Explanation: What does the Andromeda galaxy really look like? The featured image shows how our Milky Way Galaxy's closest major galactic neighbor really appears in a long exposure through Earth's busy skies and with a digital camera that introduces normal imperfections. The picture is a stack of 223 images, each a 300 second exposure, taken from a garden observatory in Portugal over the past year. Obvious image deficiencies include bright parallel airplane trails, long and continuous satellite trails, short cosmic ray streaks, and bad pixels. These imperfections were actually not removed with Photoshop specifically, but rather greatly reduced with a series of computer software packages that included Astro Pixel Processor, DeepSkyStacker, and PixInsight. All of this work was done not to deceive you with a digital fantasy that has little to do with the real likeness of the Andromeda galaxy (M31), but to minimize Earthly artifacts that have nothing to do with the distant galaxy and so better recreate what M31 really does look like.
I like the photo at left. The pale yellowish color is just the way I remember it from the time when I first saw M31, back in 1972(?) on a cold February evening in my parents' back yard.
Of course, I didn't see even a trace of the disk of Andromeda.
Alex_g444 wrote: ↑Mon Oct 14, 2019 4:39 am
It looks like some stars have disappeared in the process.
No, nothing was changed, this is the raw stack, the histogram is stretched with everything in it without any editing. There is also a "mouse over" function to show a version with all trails and tracks removed with LN Winsor Sigma clipping in Astropixelprocessor.
The 223 Luminance images with 300 second exposure where made between 12 nov 2018 and 4 oct 2019. I make around 12 images per night when there is no Moon above the horizon and M31 is at least at 30 degrees altitude. For each night i make a stack and use that stack to do a blink comparison with a reference stack to find novae in M31. And so after a while you end up with a lot of subs and every now and then i make a stack with all the collected subs. To find Asteroids i also make stacks without "sigma clipping". The Asteroid will show up as a short line, there is 1 in the middle top in this image, just below the letter "c" of "October". It is Asteroid 334168 ST119 imaged on 4 october 2019 (6.3km diameter, magnitude 20.8). The image featured here showed a nice airplane/ Satellite trail frame around M31.
Hi Kees. Nice photo. About the disappearing "stars". I noticed by flipping between the regular photo and the "annotated" photo that there are about seven or eight "stars" that vanish, and they are all in a straight line parallel to the lines on the left. Also most appear to be the same, a bright point of light with a dimmer companion on the left. They may be the same object. However with an exposure time of five minutes they likely aren't planes, but more distant Earth objects. Could they be artificial satellites or even a passing asteroid?
De58te wrote: ↑Mon Oct 14, 2019 12:22 pm
Hi Kees. Nice photo. About the disappearing "stars". I noticed by flipping between the regular photo and the "annotated" photo that there are about seven or eight "stars" that vanish, and they are all in a straight line parallel to the lines on the left. Also most appear to be the same, a bright point of light with a dimmer companion on the left. They may be the same object. However with an exposure time of five minutes they likely aren't planes, but more distant Earth objects. Could they be artificial satellites or even a passing asteroid?
They are the flashing lights of the airplane, that is why they have disappeared in the final image. They appear only on 1 subframe and will be removed by the outlier rejection.
Last edited by keesscherer on Mon Oct 14, 2019 1:09 pm, edited 1 time in total.
E Fish wrote: ↑Mon Oct 14, 2019 12:59 pm
It seems to me that the central bulge gets brighter in the after image and the disk is more defined. Did you also remove atmospheric distortions?
The "before" and "after" images are made with the same data, but both have to be stretched so that the histogram fits the brightness levels from 0 to 1.
An unstretched image looks like the one attached, only the core of M31 is visible, the rest is way too dark.
The levels in both images are different to begin with including background level. With both histrogram stretches i have tried to make the backgroundlevels equal as much as possible so "the mouse over" would not flicker too much. I did not match all the other levels as i wanted to keep the processing to a minimum.
E Fish wrote: ↑Mon Oct 14, 2019 12:59 pm
It seems to me that the central bulge gets brighter in the after image and the disk is more defined. Did you also remove atmospheric distortions?
The "before" and "after" images are made with the same data, but both have to be stretched so that the histogram fits the brightness levels from 0 to 1.
An unstretched image looks like the one attached, only the core of M31 is visible, the rest is way too dark.
M31-unstretched.jpgThe levels in both images are different to begin with including background level. With both histrogram stretches i have tried to make the backgroundlevels equal as much as possible so "the mouse over" would not flicker too much. I did not match all the other levels as i wanted to keep the processing to a minimum.
I love it! That's exactly what M31 looked to me back in 1972(?), except that the core of the galaxy was yellowish!
E Fish wrote: ↑Mon Oct 14, 2019 12:59 pm
It seems to me that the central bulge gets brighter in the after image and the disk is more defined. Did you also remove atmospheric distortions?
The "before" and "after" images are made with the same data, but both have to be stretched so that the histogram fits the brightness levels from 0 to 1.
An unstretched image looks like the one attached, only the core of M31 is visible, the rest is way too dark.
M31-unstretched.jpgThe levels in both images are different to begin with including background level. With both histrogram stretches i have tried to make the backgroundlevels equal as much as possible so "the mouse over" would not flicker too much. I did not match all the other levels as i wanted to keep the processing to a minimum.
I love it! That's exactly what M31 looked to me back in 1972(?), except that the core of the galaxy was yellowish! :D
Telescopically, the core of M31 isn't bright enough to stimulate color vision. So your impression of color was coming from your brain, not from the galaxy.
Chris
*****************************************
Chris L Peterson
Cloudbait Observatory https://www.cloudbait.com
The "before" and "after" images are made with the same data, but both have to be stretched so that the histogram fits the brightness levels from 0 to 1.
