TCampbell
Been spending a lot of time on here!
- Joined
- Mar 31, 2012
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- 3,614
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- Location
- Dearborn, MI
- Can others edit my Photos
- Photos OK to edit
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.
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.
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...
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:
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.)
Enjoy!
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.
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.
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.
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:
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.)
Enjoy!
![[No title]](/data/xfmg/thumbnail/33/33360-ff0b69685c94740bde3f53b6d7aa9af1.jpg?1619735924)
![[No title]](/data/xfmg/thumbnail/33/33362-84aacb865117bf8cba89104b89e9b36c.jpg?1619735927)
