Andromeda Galaxy - some astrophotography comparisons


Been spending a lot of time on here!
Mar 31, 2012
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Dearborn, MI
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As some of you know, I have a passion for astronomy and, when I can get to a nice dark sky location, I can't resist the temptation to do some astrophotography.

I took the image below from a dark sky site last Saturday (I had to wait for the moon to set so the conditions would be dark enough.) To get this, I actually had to capture eight exposures. Each exposure is 8 minutes long. I also had to capture three "dark" frames (also 8 minutes long) and these are to help the stacking and processing software produce a good result.

First... here's what you get out of the camera without much processing (ok, I did tone down the "red" since my astrophotography camera is PARTICULARLY sensitive to reds (about 4x more sensitive than a typical DSLR -- and this is deliberate, that extra sensitive really helps on several deep-space nebulae even though this object is a galaxy and not a nebula.) This is one single 8 minute exposure at ISO 800 using a 4" f/5.4 telescope (TeleVue NP101is). Note that to get this, the telescope MUST be mounted to a tracking mount with an extremely good "polar" alignment -- otherwise you get blur because the object will appear to drift in the frame as you capture the image.

M31 camera normal.jpg

This was shot as a "RAW" image so there actually is a lot more "data" in here than the eye can see (more on that in a moment.)

And speaking of what the eye can see... here's a sample. Notice how in the image above, the disk of the galaxy is extremely dim. It doesn't take much light pollution before the sky is lit up brighter than the disk. The central core is bright... but the not the disk. So the disk washes out and this is typical of what you might see looking through a telescope with a human eye.

M31 visual dim.jpg

Not very impressive, is it?

This is what astronomers have to deal with regularly and THE reason why astronomers are always trying to find dark skies and encourage the public to make sure that unnecessary outdoor lighting is turned off (avoid gratuitous lighting) and make sure that lighting is "shielded" so that no light can shine upward toward the sky.

However... if you do have nice dark sky locations, you can collect enough data to create a much more impressive result. The image below is the result of combining and processing...
  • 8x "light" frames (these are the normal frames such as the first image above) where each frame in this case was 8 minutes long at ISO 800 on the f/5.4 scope.
  • 3x "dark" frames (these are identical in exposure except the front-cap is on the telescope to avoid collecting any light. The purpose of these is to grab representative samples of the amount of image "noise" that will appear in an identical exposure and at the identical temperature.
  • A master "bias" frame. I created the bias frame ahead of time by capturing a number of 0-time length exposures (you set the camera to the fastest shutter speed and capture some sample frames with the front cap on the camera.) These are similar to "dark" frames except we're trying to get a sample of how information comes out of the camera simply by powering up the sensor and immediately doing a "read-out" ... that's the "bias" level. Anything stronger than the bias level represents either real signal (image data) or "noise". The computer processing software can do a much better job processing the images (identifying the real data) when it can subtract the bias data and noise.
Normally I would have collected "flat" frames as well, but this scope is actually extremely good (exceptionally low vignetting) so I didn't bother with the flats this time.

All of this is combined and integrated. The software (I use something called "PixInsight") aligns each frame using bright stars to make sure everything is stacked nicely, then integrates them and processes out the noise to create stronger signal data and minimize noise.

At this point I have to "process" the image. In the first image, the data is there but your eye can't see it. Differences in color, brightness, etc. are extremely subtle and your eye cannot perceive the differences, but the computer can. Processing involves "stretching" those subtle differences to become more obvious differences.

This processing is more "art" than "science" because you get to a point at which your are making artistic decisions in an effort to achieve a visually appealing result -- but the decisions are subjective. There is no "correct" final result.

Here is my result:

M31 processed.jpg

This was the result of about 2 hours worth of work capturing data at the telescope, and another several hours working on the data on the computer. I reprocessed this image twice before I was happy with it and even now I see subtle things that I'd like to change (I will certainly reprocess this image one or two more times in the future.)

Very fascinating! Thank you for sharing your beautiful results!
Absolutely fascinating and an incredible result! Thank you so much for taking the time to share the details and the photograph.
Incidentally, there are approximately 1 Trillion (yes, that's with a "T") suns in that galaxy. The bright spot located on the edge of the disk (upper left area of the disk) is a companion galaxy designated "M32" (Andromeda's designation is M31) and the lower right oval is another companion galaxy designated M110. The "M" stands for "Messier Catalog". Charles Messier was a comet hunter from France in the 1600's. In the 1600's, downtown Paris qualifies as a "dark sky" location. Anyway... his telescope was basically junk compared to even moderate amateur grade equipment. So he could not resolve detail -- he only observed fuzzy spots in the sky. You find comets by looking for "fuzzy spots" in the sky and then tracking them night after night for a week or two to see if they move. If they do move, you found a comet. If they don't move, you didn't.

Messier kept finding these fuzzy spots that did not move. He called them "comet imposters"... one of his contemporaries called them "embarrassing objects". Frustrated, he made a catalog to identify the location of all the objects in the night sky that are NOT COMETS and thus you should not waste your time looking at them. It's ironic... because now that we have good enough telescopes to resolve the detail, these are actually many of the most beautiful objects in the night sky.


This galaxy is located approximately 2.5 Million Light Years (Mly) away from us... and it's headed our way in a big hurry! We see the galaxy as it looked 2.5 million years ago. Humans didn't even evolve until somewhere between 95,000 - 250,000 years ago (depending on which anthropologist you ask). So the light from this galaxy started it's journey more than 10 times farther back in time before humans even existed ... and in that time, we evolved, we learned to use tools, we developed technology, we made telescopes, and then we made cameras -- and we started taking pictures of the sky "just in time" to catch that light arriving at the end of it's journey (I find this somehow mind-blowing and somewhat deeply spiritual all at the same time.)

A telescope is a kind of time machine with a "look but don't touch" rule. Even though the telescope will not let us visit or change the past, it will let us look back to view the past.

In approximately 4 billion years (which makes this "somebody else's problem") it will collide with our galaxy and our Milky Way galaxy (about 300 Billion suns) will merge with this galaxy (of about 1 Trillion suns) making a new larger galaxy which not only has 1.3 Trillion suns... the merging gases will likely trigger substantial new star formation.

Very few stars (if any) will actually collide with any other stars because the space between the stars is significantly larger than most people can imagine. If our own sun was reduced to the size of a basketball, then the closest neighboring star would be reduced to the size of a foosball -- and would need to be placed about 4500 miles away from the basketball in order to get the "scale" correct.) That's a lot of space between relatively small "sports balls" so you can imagine why there will be almost no collisions.
You lost me with all the technical stuff but, the final result is awesome!
wonderful pictures and explanation.

when two galaxies start to interact, does everything go to hell or is there kind of a gentle adjustment to a new situation?
That is a great it

Beautiful processing to get that photo. As for the "old light" , that's the favourite activity of my daughter and me, out in the country, looking for Andromeda by eye, usually after a nice relaxing sitdown around the campfire.
That to me was sci fi fiction.
The pics surreal. Exciting stuff Tim and you are quite right when you call it " Spiritual"

Very Impressive,beam me up scotty.
wonderful pictures and explanation.

when two galaxies start to interact, does everything go to hell or is there kind of a gentle adjustment to a new situation?

something like this...

The stars, for the most part, as Tim noted, won't be affected much. The gas clouds however, will collide and that will stimulated star formation, with the largest ones being very large and bright, and perhaps living very short lives before going supernova. What these galactic collisions will do is also disturb the O0rt clouds around stars, potentially drawing in comets and planetismals into the inner solar systems. In short, over geologic times, chaos.
That is amazing! Thanks for the explanations!

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