APOD: The Spanish Dancer Spiral Galaxy (2023 May 08)

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APOD: The Spanish Dancer Spiral Galaxy (2023 May 08)

Post by APOD Robot » Mon May 08, 2023 4:05 am

Image The Spanish Dancer Spiral Galaxy

Explanation: If not perfect, then this spiral galaxy is at least one of the most photogenic. An island universe containing billions of stars and situated about 40 million light-years away toward the constellation of the Dolphinfish (Dorado), NGC 1566 presents a gorgeous face-on view. Classified as a grand design spiral, NGC 1566 shows two prominent and graceful spiral arms that are traced by bright blue star clusters and dark cosmic dust lanes. Numerous Hubble Space Telescope images of NGC 1566 have been taken to study star formation, supernovas, and the spiral's unusually active center. Some of these images, stored online in the Hubble Legacy Archive, were freely downloaded, combined, and digitally processed by an industrious amateur to create the featured image. NGC 1566's flaring center makes the spiral one of the closest and brightest Seyfert galaxies, likely housing a central supermassive black hole wreaking havoc on surrounding stars and gas.

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Re: APOD: The Spanish Dancer Spiral Galaxy (2023 May 08)

Post by AVAO » Mon May 08, 2023 4:47 am

APOD Robot wrote: Mon May 08, 2023 4:05 am Image The Spanish Dancer Spiral Galaxy

Explanation: If not perfect, then this spiral galaxy is at least one of the most photogenic. An island universe containing billions of stars and situated about 40 million light-years away toward the constellation of the Dolphinfish (Dorado), NGC 1566 presents a gorgeous face-on view. Classified as a grand design spiral, NGC 1566 shows two prominent and graceful spiral arms that are traced by bright blue star clusters and dark cosmic dust lanes. Numerous Hubble Space Telescope images of NGC 1566 have been taken to study star formation, supernovas, and the spiral's unusually active center. Some of these images, stored online in the Hubble Legacy Archive, were freely downloaded, combined, and digitally processed by an industrious amateur to create the featured image. NGC 1566's flaring center makes the spiral one of the closest and brightest Seyfert galaxies, likely housing a central supermassive black hole wreaking havoc on surrounding stars and gas.

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Hmmm ...If we discuss this image - maybe we should also discuss about the appropriateness of the up-to-dateness of APOD pictures... and the text (https://apod.nasa.gov/apod/ap190702.html) ....

This image was taken by Hubble’s Wide Field Camera 3 (WFC3) in the near-infrared part of the spectrum. A version of the image was entered into the Hubble’s Hidden Treasures image processing competition by Flickr user Det58.
Acknowledgement: Flickr user Det58 Release date: 2 June 2014

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Re: APOD: The Spanish Dancer Spiral Galaxy (2023 May 08)

Post by Ann » Mon May 08, 2023 6:20 am


NGC 1566, the Spanish Dancer galaxy, sure is spectacularly beautiful! :D


And it sure isn't any less spectacular when JWST's MIRI instrument has photographed it, and the great, great Geck (Judy Schmidt) has had a go at processing the raw data from MIRI!!! :shock: :D

In case you're wondering, the JWST/Geck image reveals the underlying dust structure of NGC 1566.

And if you are interested in such things, you may note that a small beige-colored ring surrounds the bright core of NGC 1566, and two short stubby mostly beige-colored dust lanes connect this small ring to the two magnificent spiral arms of NGC 1566. This inner ring and its two dust lanes are typical of barred galaxies. However, note that the two great arms are mostly unaffected by the two dust lanes reaching out to them, and the arms mostly curve past them as if "nothing had happened".

This is why 1566 is called an intermediate galaxy, a galaxy that is midway between a barred and an unbarred galaxy.

Anyway, there's a lot more than meets the eye in NGC 1566! Do have a look at this ultraviolet GALEX image:


In this picture, the colors probably mean ultraviolet flux, with red being highest and green and blue lowest. The fact that this picture is an ultraviolet image means that it detects hot stars, from blisteringly hot O and B-type stars down to modest A, F and even G-type stars like Sirius, Procyon and the Sun. This means that the spiral shape of NGC 1566 continues outwards beyond the bright visual parts of it, and even in this thinly populated outer reaches there are hot stars being born. Note that the two-armed spiral shape continues all the way out as far as the sensitivity of GALEX can detect it.


