by Ann » Mon Jan 01, 2024 9:43 am
First of all, A Happy New Year everyone!
And indeed, the date of this image must be changed, from January 3 to January 1.
And... oh well. There is finally a galaxy image as an APOD, and I'm critical of it.
First of all, this is a very old image, from 1998. It's not wrong to show old images, but if an image is old you should acknowledge its age and give a reason for why the picture is still relevant.
Second, the colors are weird. They really are. We can see that there are emission nebulas in this galaxy, and we should be able to see red or pink color in at least some of them. Yet we don't. And the arms are very harshly, unnaturally blue.
The reason for the weird colors is the filters chosen for the image. The filters were 360 nm, an ultraviolet filter, 420 nm, a fairly deep dark purple filter,
███, and 600 nm, a yellow-orange filter,
███.
It is clear that you can't construct a picture from ultraviolet, purple and orange filters that matches what our eyes would see, if our organs of vision were enormously sensitive to the faint light emitted by galaxies. You would need RGB filters for that:
███,
███,
███.
By using RGB filters, you could indeed create a picture of NGC 1232 that matches what our eyes would see, if they were thousands of times more sensitive.
Interestingly, there exists another ESO picture (well, an ESO/IDA/Danish 1.5 meter telescope picture) of NGC 1232. That picture is a bit younger too, from 2009. I definitely like that picture better than the APOD:
If you look carefully at the ESO/IDA/Danish telescope image, you can indeed see little pink spots in the arms of NGC 1232, marking the presence of emission nebulas. Also note that the arms are not as harshly unnaturally blue as they are in the APOD. That is because this image has been created from exposures through RVB filters, which closely resemble RGB filters.
My second complaint regarding the APOD is this:
NGC 1232 is not a grand design galaxy!
Yes, I know that the Wikipedia entry on NGC 1232 claims that it is a grand design galaxy. Well, I'd say they are wrong:
Wikipedia wrote:
NGC 1232 is a face-on spiral galaxy.
It can be technically considered a Grand-design galaxy and is considered a prototype for multi-arm spiral galaxies.
That is a contradiction in terms!
Wikipedia wrote:
A grand design spiral galaxy is a type of spiral galaxy with prominent and well-defined spiral arms,
as opposed to multi-arm and flocculent spirals which have subtler structural features.
I wouldn't go so far as to call NGC 1232 a
flocculent galaxy, because it clearly is not. But a multi-armed galaxy shouldn't be described as a grand design galaxy.
If you want to see a beautiful grand design galaxy, then consider NGC 1566, the Spanish Dancer galaxy:
A stunning image, isn't it? Note how well-defined the two main arms of NGC 1566 really are. That's a grand design galaxy if I ever saw one!
Note all the red light from emission nebulas, which have become so visible thanks to the use of an NII filter at 658 nm, very close to hydrogen alpha at 656 nm. The image has also been constructed from exposures through two ultraviolet filter, one image through a 438 nm filter (close to the "ideal " 440 nm blue filter) one image through a 555 nm filter (close to the "ideal" green filter) and one image through a near-infrared 814 nm filter. That's a truly great choice of filters for creating an image that closely matches what our eyes would see if they were thousands of times more sensitive to the faint light of galaxies!
I hope the APOD will feature a beautiful galaxy picture soon that is more recent than from 1998 and which has been constructed from a more RGB-like set of filters than today's APOD!
Ann
First of all, A Happy New Year everyone! 🌟 💫 ⚡️ And indeed, the date of this image must be changed, from January 3 to January 1.
And... oh well. There is finally a galaxy image as an APOD, and I'm critical of it. :(
[img3="NGC 1232. Credit: ESO."]https://www.eso.org/public/archives/images/screen/eso9845d.jpg[/img3]
First of all, this is a very old image, from 1998. It's not wrong to show old images, but if an image is old you should acknowledge its age and give a reason for why the picture is still relevant.
Second, the colors are weird. They really are. We can see that there are emission nebulas in this galaxy, and we should be able to see red or pink color in at least some of them. Yet we don't. And the arms are very harshly, unnaturally blue.
