by Ann » Sat Nov 04, 2017 6:08 pm
geckzilla wrote:NoelC wrote:This is a false color image where the blue channel was indeed photographed through a blue filter, but the green channel is really red data and the red channel is infrared data.
Photography specifics here:
https://www.spacetelescope.org/images/potw1118a/
Also, the Hubble itself was apparently rotated when shooting the infrared data (shown as the red channel) as compared to the blue and red data (shown as blue and green channels) since we can see that the red diffraction spikes don't align with the blue and green ones. Some elbow grease could correct that by removing the optical spikes, or by removing just the red ones and regenerating equivalent spikes with the same rotation as the blue/green channels.
Lastly, the three color channels are not very well aligned with one another, as you can see red/cyan fringes on virtually every star.
It would make very little difference if the red channel was a real red or the near-infrared channel that was used in this case. That only makes a big difference for galaxies, where an image like this would have green ionized hydrogen clouds instead of the preferable pinkish ones, and more background galaxies would be revealed. In this case, the infrared actually helps separate the colors more. If you wanted the globular to become very red, then you would have to include some near-UV data for the blue channel. That would turn the entire globular yellow, and then the most yellowish stars would become red.
As a side note of possible interest, here* are some clusters I picked out from a near-uv / visible blue image of the Triangulum galaxy. Note the distinct difference between a globular cluster and the young ones. The globulars nearly became invisible!
https://twitter.com/SpaceGeck/status/925233406455070723
Very interesting, Geck!
I am, however, surprised that so many globulars don't show up as more than orange smudges through near UV filters. A globular like M5 contains a healthy population of blue horizontal stars. I'm too laze to check up the typical spectral classes of blue horizontal stars, but I'd say that most of them belong to spectral class B8 or so. In other words, they are generally B-class stars, whose energy output peaks in the ultraviolet part of the spectrum. Surely such stars would stand out like sore thumbs through a near UV filter?
Other globulars, like for example 47 Tuc, don't contain any blue horizontal stars at all (though they do contain som blue straggler stars, which do not necessarily belong to spectral class B), and I can easily imagine that such globulars only look like orange smudges through near UV filters.
I quite agree that the use of a UV filter instead of a blue one will make star clusters and galaxies look redder. A good example is the Hubble picture of blue starburst galaxy NGC 4214. The filters used for this image are F225W (UV), F336W (UV), F438 W (indigo), F487N (a narrowband filter that will detect shortwave nebular emission), F502N ([O III]), F547M (y), F657N (H-alpha+[N II]), and F814W (I). In the finished image, the only the stars strongly detected through the UV and indigo filters are shown as blue. By contrast, the underlying population of old red (make that yellow) stars are strongly detected through the infrared filter, much more strongly than they are detected through the F547M (green) filter. These small cool stars are therefore mapped as very red in the Hubble image.
Ann
[quote="geckzilla"][quote="NoelC"]This is a false color image where the blue channel was indeed photographed through a blue filter, but the green channel is really red data and the red channel is infrared data.
Photography specifics here: https://www.spacetelescope.org/images/potw1118a/
Also, the Hubble itself was apparently rotated when shooting the infrared data (shown as the red channel) as compared to the blue and red data (shown as blue and green channels) since we can see that the red diffraction spikes don't align with the blue and green ones. Some elbow grease could correct that by removing the optical spikes, or by removing just the red ones and regenerating equivalent spikes with the same rotation as the blue/green channels.
Lastly, the three color channels are not very well aligned with one another, as you can see red/cyan fringes on virtually every star.[/quote]
It would make very little difference if the red channel was a real red or the near-infrared channel that was used in this case. That only makes a big difference for galaxies, where an image like this would have green ionized hydrogen clouds instead of the preferable pinkish ones, and more background galaxies would be revealed. In this case, the infrared actually helps separate the colors more. If you wanted the globular to become very red, then you would have to include some near-UV data for the blue channel. That would turn the entire globular yellow, and then the most yellowish stars would become red.
As a side note of possible interest, here* are some clusters I picked out from a near-uv / visible blue image of the Triangulum galaxy. Note the distinct difference between a globular cluster and the young ones. The globulars nearly became invisible!
https://twitter.com/SpaceGeck/status/925233406455070723
[/quote]
Very interesting, Geck!
I am, however, surprised that so many globulars don't show up as more than orange smudges through near UV filters. A globular like M5 contains a healthy population of blue horizontal stars. I'm too laze to check up the typical spectral classes of blue horizontal stars, but I'd say that most of them belong to spectral class B8 or so. In other words, they are generally B-class stars, whose energy output peaks in the ultraviolet part of the spectrum. Surely such stars would stand out like sore thumbs through a near UV filter?
Other globulars, like for example 47 Tuc, don't contain any blue horizontal stars at all (though they do contain som blue straggler stars, which do not necessarily belong to spectral class B), and I can easily imagine that such globulars only look like orange smudges through near UV filters.
[float=right][img2]https://astronomicamens.files.wordpress.com/2012/07/dwarf_galaxy_ngc4214.jpg?w=800&h=576&crop=1[/img2][c][size=85]NGC 4214. Source: http://heritage.stsci.edu/2011/14/fast_facts.html[/size][/c][/float]I quite agree that the use of a UV filter instead of a blue one will make star clusters and galaxies look redder. A good example is the Hubble picture of blue starburst galaxy NGC 4214. The filters used for this image are F225W (UV), F336W (UV), F438 W (indigo), F487N (a narrowband filter that will detect shortwave nebular emission), F502N ([O III]), F547M (y), F657N (H-alpha+[N II]), and F814W (I). In the finished image, the only the stars strongly detected through the UV and indigo filters are shown as blue. By contrast, the underlying population of old red (make that yellow) stars are strongly detected through the infrared filter, much more strongly than they are detected through the F547M (green) filter. These small cool stars are therefore mapped as very red in the Hubble image.
Ann