by geckzilla » Fri Dec 06, 2013 2:00 pm
Ann wrote:geckzilla wrote:Chris Peterson wrote:Certainly, the star is very blue in this image. But that is an imaging artifact. The color we see here is nowhere near what we'd expect for the object itself.
To further illuminate why this is an imaging artifact (specifically, a result of processing), I have attached an image of the red channel (cropped to just the nebula) so that anyone can see the dark black ring around the central star as well as some other small ones due to sharpening and saturation adjustments which adversely affect the integrity of the objects in favor of aesthetics. Note that the adjustments were applied locally to just the area of the nebula.
goldman_abell7_red_only.jpg
Geckzilla, I don't question your skill an dyour ability to tell whether or not a picture has been somewhat manipulated for aesthetic or other reasons. But should we conclude, then, that the central stars of planetary nebulae don't typically stand out in LRGB images?
Take a look at
Adam Block's collection of planetary nebulae in RGB. (These images are rather old, and I don't know how proud Adam is that I call attention to them. Forgive me, Adam.)
Note that very many of these planetary nebulae have strikingly blue central stars. Check out, for example,
PK205+14.1: The Medusa Nebula. The color of the central star is the only thing that gives it away.
Ann
Ok, full disclosure before you read my following post. A lot of these assertions are based on my experience with Hubble data and they are anecdotal. This is not something I have any formal training or education in. I hope if I am wrong that someone with such education will correct it (Hi, Chris!).
Those [Adam Block's PNs] all look like they are emphasized by similar processing. If you look at the red channel there is usually a black ring around the central star which indicates that the saturation has been increased substantially. I used to use saturation adjustments a lot when I first started processing but when I realized that it is destructive to the integrity of the image, I began avoiding it. Nowadays, if I decide it is necessary, I make a note of it in the description. If a saturation adjustment isn't to blame then some kind of sharpening algorithm is. I still use a couple of sharpening algorithms and I know that sometimes unnatural rings or halos can appear which can have a similar effect of creating color artifacts even though I do my best to avoid it.
Anyway, with a saturation modification you are seeing what was formerly a very slight blue turn into a very blue blue. Furthermore, in my experience with Hubble data, even though oftentimes the blue wavelengths have received more exposure time, I still have to increase the signal significantly to make the colors look balanced. I assume that you look at Adam Block's photos and see that he has given all of his channels a 1:1:1 and you think this means all of the colors are getting fair treatment. Right? Sort of.
What's happening during processing is something that you are not fully understanding, which is the reason I suggested months ago that you attempt processing in order for you to attain a less superficial understanding. At this point, I assert that the blue is always getting a boost somehow for wideband RGB images (narrowband is another story). This happens either during processing or elsewhere. For Adam Block's Abell 39, it seems to happen at the filter level. I took a look at his AstroDon Gen II filter set
spectra. As you can see, blue is significantly wider. This means that during processing less adjustment (or none at all) needs to be made to the color balance but it's still happening at a lower level. Another feature mentioned for the AstroDon Gen II filter set is "Better color rendition for galaxies based upon color theory" which, as you should understand, is a human, subjective way of looking at colors and deciding that some combinations simply look better than others. This is awesome, but it is important to understand that this is an aesthetic technique. I mean, science and math is involved, of course, but there is bias for aesthetics.
What do things look like
before blue gets a boost? Various shades of red, orange, yellow, and then up to white. The white or less yellow ones can turn blue once blue gets its signal increase. This is why it is more important to say that a star, relative to its surrounding stars, emits shorter wavelength light than its neighbors, and NOT that the star is truly blue. It
might be blue. But the only thing you can gather for certain is that some things are redder and some things are bluer, not that they are necessarily blue. Blue things might actually be white and red things might actually be infrared depending on the imaging technique, but you knew about the infrared thing.
Anyway, that is my explanation and how I understand that an apparently blue star is actually white. Hopefully I didn't land too far off the mark.
