Starship Asterisk*

Milky Way over French Alp Hoodoos

Explanation: Real castles aren't this old. And the background galaxy is even older. Looking a bit like an alien castle, the pictured rock spires are called hoodoos and are likely millions of years old. Rare, but found around the world, hoodoos form when dense rocks slow the erosion of softer rock underneath. The pictured hoodoos survive in the French Alps and are named Demoiselles Coiffées -- which translates to English as "Ladies with Hairdos". The background galaxy is part of the central disk of our own Milky Way galaxy and contains stars that are typically billions of years old. The photogenic Cygnus sky region -- rich in dusty dark clouds and red glowing nebulas -- appears just above and behind the hoodoos. The featured image was taken in two stages: the foreground was captured during the evening blue hour, while the background was acquired from the same location later that night.

<< Previous APOD

This Day in APOD

Next APOD >>

Cool photo, great composition but a little too saturated for my taste.

Hoodoo? do what? Hoodoo! Voodoo; you do? Adorable cat modeling hair styles! Cygnus Veil--- Just beautiful!!!

XgeoX wrote: ↑Mon May 16, 2022 7:47 am Cool photo, great composition but a little too saturated for my taste.

Yes, the image is quite saturated in Hydrogen Alpha, but then again, there really is a lot of Hα in parts of Cygnus (near Deneb and Sadr), and there is also quite a lot of Hα in Cepheus.


You can find the North America Nebula and the Pelican Nebula in my annotated version of the APOD. In Alistair Symon's image, you can find the North America Nebula and the Pelican Nebula right of center.

In the APOD, you can see IC 1396 at far left. In Alistair Symon's image, you can see a whole complex of nebulas in the upper left part of the picture. The nebula farthest to the right if these (except a smaller nebula above it) is IC 1396 in Cepheus.

Note a stretch of Milky Way between the Cygnus and Cepheus that contains no visible Hα at all. Most of the visible Milky Way is like that: it contains no visible nebulas and no visible Hα.

One large area in Cygnus and one large area in Cepheus are special, because they contain so much Hα. Photographers Benjamin Barakat and Alistair Symon decided to highlight this Hα by imaging this part of the Milky for many hours through an Hα filter.

So yes, today's APOD is saturated in Hα, but then again, you can't bring out what isn't there (in more than trace amounts).

Ann

You say, "you can't bring out what isn't there." The text does not suggest that this image is enhanced. Does that go without saying?

My dad and I are going around for the second day about whether you can collect that much red light out of the night sky.

EricaT wrote: ↑Tue May 17, 2022 4:46 pm You say, "you can't bring out what isn't there." The text does not suggest that this image is enhanced. Does that go without saying?

My dad and I are going around for the second day about whether you can collect that much red light out of the night sky.

Erica,

Please note that I'm most certainly not saying that the APOD is lying in any way. All the photographer has done is make a very long exposure (or perhaps a long series of short exposures) of this part of the Milky Way through an Hα filter. There is nothing wrong or dishonest about that! But please note that you can't bring out so much red Hα if you don't use an Hα filter.

I googled "Cygnus Milky Way" and found this image:


Compare the APOD (the picture at right) with Edoardo Dusina's image from Wikipedia. It is not nearly as red as the APOD, and it seem obvious to me that Edoardo Dusina has not used an Hα filter, only a red filter (as well as a green and a blue filter, or, alternatively, an "all-color filter", if you get what I mean by that).

You can't bring out so much Hα in the sky as Benjamin Barakat has done if you don't use an Hα filter. Obviously, though, a good deal of Hα has to be there in the first place.


As you can see, Richard Crisp's image is redder than Edoardo Dusina's image, but less red than the APOD. Almost certainly Crisp collect as much Hα light through his Hα filter as Benjamin Barakat did.

Note too in Richard Crisp's image that long stretches of Milky Way don't look red. There is little Hα in these parts.

In case Richard Crisp's image disappears, which it might go this this address to see it.

Ann

Ann wrote: ↑Tue May 17, 2022 5:10 pm

EricaT wrote: ↑Tue May 17, 2022 4:46 pm You say, "you can't bring out what isn't there." The text does not suggest that this image is enhanced. Does that go without saying?

My dad and I are going around for the second day about whether you can collect that much red light out of the night sky.

Erica,

Please note that I'm most certainly not saying that the APOD is lying in any way...

The reality is, the only accurate statement is that the "true color" is gray. That is all our eyes can see, and what our eyes see is the only perfectly accurate way to talk about "true color".

The data is real. The presentation of the data- the color mapping, the balance between the channels, the brightness and saturation- all can dramatically alter the appearance of the image... which is not the same as the actual visual scene. Those choices can be designed to emphasize some aspect or aspects of the data, or they may be chosen for aesthetic purposes. But it's why we see so many very different looking images of the same targets. And realistically, none are "accurate" in a visual sense.

Chris Peterson wrote: ↑Tue May 17, 2022 11:32 pm

Ann wrote: ↑Tue May 17, 2022 5:10 pm

EricaT wrote: ↑Tue May 17, 2022 4:46 pm You say, "you can't bring out what isn't there." The text does not suggest that this image is enhanced. Does that go without saying?

My dad and I are going around for the second day about whether you can collect that much red light out of the night sky.

