by Chris Peterson » Tue Oct 22, 2013 2:52 pm
Ann wrote:Of course I keep wondering about the colors. As can be seen from
this larger photo of the same area, the star that is seen "inside the nebular cave", so that it should be more reddened than the other stars, is instead seen to be the bluest. Is that because of the filters used and how the filters were mapped?
This page lists several filters used for this image, among them a 550 nm filter mapped as blue, and a 547 nm filter, very slightly bluer than the first one, which is mapped as green. Does this have anything to do with the confusing colors?
The short answer is yes, that's why the colors aren't natural.
There's really no excuse for them to put together such a poor page as that, which doesn't properly identify the filters used. If forces anybody who is interested to go out and track them down. The actual filters used, with their central wavelength and passband, were:
ACS 550M - 558 nm (54.7 nm) mapped to blue
ACS 658N - 658.4 nm (7.3 nm) mapped to red
ACS 850LP - 944.5 nm (122.9 nm) mapped to red
WF 547M - 548.3 nm (48.4 nm) mapped to green
WF 656N - 656.4 nm (2.2 nm) mapped to red
To fully understand the represented color, you need to consider the passband of each filter (a broad filter with a central wavelength associated with a particular color may not appear that color at all). You also need to understand how the individual channels were weighted before being mapped to RGB color channels- information which isn't available on the image page.
Of course, just noting that a moderately narrow "green" filter is being mapped to blue, that the blue and green channels are almost identical, and that invisible IR wavelengths are being mapped to red should tell you that you aren't seeing an image reflective of natural colors.
[quote="Ann"]Of course I keep wondering about the colors. As can be seen from [url=http://www.spacetelescope.org/static/archives/images/screen/heic0619a.jpg]this larger photo of the same area[/url], the star that is seen "inside the nebular cave", so that it should be more reddened than the other stars, is instead seen to be the bluest. Is that because of the filters used and how the filters were mapped? [url=http://www.spacetelescope.org/images/heic0619a/]This page[/url] lists several filters used for this image, among them a 550 nm filter mapped as blue, and a 547 nm filter, very slightly bluer than the first one, which is mapped as green. Does this have anything to do with the confusing colors?[/quote]
The short answer is yes, that's why the colors aren't natural.
There's really no excuse for them to put together such a poor page as that, which doesn't properly identify the filters used. If forces anybody who is interested to go out and track them down. The actual filters used, with their central wavelength and passband, were:
ACS 550M - 558 nm (54.7 nm) mapped to blue
ACS 658N - 658.4 nm (7.3 nm) mapped to red
ACS 850LP - 944.5 nm (122.9 nm) mapped to red
WF 547M - 548.3 nm (48.4 nm) mapped to green
WF 656N - 656.4 nm (2.2 nm) mapped to red
To fully understand the represented color, you need to consider the passband of each filter (a broad filter with a central wavelength associated with a particular color may not appear that color at all). You also need to understand how the individual channels were weighted before being mapped to RGB color channels- information which isn't available on the image page.
Of course, just noting that a moderately narrow "green" filter is being mapped to blue, that the blue and green channels are almost identical, and that invisible IR wavelengths are being mapped to red should tell you that you aren't seeing an image reflective of natural colors.