by Ann » Fri Feb 26, 2016 10:05 am
As I'm watching today's APOD, bursting with weird and vibrant colors, I'm reminded of my childhood, when I used to stare at my parents' print of Vincent van Gogh's painting
The Sower, and ask myself, "Why is the sky green?"
Watching today's vibrant APOD, I ask myself, "Why is the Tarantula Nebula yellow?"
As some of you know, colors are exceedingly important to me. In astronomical images, I want colors to represent some sort of "reality". In David Malin's color images from the 1980s, wideband red, green and blue photographic glass plates were used to create
very "natural" (if sometimes probably saturated) images. In Hubble's famous
Pillars of Creation image, blue color represents OIII emission at 501 nm, green represents Ha at 656 nm, and red represents SII emission at about 672 nm. The longer the wavelength, the redder the mapped color: it makes sense.
Other composite, multiwavelength images are mapped in ways that are far less intuitive, but here the colors are
carefully explained.
But I was unable to make sense of the colors in today's APOD until I found the picture at right, made with the Wide Field Imager (WFI) on the MPG/ESO 2.2-metre telescope at the European Southern Observatory’s site at La Silla, Chile.
The filters used for the image were
B,
V,
OIII and
H-alpha.
Hβ, OIII, Hα, SII. Illustration: laser_jock99
I think the ESO picture of the Tarantula region has been used as a a starting point for today's APOD. The colors in today's APOD are different from the ESO image, but they are clearly influenced by it. The "color weirdness" of today's APOD arises, I think, from the the fact that OIII emission was mapped as yellow-orange and the wideband V image as green in the ESO image. As can be seen in the diagram at left, OIII is a blue-green hue, located at the "blue edge" of the wideband green V filter. It is this mapping of OIII as yellow-orange that creates the strange yellow color of the Tarantula Nebula in today's APOD.
But it is a great image, nevertheless.
Now I only have to figure out why the sky is green in that Vincent van Gogh painting... or else I can enjoy the painting just the way it is.
Ann
[float=left][img2]https://news.artnet.com/wp-content/news-upload/2015/03/TheSowerVanGogh.jpg[/img2][/float] As I'm watching today's APOD, bursting with weird and vibrant colors, I'm reminded of my childhood, when I used to stare at my parents' print of Vincent van Gogh's painting [i]The Sower[/i], and ask myself, "Why is the sky green?"
Watching today's vibrant APOD, I ask myself, "Why is the Tarantula Nebula yellow?"
As some of you know, colors are exceedingly important to me. In astronomical images, I want colors to represent some sort of "reality". In David Malin's color images from the 1980s, wideband red, green and blue photographic glass plates were used to create [url=http://ftp.aao.gov.au/images/image/uks015.jpg]very "natural"[/url] (if sometimes probably saturated) images. In Hubble's famous [url=http://www.abc.net.au/reslib/201501/r1378025_19495710.jpg]Pillars of Creation image[/url], blue color represents OIII emission at 501 nm, green represents Ha at 656 nm, and red represents SII emission at about 672 nm. The longer the wavelength, the redder the mapped color: it makes sense.
Other composite, multiwavelength images are mapped in ways that are far less intuitive, but here the colors are [url=http://astronomynow.com/2015/08/28/radio-phoenix-rises-from-the-ashes-of-galaxy-cluster-collision/]carefully explained[/url].
[float=right][img2]http://cdn.spacetelescope.org/archives/images/thumb300y/heic1105c.jpg[/img2][/float] But I was unable to make sense of the colors in today's APOD until I found the picture at right, made with the Wide Field Imager (WFI) on the MPG/ESO 2.2-metre telescope at the European Southern Observatory’s site at La Silla, Chile. [url=https://www.spacetelescope.org/images/heic1105c/]The filters used for the image[/url] were [b][color=#0040FF]B[/color][/b], [b][color=#008000]V[/color][/b], [b][color=#FFBF00]OIII[/color][/b] and [b][color=#FF0000]H-alpha[/color][/b].
[float=left][img2]http://starizona.com/acb/ccd/advimages/Narrowband%20Diagram.jpg[/img2][c][size=85]Hβ, OIII, Hα, SII. Illustration: laser_jock99[/size][/c][/float]
I think the ESO picture of the Tarantula region has been used as a a starting point for today's APOD. The colors in today's APOD are different from the ESO image, but they are clearly influenced by it. The "color weirdness" of today's APOD arises, I think, from the the fact that OIII emission was mapped as yellow-orange and the wideband V image as green in the ESO image. As can be seen in the diagram at left, OIII is a blue-green hue, located at the "blue edge" of the wideband green V filter. It is this mapping of OIII as yellow-orange that creates the strange yellow color of the Tarantula Nebula in today's APOD.
But it is a great image, nevertheless.
Now I only have to figure out why the sky is green in that Vincent van Gogh painting... or else I can enjoy the painting just the way it is. :wink:
Ann