APOD: The Wizard Nebula (2014 Aug 29)

Comments and questions about the APOD on the main view screen.
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APOD: The Wizard Nebula (2014 Aug 29)

Post by APOD Robot » Fri Aug 29, 2014 4:06 am

Image The Wizard Nebula

Explanation: Open star cluster NGC 7380 is still embedded in its natal cloud of interstellar gas and dust popularly known as the Wizard Nebula. Seen with foreground and background stars along the plane of our Milky Way galaxy it lies some 8,000 light-years distant, toward the constellation Cepheus. A full moon would easily fit inside this telescopic view of the 4 million year young cluster and associated nebula, normally much too faint to be seen by eye. Made with telescope and camera firmly planted on Earth, the image reveals multi light-year sized shapes and structures within the Wizard in a color palette made popular in Hubble Space Telescope images. Recorded with narrowband filters, the visible wavelength light from the nebula's hydrogen, oxygen, and sulfur atoms is transformed into green, blue, and red colors in the final digital composite. But there is still a trick up the Wizard's sleeve. Sliding your cursor over the image (or following this link) will make the stars disappear, leaving only the cosmic gas and dust of the Wizard Nebula.

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Re: APOD: The Wizard Nebula (2014 Aug 29)

Post by Beyond » Fri Aug 29, 2014 4:24 am

Well, that was neat. I put my cursor on the picture and all the stars disappeared. All of a sudden it was quiet and peaceful. I didn't want to put the stars back.
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Re: APOD: The Wizard Nebula (2014 Aug 29)

Post by Boomer12k » Fri Aug 29, 2014 5:49 am

AWESOME....I LIKE IT....the idea of taking out the stars....would love to see more of this, especially for some other galaxy images.

Really interesting.

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Re: APOD: The Wizard Nebula (2014 Aug 29)

Post by Markus Schwarz » Fri Aug 29, 2014 7:21 am

Interesting that "The Sorcerer's Apprentice", which I had to memorize in school, shows up here :)

Now, I wonder how the wizard has done the trick, i.e. how did he remove the stars? My naive guess is that you first use some kind of peak finder and then subtract it from the image. But that would leave a black spot, wouldn't it? The nebula looks pretty smooth, so maybe he did an interpolation afterward?

Also, there seem to be some stars left inside the "blue lagoon". Are these stars that belong to the nebula, or have they been overlooked by the algorithm?

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Re: APOD: The Wizard Nebula (2014 Aug 29)

Post by geckzilla » Fri Aug 29, 2014 7:25 am

Markus Schwarz wrote:Now, I wonder how the wizard has done the trick, i.e. how did he remove the stars? My naive guess is that you first use some kind of peak finder and then subtract it from the image. But that would leave a black spot, wouldn't it? The nebula looks pretty smooth, so maybe he did an interpolation afterward?

Also, there seem to be some stars left inside the "blue lagoon". Are these stars that belong to the nebula, or have they been overlooked by the algorithm?
I've found Photoshop's Dust and Scratches filter, when used alongside what you describe as a peak filter (I just call it masking off everything but the bright spots) works wonders for this sort of thing. It's easy to get rid of the bright points. It's a bit of work getting rid of the glow surrounding the stars if the point spread function is expansive, though.
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Re: APOD: The Wizard Nebula (2014 Aug 29)

Post by Peeratz » Fri Aug 29, 2014 2:14 pm

I love it. I asked about this being done a couple of years ago! http://asterisk.apod.com/viewtopic.php? ... 90#p188220
What I would like to see are pictures of galaxies that have foreground stars removed to better show the vastness of space. With foreground stars mixed with galaxies, the separation of the Milky Way to the other galaxies is lost.

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Re: APOD: The Wizard Nebula (2014 Aug 29)

Post by Chilimac45 » Fri Aug 29, 2014 2:33 pm

Nery nice surprise! It looks great. No need for changes. Thanks for all the effort!