[quote="Ann"][quote="geckzilla"][quote="Chris Peterson"]Certainly, the star is very blue in this image. But that is an imaging artifact. The color we see here is nowhere near what we'd expect for the object itself.[/quote]
To further illuminate why this is an imaging artifact (specifically, a result of processing), I have attached an image of the red channel (cropped to just the nebula) so that anyone can see the dark black ring around the central star as well as some other small ones due to sharpening and saturation adjustments which adversely affect the integrity of the objects in favor of aesthetics. Note that the adjustments were applied locally to just the area of the nebula.
[attachment=0]goldman_abell7_red_only.jpg[/attachment][/quote]
Geckzilla, I don't question your skill an dyour ability to tell whether or not a picture has been somewhat manipulated for aesthetic or other reasons. But should we conclude, then, that the central stars of planetary nebulae don't typically stand out in LRGB images?
Take a look at [url=http://www.caelumobservatory.com/obs/pn.shtml]Adam Block's collection of planetary nebulae in RGB[/url]. (These images are rather old, and I don't know how proud Adam is that I call attention to them. Forgive me, Adam.)
Note that very many of these planetary nebulae have strikingly blue central stars. Check out, for example, [url=http://www.caelumobservatory.com/obs/medusa.html]PK205+14.1: The Medusa Nebula[/url]. The color of the central star is the only thing that gives it away.
Ann[/quote]
Ok, full disclosure before you read my following post. A lot of these assertions are based on my experience with Hubble data and they are anecdotal. This is not something I have any formal training or education in. I hope if I am wrong that someone with such education will correct it (Hi, Chris!).
Those [Adam Block's PNs] all look like they are emphasized by similar processing. If you look at the red channel there is usually a black ring around the central star which indicates that the saturation has been increased substantially. I used to use saturation adjustments a lot when I first started processing but when I realized that it is destructive to the integrity of the image, I began avoiding it. Nowadays, if I decide it is necessary, I make a note of it in the description. If a saturation adjustment isn't to blame then some kind of sharpening algorithm is. I still use a couple of sharpening algorithms and I know that sometimes unnatural rings or halos can appear which can have a similar effect of creating color artifacts even though I do my best to avoid it.
Anyway, with a saturation modification you are seeing what was formerly a very slight blue turn into a very blue blue. Furthermore, in my experience with Hubble data, even though oftentimes the blue wavelengths have received more exposure time, I still have to increase the signal significantly to make the colors look balanced. I assume that you look at Adam Block's photos and see that he has given all of his channels a 1:1:1 and you think this means all of the colors are getting fair treatment. Right? Sort of.
What's happening during processing is something that you are not fully understanding, which is the reason I suggested months ago that you attempt processing in order for you to attain a less superficial understanding. At this point, I assert that the blue is always getting a boost somehow for wideband RGB images (narrowband is another story). This happens either during processing or elsewhere. For Adam Block's Abell 39, it seems to happen at the filter level. I took a look at his AstroDon Gen II filter set [url=http://www.astrodon.com/custom/_2e2a/content/images/AstrodonG2ESeriesScansforWeb.jpg]spectra[/url]. As you can see, blue is significantly wider. This means that during processing less adjustment (or none at all) needs to be made to the color balance but it's still happening at a lower level. Another feature mentioned for the AstroDon Gen II filter set is "Better color rendition for galaxies based upon color theory" which, as you should understand, is a human, subjective way of looking at colors and deciding that some combinations simply look better than others. This is awesome, but it is important to understand that this is an aesthetic technique. I mean, science and math is involved, of course, but there is bias for aesthetics.
What do things look like [i]before[/i] blue gets a boost? Various shades of red, orange, yellow, and then up to white. The white or less yellow ones can turn blue once blue gets its signal increase. This is why it is more important to say that a star, relative to its surrounding stars, emits shorter wavelength light than its neighbors, and NOT that the star is truly blue. It [i]might[/i] be blue. But the only thing you can gather for certain is that some things are redder and some things are bluer, not that they are necessarily blue. Blue things might actually be white and red things might actually be infrared depending on the imaging technique, but you knew about the infrared thing.
Anyway, that is my explanation and how I understand that an apparently blue star is actually white. Hopefully I didn't land too far off the mark.