And if you get a kick out of black holes, here is a portrait of the hot gas and dust swirling around the black hole of NGC 1566:


NGC 1566 is some galaxy all right!

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Re: APOD: The Spanish Dancer Spiral Galaxy (2023 May 08)

Post by Guest » Mon May 08, 2023 10:47 am

I post some of these pics in my social media feed. When I see pink, or other outstanding colors, I wonder if they are there due to filtering and highlighting from a photo process. I suspect many people who may see that in my feed would feel the photos were doctored for some reason. It would be great if colorizing was mentioned for colorful space pics going forward.

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Re: APOD: The Spanish Dancer Spiral Galaxy (2023 May 08)

Post by Chris Peterson » Mon May 08, 2023 1:39 pm

Guest wrote: Mon May 08, 2023 10:47 am I post some of these pics in my social media feed. When I see pink, or other outstanding colors, I wonder if they are there due to filtering and highlighting from a photo process. I suspect many people who may see that in my feed would feel the photos were doctored for some reason. It would be great if colorizing was mentioned for colorful space pics going forward.
Pink is the color of ionized hydrogen, the most common thing in the Universe.
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Re: APOD: The Spanish Dancer Spiral Galaxy (2023 May 08)

Post by Guest » Mon May 08, 2023 2:09 pm

Ok. So this photo must have enhanced the color since we basically only see black and white. This is what I’m talking about. Yes, I’m a layman but the people who see these pics in my social media feed are even more so.

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Re: APOD: The Spanish Dancer Spiral Galaxy (2023 May 08)

Post by Chris Peterson » Mon May 08, 2023 2:21 pm

Guest wrote: Mon May 08, 2023 2:09 pm Ok. So this photo must have enhanced the color since we basically only see black and white. This is what I’m talking about. Yes, I’m a layman but the people who see these pics in my social media feed are even more so.
While the idea of "true color" has certain problems, it is generally taken to mean that what we see in the image is what we'd see if our eyes were more sensitive. The "colors" are the same as we'd see if the object simply became brighter without any other changes. No deliberate "enhancement" of the colors in these images is necessary.
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Re: APOD: The Spanish Dancer Spiral Galaxy (2023 May 08)

Post by Stan23 » Mon May 08, 2023 3:35 pm

Layman's question: these pictures often leave me awe-struck. The idea of star clusters, dust lanes, star nurseries, etc. But I can't form an image of what the "hot" golden centre and surrounding yellow area inside the main dust lanes are made of. I've seen the "closeup" images of a super massive black hole, but I assume that just makes up a small central area, perhaps a light year or less? What makes up the golden and yellow area in the centre of the galaxy? Is it just a huge area of dense hot gas and dust swirling around the black hole as shown in Ann's post? Thanks.

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Re: APOD: The Spanish Dancer Spiral Galaxy (2023 May 08)

Post by Ann » Mon May 08, 2023 3:56 pm

Guest wrote: Mon May 08, 2023 2:09 pm Ok. So this photo must have enhanced the color since we basically only see black and white. This is what I’m talking about. Yes, I’m a layman but the people who see these pics in my social media feed are even more so.
Today's APOD is "true color". The pink color of the emission nebulas in the arms of NGC 1566 is correct. Yet we humans can never see these nebulas the way they are, namely "optically pink", because of the limitations of our eyes.

Take a look at this illustration of the color sensitivity of the human eye:


As you can see, our human eyes are very sensitive to green light, but far, far less sensitive to blue and red light.

In galaxies like NGC 1566, the brightest stars are hot blue giants. However, stars in other galaxies are way, way too faint to stimulate color vision in our eyes. Admittedly though, the first time I ever saw Andromeda, it looked something like this:

Andromeda Alfeuerstein Cloudy Nights.png
Credit: Alfeuerstein at Cloudy Nights

I was 15 years old when I saw Andromeda like this, and my eyesight was much, much sharper then than it is now. I have never seen Andromeda like this again. Nevertheless, I'd say that this is a relatively true color portrait of the inner bulge of Andromeda. This bulge really is dominated by huge numbers of yellow-orange stars.