Erica,

Please note that I'm most certainly not saying that the APOD is lying in any way...

The reality is, the only accurate statement is that the "true color" is gray. That is all our eyes can see, and what our eyes see is the only perfectly accurate way to talk about "true color".

The data is real. The presentation of the data- the color mapping, the balance between the channels, the brightness and saturation- all can dramatically alter the appearance of the image... which is not the same as the actual visual scene. Those choices can be designed to emphasize some aspect or aspects of the data, or they may be chosen for aesthetic purposes. But it's why we see so many very different looking images of the same targets. And realistically, none are "accurate" in a visual sense.

I get your point, Chris, which is certainly accurate when it comes to the red color of Hα. The human eye's comparative low sensitivity to red color, coupled with the fact that the Hα intensity as perceived by us in all known deep-space objects is below our sensitivity threshold for color vision, means that we can't see the red color of Hα in space. Period. That said, there is certainly a lot more "Hα data" to be picked up by photographic equipment in the Cygnus region near Deneb and Sadr than in most places along the plane of the Milky Way.

So no, humans can't see the red color of Hα anywhere in space, but yes, we can photograph it and bring a lot of it out in the Cygnus region near Deneb and Sadr, because there is a lot of Hα data to be picked out in that particular place of the Milky Way.

And if our eyes were many times more sensitive to faint red light than they are, we would see that Hα light is red.


There is sure a lot more Hα to be detected in some parts of the Milky Way than in others.

A question, though, Chris. It is possible to see the North America nebula as a large gray patch. What are we seeing, though? Are we picking up the red Hα light, even though we "interpret" it as gray? Or are we seeing OIII, or perhaps Hβ?

Ann

Ann wrote: ↑Wed May 18, 2022 4:20 am

Chris Peterson wrote: ↑Tue May 17, 2022 11:32 pm

Ann wrote: ↑Tue May 17, 2022 5:10 pm

Erica,

Please note that I'm most certainly not saying that the APOD is lying in any way...

The reality is, the only accurate statement is that the "true color" is gray. That is all our eyes can see, and what our eyes see is the only perfectly accurate way to talk about "true color".

The data is real. The presentation of the data- the color mapping, the balance between the channels, the brightness and saturation- all can dramatically alter the appearance of the image... which is not the same as the actual visual scene. Those choices can be designed to emphasize some aspect or aspects of the data, or they may be chosen for aesthetic purposes. But it's why we see so many very different looking images of the same targets. And realistically, none are "accurate" in a visual sense.

I get your point, Chris, which is certainly accurate when it comes to the red color of Hα. The human eye's comparative low sensitivity to red color, coupled with the fact that the Hα intensity as perceived by us in all known deep-space objects is below our sensitivity threshold for color vision, means that we can't see the red color of Hα in space. Period. That said, there is certainly a lot more "Hα data" to be picked up by photographic equipment in the Cygnus region near Deneb and Sadr than in most places along the plane of the Milky Way.

So no, humans can't see the red color of Hα anywhere in space, but yes, we can photograph it and bring a lot of it out in the Cygnus region near Deneb and Sadr, because there is a lot of Hα data to be picked out in that particular place of the Milky Way.

And if our eyes were many times more sensitive to faint red light than they are, we would see that Hα light is red.

Whatever that means. Because the color of Hα light depends on how bright it is. It is 1000 different colors to our eyes.

A question, though, Chris. It is possible to see the North America nebula as a large gray patch. What are we seeing, though? Are we picking up the red Hα light, even though we "interpret" it as gray? Or are we seeing OIII, or perhaps Hβ?

Not sure. But through a telescope, [O III] filters generally provide the best views of emission nebulas. So I think most of the gray we see is coming from that. Which is consistent with our rod peak response at about 500 nm, and the fact that we have almost no rod response at Hα.

Chris Peterson wrote: ↑Wed May 18, 2022 4:56 am

Ann wrote: ↑Wed May 18, 2022 4:20 am

Chris Peterson wrote: ↑Tue May 17, 2022 11:32 pm
The reality is, the only accurate statement is that the "true color" is gray. That is all our eyes can see, and what our eyes see is the only perfectly accurate way to talk about "true color".

The data is real. The presentation of the data- the color mapping, the balance between the channels, the brightness and saturation- all can dramatically alter the appearance of the image... which is not the same as the actual visual scene. Those choices can be designed to emphasize some aspect or aspects of the data, or they may be chosen for aesthetic purposes. But it's why we see so many very different looking images of the same targets. And realistically, none are "accurate" in a visual sense.

And if our eyes were many times more sensitive to faint red light than they are, we would see that Hα light is red.

Whatever that means. Because the color of Hα light depends on how bright it is. It is 1000 different colors to our eyes.

A question, though, Chris. It is possible to see the North America nebula as a large gray patch. What are we seeing, though? Are we picking up the red Hα light, even though we "interpret" it as gray? Or are we seeing OIII, or perhaps Hβ?

Not sure. But through a telescope, [O III] filters generally provide the best views of emission nebulas. So I think most of the gray we see is coming from that. Which is consistent with our rod peak response at about 500 nm, and the fact that we have almost no rod response at Hα.

Yes, yes... I know you insist that hue is about wavelength and color is a combination of wavelength and intensity.

Thank you for the info on OIII.

Ann