An unstretched image looks like the one attached, only the core of M31 is visible, the rest is way too dark.
M31-unstretched.jpgThe levels in both images are different to begin with including background level. With both histrogram stretches i have tried to make the backgroundlevels equal as much as possible so "the mouse over" would not flicker too much. I did not match all the other levels as i wanted to keep the processing to a minimum.
I love it! That's exactly what M31 looked to me back in 1972(?), except that the core of the galaxy was yellowish!
Telescopically, the core of M31 isn't bright enough to stimulate color vision. So your impression of color was coming from your brain, not from the galaxy.
All I can say is that I had no expectations whatsoever as to what Andromeda would look like. I was startled and delighted by the pale yellowish color. Not that I wouldn't have loved it even better if the color had been blue, but my point is, I didn't expect any color but I did see it. And I didn't know back then that galaxies typically have yellow cores.
You say I didn't see the yellow color of the center of Andromeda. I say I did.
I love it! That's exactly what M31 looked to me back in 1972(?), except that the core of the galaxy was yellowish! :D
Telescopically, the core of M31 isn't bright enough to stimulate color vision. So your impression of color was coming from your brain, not from the galaxy.
All I can say is that I had no expectations whatsoever as to what Andromeda would look like. I was startled and delighted by the pale yellowish color. Not that I wouldn't have loved it even better if the color had been blue, but my point is, I didn't expect any color but I did see it. And I didn't know back then that galaxies typically have yellow cores.
You say I didn't see the yellow color of the center of Andromeda. I say I did. :wink:
I suspect you have atypical color vision.
Chris
*****************************************
Chris L Peterson
Cloudbait Observatory https://www.cloudbait.com
Chris Peterson wrote: ↑Mon Oct 14, 2019 2:27 pm
Telescopically, the core of M31 isn't bright enough to stimulate color vision. So your impression of color was coming from your brain, not from the galaxy.
All I can say is that I had no expectations whatsoever as to what Andromeda would look like. I was startled and delighted by the pale yellowish color. Not that I wouldn't have loved it even better if the color had been blue, but my point is, I didn't expect any color but I did see it. And I didn't know back then that galaxies typically have yellow cores. You say I didn't see the yellow color of the center of Andromeda. I say I did.
Telescopically, the core of M31 isn't bright enough to stimulate color vision. So your impression of color was coming from your brain, not from the galaxy.
All I can say is that I had no expectations whatsoever as to what Andromeda would look like. I was startled and delighted by the pale yellowish color. Not that I wouldn't have loved it even better if the color had been blue, but my point is, I didn't expect any color but I did see it. And I didn't know back then that galaxies typically have yellow cores.
You say I didn't see the yellow color of the center of Andromeda. I say I did.
I suspect you have atypical color vision.
I'm just glad that as a child, your reaction to the backyard viewing of Andromeda through a small telescope was one of wonder and careful observation, as opposed to "Mom[Dad], it's too cold out here. I want to go inside."
keesscherer wrote: ↑Mon Oct 14, 2019 7:38 am
The 223 Luminance images with 300 second exposure where made between 12 nov 2018 and 4 oct 2019. I make around 12 images per night when there is no Moon above the horizon and M31 is at least at 30 degrees altitude. For each night i make a stack and use that stack to do a blink comparison with a reference stack to find novae in M31. And so after a while you end up with a lot of subs and every now and then i make a stack with all the collected subs. To find Asteroids i also make stacks without "sigma clipping". The Asteroid will show up as a short line, there is 1 in the middle top in this image, just below the letter "c" of "October". It is Asteroid 334168 ST119 imaged on 4 october 2019 (6.3km diameter, magnitude 20.8). The image featured here showed a nice airplane/ Satellite trail frame around M31.
I really like this APOD! Shows a little of what's involved. And it's especially nice that Kees is answering questions here. Thanks for explaining and pointing out that asteroid line. If you go further to the right, still very near the top of the image, there is another, longer, line segment, but it bulges in the middle and both tails are fine points. I'm guessing that was a meteor, is that right?
keesscherer wrote: ↑Mon Oct 14, 2019 7:38 am
The 223 Luminance images with 300 second exposure where made between 12 nov 2018 and 4 oct 2019. I make around 12 images per night when there is no Moon above the horizon and M31 is at least at 30 degrees altitude. For each night i make a stack and use that stack to do a blink comparison with a reference stack to find novae in M31. And so after a while you end up with a lot of subs and every now and then i make a stack with all the collected subs. To find Asteroids i also make stacks without "sigma clipping". The Asteroid will show up as a short line, there is 1 in the middle top in this image, just below the letter "c" of "October". It is Asteroid 334168 ST119 imaged on 4 october 2019 (6.3km diameter, magnitude 20.8). The image featured here showed a nice airplane/ Satellite trail frame around M31.
I really like this APOD! Shows a little of what's involved. And it's especially nice that Kees is answering questions here. Thanks for explaining and pointing out that asteroid line. If you go further to the right, still very near the top of the image, there is another, longer, line segment, but it bulges in the middle and both tails are fine points. I'm guessing that was a meteor, is that right?
Thank you! That trail is a tumbling upper rocket stage or tumbling satellite. There are more of these trails in the image and when you zoom out you will see that they are brightening and fading.
Andromeda before Photoshop
I also made full color versions, here is an example: 
While some Galaxies hide their "tops" in their "trunks".
![counterclockwise [top-wise?]](https://en.wikipedia.org/wiki/Andromeda_Galaxy#/media/File:Andromeda_galaxy_Ssc2005-20a1.jpg){kind=link}