We humans can easily discern individual and reasonably bright yellow-orange stars in the sky. It is not hard for us to see that Betelgeuse is a yellow-orange star that is different in color from all the other bright stars in Orion:


But even though we can easily see that Betelgeuse is yellow-orange, it is so, so very much harder to see that the center of a galaxy is yellow. This is for two reasons. First, even in the center of a galaxy, the stars are spread out, and their combined light is fuzzy and "diluted" and not concentrated in a single point. Second, galactic bulges are never as orange as Betelgeuse, because galactic centers are typically dominated by stars similar to Pollux, which are a lot paler in color than Betelgeuse


So we can't see the yellow color of the bulges of galaxies, and we really can't see the blue color of the brilliant hot stars that are typically found in the arms of other spiral galaxies. It is often too hard for us to see even the blue color of individual bright stars in the sky. Stars never look as blue in real life as Castor does in the picture above of Pollux and Castor. Your best chance to see blue color in a star may be to look at bright northern star Vega, preferably through a telescope.


To summarize star colors, let's have alook at the blackbody curves of stars:


Hot stars have blackbody curves that peak "beyond the blue (or violet) edge" of the visible spectrum. These stars are blue, or blue-white. Cool stars have blackbody curves that peak "beyond the red edge of" the visible spectrum. These stars are red, or yellow-orange. Stars like the Sun peak near the green part of the visible spectrum. Technically, they could be described as green stars. We humans see them as white.


What about the pink nebulas that often dot the arms of spiral galaxies? They really are pink. And we can't see the color of them.

Most bright nebulas in the sky are dominated by the emission lines of the Balmer series of hydrogen:


Nebulas are almost always dominated by hydrogen, and hydrogen is made to emit light when it has been ionized by the ultraviolet light of hot stars. By far the dominant light emitted by hydrogen nebulas is the hydrogen alpha line at 656 nm, ███. This spectral line corresponds to a deep red color, and our eyes are quite insensitive to it. However, there are other hydrogen emission lines as well, the most important of which is hydrogen beta at 486 nm, ███. This emission line corresponds to a bluish cyan hue, and even though it is much, much fainter than red hydrogen alpha, this cyan color of hydrogen beta dilutes the color of hydrogen emission nebulas to a pink hue.


However, the most central parts of nebulas very close to very hot stars are typically not pink, because there is typically a significant amount of doubly ionized oxygen, OIII, at 501 nm, ███, in the center of emission nebulas. This nebulosity is green, and our eyes are sensitive to green light. That is why we can see some green color near the Trapezium in Orion. This is what the Trapezium has looked like to me:


The nebulosity of the innermost part of the Orion Nebula is only very faintly greenish, but it is greenish.

Okay, I have spent more time than I had at my disposal on this post, so I'd better stop now!

Ann
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Re: APOD: The Spanish Dancer Spiral Galaxy (2023 May 08)

Post by Chris Peterson » Mon May 08, 2023 4:34 pm

Stan23 wrote: Mon May 08, 2023 3:35 pm Layman's question: these pictures often leave me awe-struck. The idea of star clusters, dust lanes, star nurseries, etc. But I can't form an image of what the "hot" golden centre and surrounding yellow area inside the main dust lanes are made of. I've seen the "closeup" images of a super massive black hole, but I assume that just makes up a small central area, perhaps a light year or less? What makes up the golden and yellow area in the centre of the galaxy? Is it just a huge area of dense hot gas and dust swirling around the black hole as shown in Ann's post? Thanks.
Not gas, not dust (mostly). Just relatively cool stars. Hundreds of millions, even billions of them. And they're not really swirling around the black hole, because the black hole is only a tiny gravitational influence. They're orbiting around the center of mass of the entire galaxy.
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Re: APOD: The Spanish Dancer Spiral Galaxy (2023 May 08)

Post by AVAO » Mon May 08, 2023 4:42 pm

Ann wrote: Mon May 08, 2023 3:56 pm
Today's APOD is "true color". The pink color of the emission nebulas in the arms of NGC 1566 is correct. Yet we humans can never see these nebulas the way they are, namely "optically pink", because of the limitations of our eyes.