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Re: APOD: The Wizard Nebula (2014 Aug 29)

Post by Chris Peterson » Fri Aug 29, 2014 3:11 pm

geckzilla wrote:I've found Photoshop's Dust and Scratches filter, when used alongside what you describe as a peak filter (I just call it masking off everything but the bright spots) works wonders for this sort of thing. It's easy to get rid of the bright points. It's a bit of work getting rid of the glow surrounding the stars if the point spread function is expansive, though.
The stars in this image are very bright and overexposed for a narrowband construction. Makes me wonder if they weren't largely provided by RGB or luminance channels. Normally, stars are reduced in significance by using narrowband images (since stars are very weak in those narrow wavelengths). That makes removing them much easier, since they aren't bloated messes.
Chris

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Re: APOD: The Wizard Nebula (2014 Aug 29)

Post by starsurfer » Fri Aug 29, 2014 3:18 pm

Chris Peterson wrote:
geckzilla wrote:I've found Photoshop's Dust and Scratches filter, when used alongside what you describe as a peak filter (I just call it masking off everything but the bright spots) works wonders for this sort of thing. It's easy to get rid of the bright points. It's a bit of work getting rid of the glow surrounding the stars if the point spread function is expansive, though.
The stars in this image are very bright and overexposed for a narrowband construction. Makes me wonder if they weren't largely provided by RGB or luminance channels. Normally, stars are reduced in significance by using narrowband images (since stars are very weak in those narrow wavelengths). That makes removing them much easier, since they aren't bloated messes.
For his other narrowband images, he has also taken LRGB exposures. The page for the starless version can be found here: http://www.astrobin.com/115159/
I much prefer seeing this nebula in true colour, so I prefer his other image: http://www.astrobin.com/116751/
I remember seeing Rob Gendler's image of this in 2006 and was amazed!

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Re: APOD: The Wizard Nebula (2014 Aug 29)

Post by starsurfer » Fri Aug 29, 2014 3:23 pm

Boomer12k wrote:AWESOME....I LIKE IT....the idea of taking out the stars....would love to see more of this, especially for some other galaxy images.

Really interesting.

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Check out this Ha image of M33 by Adam Block: http://www.caelumobservatory.com/mlsc/m33_Ha.jpg
I wish more people took Ha and OIII for galaxies.
Exposures taken through a special red continuum filter can be used to subtract stars from a Ha image, this method was used in the processing of the Ha outflow of M82 by Ken Crawford: http://www.imagingdeepsky.com/Galaxies/M82/M82.htm

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Re: APOD: The Wizard Nebula (2014 Aug 29)

Post by MarkBour » Fri Aug 29, 2014 3:39 pm

I have a basic question about what I see in many APODs. If you look at the small excerpt here, for example, the stars appear to come in many sizes and levels of brightness. Naturally, my brain wants to put the larger and brighter ones as closer (I assume that strategy is generally correct, but would result in occasional mistakes.)

And in this excerpt, a few of the larger stars appear to have a discernable radius; they appear larger than the pixel size of the image. I believe this is what Chris has in the past referred to as "resolved", but I'm not sure. So my question is, when viewing images on APOD in general, should I be wary of this ... is it an illusion created somehow just by the brightness of stars and the equipment in use? Or can I believe, as I would initially think, that one can roughly measure a radius for some of the larger stars in the image?
NGC7380Excerpt.gif
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Re: APOD: The Wizard Nebula (2014 Aug 29)

Post by Chris Peterson » Fri Aug 29, 2014 3:47 pm

MarkBour wrote:I have a basic question about what I see in many APODs. If you look at the small excerpt here, for example, the stars appear to come in many sizes and levels of brightness. Naturally, my brain wants to put the larger and brighter ones as closer (I assume that strategy is generally correct, but would result in occasional mistakes.)

And in this excerpt, a few of the larger stars appear to have a discernable radius; they appear larger than the pixel size of the image. I believe this is what Chris has in the past referred to as "resolved", but I'm not sure. So my question is, when viewing images on APOD in general, should I be wary of this ... is it an illusion created somehow just by the brightness of stars and the equipment in use? Or can I believe, as I would initially think, that one can roughly measure a radius for some of the larger stars in the image?
With just a handful of exceptions, and big professional telescopes, no stars have resolvable sizes in astronomical images. Generally, when "resolved" is used in discussing stars it simply means that the star is distinct from its background.