The reason for the weird colors is the filters chosen for the image. The filters were 360 nm, an ultraviolet filter, 420 nm, a fairly deep dark purple filter, [color=#6d00fb]███[/color], and 600 nm, a yellow-orange filter, [color=#ffbb00]███[/color].
It is clear that you can't construct a picture from ultraviolet, purple and orange filters that matches what our eyes would see, if our organs of vision were enormously sensitive to the faint light emitted by galaxies. You would need RGB filters for that: [color=#ff0000]███[/color], [color=#a6ff00]███[/color], [color=#000bff]███[/color].
By using RGB filters, you could indeed create a picture of NGC 1232 that matches what our eyes would see, if they were thousands of times more sensitive.
Interestingly, there exists another ESO picture (well, an ESO/IDA/Danish 1.5 meter telescope picture) of NGC 1232. That picture is a bit younger too, from 2009. I definitely like that picture better than the APOD:
[img3="NGC 1232. Credit: ESO/IDA/Danish 1.5 m/R.Gendler and A. Hornstrup."]https://www.eso.org/public/archives/images/screen/ngc1232b.jpg[/img3]
If you look carefully at the ESO/IDA/Danish telescope image, you can indeed see little pink spots in the arms of NGC 1232, marking the presence of emission nebulas. Also note that the arms are not as harshly unnaturally blue as they are in the APOD. That is because this image has been created from exposures through RVB filters, which closely resemble RGB filters.
My second complaint regarding the APOD is this:
[b][size=120]NGC 1232 is not a grand design galaxy![/size][/b]
Yes, I know that the Wikipedia entry on NGC 1232 claims that it is a grand design galaxy. Well, I'd say they are wrong:
[quote][url=https://en.wikipedia.org/wiki/NGC_1232#Description_and_Structure]Wikipedia[/url] wrote:
NGC 1232 is a face-on spiral galaxy. [b][color=#0040FF]It can be technically considered a Grand-design galaxy[/color][/b] [b][color=#FF0000]and is considered a prototype for multi-arm spiral galaxies.[/color][/b][/quote]
[b][size=110]That is a contradiction in terms![/size][/b]
[quote][url=https://en.wikipedia.org/wiki/Grand_design_spiral_galaxy]Wikipedia[/url] wrote:
[b][color=#0040FF]A grand design spiral galaxy is a type of spiral galaxy with prominent and well-defined spiral arms[/color][/b], [b][color=#FF0000]as opposed to multi-arm and flocculent spirals which have subtler structural features.[/color][/b][/quote]
I wouldn't go so far as to call NGC 1232 a [url=https://en.wikipedia.org/wiki/Flocculent_spiral_galaxy]flocculent galaxy[/url], because it clearly is not. But a multi-armed galaxy shouldn't be described as a grand design galaxy.
If you want to see a beautiful grand design galaxy, then consider NGC 1566, the Spanish Dancer galaxy:
[img3="NGC 1566, the Spanish Dancer. Credit: Credit:
ESA/Hubble & NASA, D. Calzetti and the LEGUS team, R. Chandar"]https://cdn.esahubble.org/archives/images/screen/potw2344a.jpg[/img3]
A stunning image, isn't it? Note how well-defined the two main arms of NGC 1566 really are. That's a grand design galaxy if I ever saw one!
Note all the red light from emission nebulas, which have become so visible thanks to the use of an NII filter at 658 nm, very close to hydrogen alpha at 656 nm. The image has also been constructed from exposures through two ultraviolet filter, one image through a 438 nm filter (close to the "ideal " 440 nm blue filter) one image through a 555 nm filter (close to the "ideal" green filter) and one image through a near-infrared 814 nm filter. That's a truly great choice of filters for creating an image that closely matches what our eyes would see if they were thousands of times more sensitive to the faint light of galaxies!
I hope the APOD will feature a beautiful galaxy picture soon that is more recent than from 1998 and which has been constructed from a more RGB-like set of filters than today's APOD!
Ann