Ann
Sorry Ann and Chris. But I don't know if you actually read my post at the beginning.
AVAO wrote: Mon May 08, 2023 4:47 am ...
This image was taken by Hubble’s Wide Field Camera 3 (WFC3) in the near-infrared part of the spectrum. A version of the image was entered into the Hubble’s Hidden Treasures image processing competition by Flickr user Det58.
Acknowledgement: Flickr user Det58 Release date: 2 June 2014
True colors look different to me...
https://www.astrobin.com/8aa26h/B/
There are only a few areas in pink or Hα. Compare:
https://apod.nasa.gov/apod/ap190702.html

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Re: APOD: The Spanish Dancer Spiral Galaxy (2023 May 08)

Post by Chris Peterson » Mon May 08, 2023 5:11 pm

AVAO wrote: Mon May 08, 2023 4:42 pm
Ann wrote: Mon May 08, 2023 3:56 pm
Today's APOD is "true color". The pink color of the emission nebulas in the arms of NGC 1566 is correct. Yet we humans can never see these nebulas the way they are, namely "optically pink", because of the limitations of our eyes.

Ann
Sorry Ann and Chris. But I don't know if you actually read my post at the beginning.
AVAO wrote: Mon May 08, 2023 4:47 am ...
This image was taken by Hubble’s Wide Field Camera 3 (WFC3) in the near-infrared part of the spectrum. A version of the image was entered into the Hubble’s Hidden Treasures image processing competition by Flickr user Det58.
Acknowledgement: Flickr user Det58 Release date: 2 June 2014
True colors look different to me...
https://www.astrobin.com/8aa26h/B/
There are only a few areas in pink or Hα. Compare:
https://apod.nasa.gov/apod/ap190702.html
The pink part is true color. That doesn't mean the entire image is. Nor is it correct to say that the color is "enhanced". The image necessarily utilizes a false color palette to some degree... something that should be obvious when at least one channel is collected in a wavelength range outside of human perception!

That said, the IR in this case is a wide filter centered at 814 nm, so essentially red barely beyond our eye sensitivity, and capable of capturing Ha emissions. I don't know how the UV data was used here (if it was), but the optical data is basically red, green, and blue, so the image is still approximately "true color", possibly with the Ha emission emphasized somewhat by the filter choices or processing.

I do wish that images like this were accompanied with some processing details, but the editors don't seem to care too much about that.
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Re: APOD: The Spanish Dancer Spiral Galaxy (2023 May 08)

Post by Ann » Mon May 08, 2023 6:45 pm

AVAO wrote: Mon May 08, 2023 4:42 pm
Ann wrote: Mon May 08, 2023 3:56 pm
Today's APOD is "true color". The pink color of the emission nebulas in the arms of NGC 1566 is correct. Yet we humans can never see these nebulas the way they are, namely "optically pink", because of the limitations of our eyes.

Ann
Sorry Ann and Chris. But I don't know if you actually read my post at the beginning.
AVAO wrote: Mon May 08, 2023 4:47 am ...
This image was taken by Hubble’s Wide Field Camera 3 (WFC3) in the near-infrared part of the spectrum. A version of the image was entered into the Hubble’s Hidden Treasures image processing competition by Flickr user Det58.
Acknowledgement: Flickr user Det58 Release date: 2 June 2014
True colors look different to me...
https://www.astrobin.com/8aa26h/B/
There are only a few areas in pink or Hα. Compare:
https://apod.nasa.gov/apod/ap190702.html
When I see a galaxy picture like today's APOD, I automatically assume that a hydrogen alpha filter has been used to bring out the red (or pink) nebulosity. This also means that I think that the Hα emission has been enhanced. That is not to say that it has been "made up". I think it is really there, but it would be fainter if it wasn't enhanced. Chris said he doesn't think that "enhanced" is a good description, but I can't think of a better word for it. But I agree with Chris that there is some degree of "false color" in this picture, and probably in most color images of galaxies too.

I found an image by William Ostling that has apparently been made with just RGB and luminosity filters (so no hydrogen alpha).

