APOD: Central Cygnus Skyscape (2017 Mar 22)

Comments and questions about the APOD on the main view screen.
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APOD: Central Cygnus Skyscape (2017 Mar 22)

Post by APOD Robot » Wed Mar 22, 2017 4:05 am

Image Central Cygnus Skyscape

Explanation: In cosmic brush strokes of glowing hydrogen gas, this beautiful skyscape unfolds across the plane of our Milky Way Galaxy near the northern end of the Great Rift and the center of the constellation Cygnus the Swan. A 36 panel mosaic of telescopic image data, the scene spans about six degrees. Bright supergiant star Gamma Cygni (Sadr) to the upper left of the image center lies in the foreground of the complex gas and dust clouds and crowded star fields. Left of Gamma Cygni, shaped like two luminous wings divided by a long dark dust lane is IC 1318 whose popular name is understandably the Butterfly Nebula. The more compact, bright nebula at the lower right is NGC 6888, the Crescent Nebula. Some distance estimates for Gamma Cygni place it at around 1,800 light-years while estimates for IC 1318 and NGC 6888 range from 2,000 to 5,000 light-years.

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Re: APOD: Central Cygnus Skyscape (2017 Mar 22)

Post by heehaw » Wed Mar 22, 2017 9:27 am


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Re: APOD: Central Cygnus Skyscape (2017 Mar 22)

Post by starsurfer » Wed Mar 22, 2017 9:59 am

This wonderful image is partly based on data taken for this mosaic by Rob.

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Re: APOD: Central Cygnus Skyscape (2017 Mar 22)

Post by Fred the Cat » Wed Mar 22, 2017 3:21 pm

I noticed it on high resolution but it makes you wonder how Messier did in such a crowded area of the sky. :?:

Maybe it was a "cooling tower" of his day. :? Those dastardly comets :wink:
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Re: APOD: Central Cygnus Skyscape (2017 Mar 22)

Post by Boomer12k » Thu Mar 23, 2017 1:27 am

WOW...

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Re: APOD: Central Cygnus Skyscape (2017 Mar 22)

Post by neufer » Thu Mar 23, 2017 1:40 am

Fred the Cat wrote:
I noticed it on high resolution but it makes you wonder how Messier did in such a crowded area of the sky. :?:
Messier couldn't see hydrogen clouds since he didn't have narrow band filters.
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Re: APOD: Central Cygnus Skyscape (2017 Mar 22)

Post by starsurfer » Thu Mar 23, 2017 11:18 am

neufer wrote:
Fred the Cat wrote:
I noticed it on high resolution but it makes you wonder how Messier did in such a crowded area of the sky. :?:
Messier couldn't see hydrogen clouds since he didn't have narrow band filters.
Some of the nebulae are bright enough to be seen without narrowband filters.

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Re: APOD: Central Cygnus Skyscape (2017 Mar 22)

Post by Chris Peterson » Thu Mar 23, 2017 1:36 pm

neufer wrote:
Fred the Cat wrote: I noticed it on high resolution but it makes you wonder how Messier did in such a crowded area of the sky. :?:
Messier couldn't see hydrogen clouds since he didn't have narrow band filters.
Much of what we see visually when we observe nebulas (unfiltered) through a telescope are hydrogen clouds.
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Re: APOD: Central Cygnus Skyscape (2017 Mar 22)

Post by Ann » Fri Mar 24, 2017 1:24 am

neufer wrote:
Fred the Cat wrote:
I noticed it on high resolution but it makes you wonder how Messier did in such a crowded area of the sky. :?:
Messier couldn't see hydrogen clouds since he didn't have narrow band filters.
RGB images show many red nebulas. Of course, without the aid of an Ha filter the camera will only pick up brighter ones.

There are Ha nebulas that can be seen with the naked eye, but it is likely that the human eye reacts to the more shortwave emission lines that are present in the red nebulas, such as blue-green Hβ, rather than to the faint red Ha light. That is because the human eye is more sensitive to green than to red light.

To me the Trapezium region looks faintly greenish, for example. In the Trapezium, green OIII could also contribute to the color.