Optically, stars are point sources. Their apparent radius in images is the product of diffraction and scatter. This makes brighter stars (brighter at the camera, that is) appear larger, since you can see farther out along the wings produced by that diffraction and scatter. Every star in this image is actually much smaller in angular extent than a single pixel. All of the broadening is artifactual.
Chris

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Re: APOD: The Wizard Nebula (2014 Aug 29)

Post by Chris Peterson » Fri Aug 29, 2014 3:49 pm

starsurfer wrote:I much prefer seeing this nebula in true colour...
I seldom care much for attempts to map RGB when imaging emissive objects, so I was happy to see a good quality narrowband image on APOD. It seems like it's been a while.
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Re: APOD: The Wizard Nebula (2014 Aug 29)

Post by geckzilla » Fri Aug 29, 2014 4:22 pm

MarkBour wrote: Naturally, my brain wants to put the larger and brighter ones as closer (I assume that strategy is generally correct, but would result in occasional mistakes.)
This is actually much less accurate than one might imagine. A very small handful of the very brightest stars in a field can be very generally guessed as being closer. But between those, you would never be able to tell which one is closest. The brightest one might actually be the farthest of the closest stars. And a much dimmer star you didn't pick out to be among the brightest stars may in fact be the closest star in the field. I agree, however, that in lieu of actual measurement, guessing that the brightest stars are closer is more accurate than randomly selecting them. However, if you were to create a simulated star field out of a field of stars whose distances were all unknown, randomly choosing one or two dimmer stars to occupy the foreground layer along with the brightest ones would probably provide a closer semblance to reality, even if you picked the wrong stars.
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Re: APOD: The Wizard Nebula (2014 Aug 29)

Post by Ann » Fri Aug 29, 2014 5:36 pm

Mark Bour wrote:
I have a basic question about what I see in many APODs. If you look at the small excerpt here, for example, the stars appear to come in many sizes and levels of brightness. Naturally, my brain wants to put the larger and brighter ones as closer (I assume that strategy is generally correct, but would result in occasional mistakes.)
Take a look at this picture. The arrow points at the star that is closer to us than any other star except the Sun, Proxima Centauri. This red dwarf at about 4 light-years away is about one part in 15,000 as bright as the Sun in visual light.

Let's return to the picture again. The bright yellowish star on the left is Alpha Centauri, a double star made up of one star about one and a half times brighter than the Sun and another one about half as bright as the Sun (if I remember correctly). Apart from Proxima Centauri, the tiny red dwarf, the two Sun-like stars of Alpha Centauri are the nearest stars to us (apart from the Sun) in the universe, about four light-years away.

But take a look at the bright bluish star on the right in the picture. It looks almost as bright as Alpha Centauri. The blue star is Hadar, Beta Centauri. It is almost 400 light-years away, almost a hundred times as far away as Alpha Centauri, and it is more than 3,000 times as bright as Alpha Centauri in visual light. Its total light output, including ultraviolet light, is much higher than that.

I have read somewhere that 99% of the stars that we can easily see with the naked eye in the sky are intrinsically brighter than the Sun. But the Sun may be brighter than 95% of all the stars in the Milky Way because of the profusion in our galaxy of little red dwarfs like Proxima Centauri.

Go figure.

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Re: APOD: The Wizard Nebula (2014 Aug 29)

Post by CURRAHEE CHRIS » Fri Aug 29, 2014 6:38 pm

Great pic for today!!! Im getting ready to host a DnD event with my kids tonight!!! :D

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Re: APOD: The Wizard Nebula (2014 Aug 29)

Post by LocalColor » Fri Aug 29, 2014 8:51 pm

Wonderful Wizard - wow - just wow!

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Re: APOD: The Wizard Nebula (2014 Aug 29)

Post by starsurfer » Sat Aug 30, 2014 4:15 pm

Chris Peterson wrote:
starsurfer wrote:I much prefer seeing this nebula in true colour...
I seldom care much for attempts to map RGB when imaging emissive objects, so I was happy to see a good quality narrowband image on APOD. It seems like it's been a while.
I like images that combine both narrowband and RGB. I think it's been a while since there has been an image of a Wolf Rayet nebula that contains OIII emission.

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Re: APOD: The Wizard Nebula (2014 Aug 29)

Post by MarkBour » Mon Sep 01, 2014 4:51 pm

Thank you, Chris, geckzilla, and Ann. Very illuminating.
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