The picture looks like this, and I hope I am allowed to copy it:


And the inner spiral looks like this (I really hope I'm allowed to copy it):



As you can see, there is less pink hydrogen alpha here than there is in the APOD, and it is not distributed quite the same way, either.

For all of that, there is a good amount of pink nebulosity in William Ostling's picture, too.

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Re: APOD: The Spanish Dancer Spiral Galaxy (2023 May 08)

Post by johnnydeep » Mon May 08, 2023 8:02 pm

Well, I'll be a monkey's uncle - why am I only now understanding (via one of the links) that a Grand Design Spiral Galaxy specifically refers to one with TWO spiral arms!
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Re: APOD: The Spanish Dancer Spiral Galaxy (2023 May 08)

Post by AVAO » Mon May 08, 2023 8:37 pm

Stan23 wrote: Mon May 08, 2023 3:35 pm Layman's question: these pictures often leave me awe-struck. The idea of star clusters, dust lanes, star nurseries, etc. But I can't form an image of what the "hot" golden centre and surrounding yellow area inside the main dust lanes are made of. I've seen the "closeup" images of a super massive black hole, but I assume that just makes up a small central area, perhaps a light year or less? What makes up the golden and yellow area in the centre of the galaxy? Is it just a huge area of dense hot gas and dust swirling around the black hole as shown in Ann's post? Thanks.

Well. HUBBLE can show you the dust and WEBB can show you the stars :roll:

Close up of the NGC 1566 core:
Click to view full size image 1 or image 2
jac berne (flickr)
biggg: https://live.staticflickr.com/65535/528 ... 05c2_o.jpg
biggg: https://live.staticflickr.com/65535/528 ... 953e_o.jpg

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Re: APOD: The Spanish Dancer Spiral Galaxy (2023 May 08)

Post by orin stepanek » Mon May 08, 2023 9:04 pm

NGC1566_HubbleOdenthal_960.jpg
Wow! Beautiful!
istockphoto-1184409133-612x612.jpg
Doggy having fun! :lol2: Now! :evil:
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Re: APOD: The Spanish Dancer Spiral Galaxy (2023 May 08)

Post by Ann » Tue May 09, 2023 4:39 am

AVAO wrote: Mon May 08, 2023 8:37 pm
Well. HUBBLE can show you the dust and WEBB can show you the stars :roll:

Close up of the NGC 1566 core:
Click to view full size image 1 or image 2
jac berne (flickr)
biggg: https://live.staticflickr.com/65535/528 ... 05c2_o.jpg
biggg: https://live.staticflickr.com/65535/528 ... 953e_o.jpg

Absolutely, AVAO!

Can you imagine the boredom of a resolved JWST image of a dust free (or dust cloud free) elliptical galaxy?

You would have to agree with astronaut David Bowman of 2001 - A Space Odyssey when he exclaimed, at the end of the movie,

My God, it's full of stars!!!

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Re: APOD: The Spanish Dancer Spiral Galaxy (2023 May 08)

Post by Ann » Tue May 09, 2023 5:20 am

AVAO wrote: Mon May 08, 2023 4:42 pm
Ann wrote: Mon May 08, 2023 3:56 pm
Today's APOD is "true color". The pink color of the emission nebulas in the arms of NGC 1566 is correct. Yet we humans can never see these nebulas the way they are, namely "optically pink", because of the limitations of our eyes.

Ann
Sorry Ann and Chris. But I don't know if you actually read my post at the beginning.
AVAO wrote: Mon May 08, 2023 4:47 am ...
This image was taken by Hubble’s Wide Field Camera 3 (WFC3) in the near-infrared part of the spectrum. A version of the image was entered into the Hubble’s Hidden Treasures image processing competition by Flickr user Det58.
Acknowledgement: Flickr user Det58 Release date: 2 June 2014
I'll go out on a limb here and say that if the original Hubble data was taken by Hubble’s Wide Field Camera 3 in the near-infrared part of the spectrum, then it must have been an exposure through Hubble's beloved 814 nm filter. Because the Hubble people really use the 814 nm filter all the time.

But you are not going to construct a color picture of a galaxy from data from the 814 nm filter ONLY. If you are going to construct a color picture from infrared data only, you are going to need data from TWO infrared filters.