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Re: APOD: Central Cygnus Skyscape (2017 Mar 22)

Post by Chris Peterson » Fri Mar 24, 2017 4:28 am

Ann wrote:RGB images show many red nebulas. Of course, without the aid of an Ha filter the camera will only pick up brighter ones.
If you want the greatest sensitivity to Ha, you use a monochrome camera. Next in sensitivity is RGB (just the R channel). Least sensitivity comes from using an Ha filter (although not by a large factor compared with a wideband red filter).

What an Ha filter does is isolate just the hydrogen, blocking other emission lines and most reflected continuum light. Also valuable, it blocks most light pollution and sky glow. It reduces total signal, but increases S/N (basically, contrast).
To me the Trapezium region looks faintly greenish, for example. In the Trapezium, green OIII could also contribute to the color.
Most people see bright nebulas as greenish. As you say, we are much more sensitive to green light than red. In some cases we are seeing OIII, but with bright nebulas we may also see just the green component of white light. We also easily see Ha emissions, but we see them with our rods, not cones, so they appear gray (as do most extended astronomical sources). There are a few anecdotal reports of observers with exceptional vision seeing a pinkish tinge with a few of the brightest Ha sources.
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Re: APOD: Central Cygnus Skyscape (2017 Mar 22)

Post by starsurfer » Fri Mar 24, 2017 8:55 am

Chris Peterson wrote:
Ann wrote:RGB images show many red nebulas. Of course, without the aid of an Ha filter the camera will only pick up brighter ones.
If you want the greatest sensitivity to Ha, you use a monochrome camera. Next in sensitivity is RGB (just the R channel). Least sensitivity comes from using an Ha filter (although not by a large factor compared with a wideband red filter).

What an Ha filter does is isolate just the hydrogen, blocking other emission lines and most reflected continuum light. Also valuable, it blocks most light pollution and sky glow. It reduces total signal, but increases S/N (basically, contrast).
To me the Trapezium region looks faintly greenish, for example. In the Trapezium, green OIII could also contribute to the color.
Most people see bright nebulas as greenish. As you say, we are much more sensitive to green light than red. In some cases we are seeing OIII, but with bright nebulas we may also see just the green component of white light. We also easily see Ha emissions, but we see them with our rods, not cones, so they appear gray (as do most extended astronomical sources). There are a few anecdotal reports of observers with exceptional vision seeing a pinkish tinge with a few of the brightest Ha sources.
Depending on the bandpass of the Ha filter, NII is also included.

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Re: APOD: Central Cygnus Skyscape (2017 Mar 22)

Post by neufer » Fri Mar 24, 2017 1:12 pm

Chris Peterson wrote:
neufer wrote:
Fred the Cat wrote:
I noticed it on high resolution but it makes you wonder how Messier did in such a crowded area of the sky. :?:
Messier couldn't see hydrogen clouds since he didn't have narrow band filters.
Much of what we see visually when we observe nebulas (unfiltered) through a telescope are hydrogen clouds.
It was an Artifact specifically intended for this APOD.
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https://en.wikipedia.org/wiki/H_II_region#Observations wrote:
<<A few of the brightest H II regions are visible to the naked eye. However, none seem to have been noticed before the advent of the telescope in the early 17th century. Even Galileo did not notice the Orion Nebula when he first observed the star cluster within it (previously cataloged as a single star, θ Orionis, by Johann Bayer). The French observer Nicolas-Claude Fabri de Peiresc is credited with the discovery of the Orion Nebula in 1610. Since that early observation large numbers of H II regions have been discovered in the Milky Way and other galaxies. William Herschel observed the Orion Nebula in 1774, and described it later as "an unformed fiery mist, the chaotic material of future suns". In early days astronomers distinguished between "diffuse nebulae" (now known to be H II regions), which retained their fuzzy appearance under magnification through a large telescope, and nebulae that could be resolved into stars, now know to be galaxies external to our own.>>
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Re: APOD: Central Cygnus Skyscape (2017 Mar 22)

Post by Fred the Cat » Fri Mar 24, 2017 3:53 pm

Thanks for the replies. It might be an interesting project to re-create his instrument(s) but people have better things to do. :ssmile:
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