is too faint for us to spot the red color of Ha. I don't think there is any nebula at all in the nearby universe that is bright enough to allow us to see its Ha light. The reason for this is that the red Ha line falls in a part of the visual spectrum where the sensitivity of the human retina is really quite low.
However, a good broadband red filter will certainly detect some red Ha light, there is no question of that. David Malin, who photographed the sky through broadband red, green and blue filters, had no difficulties photographing a lot of Ha. But you will no doubt detect more Ha and more structure with a good narrowband Ha filter than with a broadband red one.
There is also no reason why good broadband green and blue filters should not detect quite a lot of OIII light. Moreover, you may spot the green color of OIII light with your own eye with the aid of a telescope, if you look at some of the more colorful planetary nebulae. The planetary known as the Blue Snowball, which to some people looks like a green snowball, is an example.
As for Thor's Helmet, I myself doubt that its surface brightness is high enough for humans to spot color in it, no matter what kind of telescope we use to look at it. However, I note that David Malin has taken a picture of it, which can be found in his book, A View of the Universe. In David Malin's picture, the blue-green "bubble" is rather whitish, though with a definite hint of blue or blue-green. The blue parts of the "horns" of Thor's Helmet are barely visible in Malin's image. Instead the horns look red or pink.
If I compare Malin's image with todays APOD, I'd say that today's APOD is much more colorful. I'd say its colors of Martin Rusterholz's image have been enhanced, but not distorted.
If our ability to detect color in faint extended nebulae were many times better than it is, then Thor's Helmet might actually look to us the way it does in Martin Rusterholz's image.
Ann
You are mostly but not completely right, I think, starsurfer.
Certainly almost any nebula in space is too faint for us to spot the red color of Ha. I don't think there is any nebula at all in the nearby universe that is bright enough to allow us to see its Ha light. The reason for this is that the red Ha line falls in a part of the visual spectrum where the sensitivity of the human retina is really quite low.
However, a good broadband red filter will certainly detect some red Ha light, there is no question of that. David Malin, who photographed the sky through broadband red, green and blue filters, had no difficulties photographing a lot of Ha. But you will no doubt detect more Ha and more structure with a good narrowband Ha filter than with a broadband red one.
There is also no reason why good broadband green and blue filters should not detect quite a lot of OIII light. Moreover, you may spot the green color of OIII light with your own eye with the aid of a telescope, if you look at some of the more colorful planetary nebulae. The planetary known as the Blue Snowball, which to some people looks like a green snowball, is an example.
As for Thor's Helmet, I myself doubt that its surface brightness is high enough for humans to spot color in it, no matter what kind of telescope we use to look at it. However, I note that David Malin has taken a picture of it, which can be found in his book, [i]A View of the Universe.[/i] In David Malin's picture, the blue-green "bubble" is rather whitish, though with a definite hint of blue or blue-green. The blue parts of the "horns" of Thor's Helmet are barely visible in Malin's image. Instead the horns look red or pink.
If I compare Malin's image with todays APOD, I'd say that today's APOD is much more colorful. I'd say its colors of Martin Rusterholz's image have been enhanced, but not distorted.
If our ability to detect color in faint extended nebulae were many times better than it is, then Thor's Helmet might actually look to us the way it does in Martin Rusterholz's image.
Ann