And I have never seen a Hubble color picture of a nearby galaxy that was taken through TWO infrared filters ONLY.


Hubble does have at least one other infrared filter too, at 775 nm, if I remember correctly. So there might be Hubble data of NGC 1566 at 814 nm and 775 nm, which could be used to create a false-color visible image of NGC 1566.

But what would be the point of Hubble observing NGC 1566 through two infrared filters but not through any visible light filters?

Take a look at this series of pictures of galaxies NGC 2207 and IC 2163, taken at different wavelengths. z is the longest wavelengths and u is the shortest:

NGC 2207 and IC 2613 at different wavelengths William C Keel et al.png
NGC 2207 and IC 2613 at different wavelengths, from longest to shortest.
Credit: William C. Keel at al.

z and i are infrared wavelengths, r and g are visible wavelengths and u is ultraviolet.

The series of pictures is low resolution, but you can see that the details in the arms get sharper and sharper at shorter wavelengths (and blurrier and blurrier at longer wavelengths). Therefore, since Hubble is able to photograph nearby starforming galaxies at shorter wavelengths, what would be the point of photographing such galaxies at two near infrared wavelengths only? For a telescope like Hubble?

I don't believe that there is Hubble data of NGC 1566 from two infrared filters only. That would practically useless. There might - MIGHT - be Hubble data for NGC 1566 at 814 nm only, but in that case, you wouldn't be able to construct a color picture of NGC 1566 from the Hubble data.

My guess is that Hubble has imaged NGC 1566 through at least three filters, where at least two are visible light filters. The near infrared filter is almost certainly 814 nm, because the Hubble people love this filter. It would detect the old yellow stars in the center of NGC 1566.

Hubble has also almost certainly imaged NGC 1566 through a 656 nm filter for hydrogen alpha. This filter would detect the pink emission nebulas.

Hubble has also very likely imaged NGC 1566 through a filter centered on a wavelength shorter than 500 nm. Alternatively it might have imaged NGC 1566 through a filter centered on a wavelength a bit longer than 500 nm, possibly as long as 606 nm (since the 606 nm filter is also used a lot by the Hubble people), but a 606 nm filter would be less useful as a stand in for a B filter for NGC 1566. In any case, this filter would detect the blue stars in the arms of NGC 1566.

My point is that today's APOD in all probability - in ALL probability!!! - is not based on near infrared data ONLY.

Ann
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Re: APOD: The Spanish Dancer Spiral Galaxy (2023 May 08)

Post by Chris Peterson » Tue May 09, 2023 2:12 pm

Ann wrote: Tue May 09, 2023 5:20 am
AVAO wrote: Mon May 08, 2023 4:42 pm
Ann wrote: Mon May 08, 2023 3:56 pm
Today's APOD is "true color". The pink color of the emission nebulas in the arms of NGC 1566 is correct. Yet we humans can never see these nebulas the way they are, namely "optically pink", because of the limitations of our eyes.

Ann
Sorry Ann and Chris. But I don't know if you actually read my post at the beginning.
AVAO wrote: Mon May 08, 2023 4:47 am ...
This image was taken by Hubble’s Wide Field Camera 3 (WFC3) in the near-infrared part of the spectrum. A version of the image was entered into the Hubble’s Hidden Treasures image processing competition by Flickr user Det58.
Acknowledgement: Flickr user Det58 Release date: 2 June 2014
I'll go out on a limb here and say that if the original Hubble data was taken by Hubble’s Wide Field Camera 3 in the near-infrared part of the spectrum, then it must have been an exposure through Hubble's beloved 814 nm filter. Because the Hubble people really use the 814 nm filter all the time.
As I noted earlier, this HST project collected data through five filters, but we don't know the details of which ones were used to construct this particular image. There were two UV filters (275nm, 336nm), two visible light filters (438nm, 555nm), and a near-IR filter (814nm). With this set it isn't difficult to construct an approximately "true color" image, as we have reasonable RGB data from the 814nm, 555nm, and 438nm filters. The UV data may not have been used, or it may have been combined with the blue channel. In any case, this mapping would yield something reasonably treated as "true color".
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Re: APOD: The Spanish Dancer Spiral Galaxy (2023 May 08)

Post by Ann » Tue May 09, 2023 2:47 pm

Chris Peterson wrote: Tue May 09, 2023 2:12 pm
Ann wrote: Tue May 09, 2023 5:20 am
AVAO wrote: Mon May 08, 2023 4:42 pm

Sorry Ann and Chris. But I don't know if you actually read my post at the beginning.


I'll go out on a limb here and say that if the original Hubble data was taken by Hubble’s Wide Field Camera 3 in the near-infrared part of the spectrum, then it must have been an exposure through Hubble's beloved 814 nm filter. Because the Hubble people really use the 814 nm filter all the time.
As I noted earlier, this HST project collected data through five filters, but we don't know the details of which ones were used to construct this particular image. There were two UV filters (275nm, 336nm), two visible light filters (438nm, 555nm), and a near-IR filter (814nm). With this set it isn't difficult to construct an approximately "true color" image, as we have reasonable RGB data from the 814nm, 555nm, and 438nm filters. The UV data may not have been used, or it may have been combined with the blue channel. In any case, this mapping would yield something reasonably treated as "true color".
Thanks, Chris, I missed that info. So no 656 hydrogen alpha filter, then.

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Re: APOD: The Spanish Dancer Spiral Galaxy (2023 May 08)

Post by Chris Peterson » Tue May 09, 2023 2:56 pm

Ann wrote: Tue May 09, 2023 2:47 pm
Chris Peterson wrote: Tue May 09, 2023 2:12 pm
Ann wrote: Tue May 09, 2023 5:20 am

I'll go out on a limb here and say that if the original Hubble data was taken by Hubble’s Wide Field Camera 3 in the near-infrared part of the spectrum, then it must have been an exposure through Hubble's beloved 814 nm filter. Because the Hubble people really use the 814 nm filter all the time.
As I noted earlier, this HST project collected data through five filters, but we don't know the details of which ones were used to construct this particular image. There were two UV filters (275nm, 336nm), two visible light filters (438nm, 555nm), and a near-IR filter (814nm). With this set it isn't difficult to construct an approximately "true color" image, as we have reasonable RGB data from the 814nm, 555nm, and 438nm filters. The UV data may not have been used, or it may have been combined with the blue channel. In any case, this mapping would yield something reasonably treated as "true color".
Thanks, Chris, I missed that info. So no 656 hydrogen alpha filter, then.

Ann
No, I think the Ha would have to come off the 555nm channel. So I'd be interested in the processing details. Sadly, they are all too often missing from APOD submissions.
Chris

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Re: APOD: The Spanish Dancer Spiral Galaxy (2023 May 08)

Post by bystander » Tue May 09, 2023 4:22 pm

Chris Peterson wrote: Tue May 09, 2023 2:56 pm
Ann wrote: Tue May 09, 2023 2:47 pm
Chris Peterson wrote: Tue May 09, 2023 2:12 pm
As I noted earlier, this HST project collected data through five filters, but we don't know the details of which ones were used to construct this particular image. There were two UV filters (275nm, 336nm), two visible light filters (438nm, 555nm), and a near-IR filter (814nm). With this set it isn't difficult to construct an approximately "true color" image, as we have reasonable RGB data from the 814nm, 555nm, and 438nm filters. The UV data may not have been used, or it may have been combined with the blue channel. In any case, this mapping would yield something reasonably treated as "true color".
Thanks, Chris, I missed that info. So no 656 hydrogen alpha filter, then.

Ann
No, I think the Ha would have to come off the 555nm channel. So I'd be interested in the processing details. Sadly, they are all too often missing from APOD submissions.
First of all I have to say I don't think this was processed by Detlev Odenthal. His version can be found here. This was processed by ESA and was the ESA Hubble POTW for 2014 Jun 02. There's not much more processing details to be found there, but you can find the color assignments.
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Ann
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Re: APOD: The Spanish Dancer Spiral Galaxy (2023 May 08)

Post by Ann » Wed May 10, 2023 5:25 am

AVAO wrote: Mon May 08, 2023 4:42 pm
Ann wrote: Mon May 08, 2023 3:56 pm
Today's APOD is "true color". The pink color of the emission nebulas in the arms of NGC 1566 is correct. Yet we humans can never see these nebulas the way they are, namely "optically pink", because of the limitations of our eyes.

Ann
Sorry Ann and Chris. But I don't know if you actually read my post at the beginning.
AVAO wrote: Mon May 08, 2023 4:47 am ...
This image was taken by Hubble’s Wide Field Camera 3 (WFC3) in the near-infrared part of the spectrum. A version of the image was entered into the Hubble’s Hidden Treasures image processing competition by Flickr user Det58.
Acknowledgement: Flickr user Det58 Release date: 2 June 2014
True colors look different to me...
https://www.astrobin.com/8aa26h/B/
There are only a few areas in pink or Hα. Compare:
https://apod.nasa.gov/apod/ap190702.html
AVAO, I apologize for dismissing you out of hand when you objected to my description of the APOD as "true color". (I did use the quotation marks.)

Anyway, your main point was that the APOD portrait of NGC 1566 could not be true color since no red filter had been used to collect the data for the image. Instead of a red filter, a near infrared filter of 814 nm had been used.

Can that sort of thing be done and we can still claim that the image is "true color"?

Yes and no.

Please go to this site to find a slider that you can move left and right to see what actual color corresponds to what wavelength.

The slider starts at 500 nm. It is this color, ███. The RGB values of this color are, r=0, g=255, b=146. The hex is #00ff92. Starship Asterisk* uses hex. If you want to colorize a word here, you click the little "paint drop" next to the word "normal" above the "box" where you actually write your reply. Hex values are used to colorize the words.

Anyway. Let's go back to the site I suggested. The "color slider" moves to the right and left by 2 nanometers at a time. So let's move to 502 nm, ███. Now the hex is #00ff7b and the RGB values are, r=0, g=255, b=123. The RGB values have changed slightly, and the hex has changed. The color has also changed, if only very little.

Now let's skip ahead by 10 nanometers from 502 to 512 nm. The color looks like this, ███, and the RGB values have really changed: r=15, g=255, b=0. The red channel has made an appearance, and the blue channel has disappeared.

Now let's skip ahead 24 nanometers from our starting point and go to 524 nm. It looks like this, ███. Okay, now let's move the other way, towards shorter wavelengths, by 24 nanometers, again starting from 500 nm. We come to 476 nm, which looks like this, ███. The RGB values here are r=0, g=196, b=255.

Why do I show you this? It's because we now need to go to the red part of the slider and see something rather remarkable. Let's start at 643 nm (yes, the slider has skipped to odd numbers here). The color looks like this, ███, and the RGB values are r=255, g=16, b=0.

Now look what happens. At 645 nm, the color looks like this, ███, and the RGB values are r=255, g=0 and b=0.

Guess what? The color, the hex and the RGB values remain absolutely the same as we move "up" all the way to 700 nm! Nothing changes for 55 nm!

Let's put 476 and 524 nm right next to one another again. That's a difference of 48 nm, and the colors look very different: ███ ███

Now let's put 645 and 700 nm next to one another. That's a difference of 55 nm, but the colors are, to our eyes and by RGB and hex values, exactly the same! ███ ███

What happens after 700 nm is that the shade of red gets darker, as the human retina gets less and less sensitive to longer and longer wavelengths. At 720 nm, the color looks like this, ███. At 740 nm, the color looks like this, ███. At 759 nm, it looks like this, ███. At 775 nm the color looks like this, ███. I'm pretty sure that Hubble has a filter of 775 nm that counts as a near infrared filter, but as you can see, there is definitely some optical red light here still.

The slider stops at 780 nm, ███, and there is still a bit of red left here. The RGB values for 780 nm is r=97, g=0 and b=0.

My point is that red is a very persistent color that "lasts for a long time". I really doubt that there is any optical red light left at 814 nm. Still, 814 nm is close enough to wavelengths that can indeed be described as red, or at least as reddish, that 814 nm can be used as a "reasonable proxy" for red light.

But the picture of NGC 1566 that was yesterday's APOD is still not really true color. As Chris explained in one of his posts, it is reasonably close to true color. But not quite. You were right about that.

Ann
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