Question about Stacking Photo's for Astrophotography

I am no expert, but recently I have been studying this same topic. Caveat emptor!

First, you are discussing image stacking, not focus stacking. Focus stacking is something completely different.

Image stacking is used to counteract sensor noise, which is a feature of every digital camera to some extent. Noise is especially amplified when using higher ISO. In general, it is good to avoid high ISO, but for night sky photos without using a tracker so you may be forced to use higher ISO in order to capture an image with modest shutter speeds. Note that stacking works best when the set of images are all taken with the same framing, so a tripod is assumed. Using an intervalometer is helpful, and many newer camera models have them built-in. An intervalometer is not a requirement, but you will need to be careful to keep the camera still and framed consistently from shot to shot.

Image stacking can be accomplished with PS or other softwares with more or less degrees of automation. Specialty softwares for stacking star images are probably the most helpful for Milky Way images. It seems Deep Sky Stacker is a popular example that is free.

The number of images to stack is a matter of practicality and diminishing returns. The noise elimination provided by stacking is essentially a statistical averaging technique. Anywhere between 10-30 images should do 90%-98% of the job. Using 120 images is probably overkill-- capturing 120 images takes a long time, it might tax the processing time on your PC, and possibly burn up storage for very little extra benefit.

Many youtubers out there describe the general process, including use of processing softwares. Check out Milky Way Mike and Nebula Photos and AstroBackyard -- they all provide helpful basic tutorials and some advanced tutorials as well.

Stacking is not going to counteract the effects of light pollution. Supposedly there are lens filters that will counteract light pollution but I don't know how well they actually work. I would not expect much from them. Light pollution can be eliminated to some extent using gradient filters in PS. One of the tutorials at Nebula Photos describes the process in detail.

As you know, nothing is better than shooting under a clear, moonless night away from light pollution. But you can get pleasing images with some light pollution, just don't let your expectations run wild!

Adding one point to dc colorado's informative post. Image stacking also tends to reduce variable atmospheric disturbances.

It’s image stacking, not focus stacking.

Stacking hundreds of exposures is technically possible, and routinely done by astrophotographers. Check my gallery. Most of my deep-sky photos are results of stacking several tens of exposures, and a good fraction of them exceed hundred.

When executed correctly (not easy), stacking can reduce noise. This includes suppressing light pollution. There are several different kinds of noises in long-exposure images. Some of them come from different parts of the camera (readout circuits and sensor, etc). Some noise comes with the signal, which is photon shot noise in photography. The bright background produced by light pollution is accompanied with strong photon shot noise. Even if you can remove the background in post-processing using tools like Photoshop’s dehaze or more sophisticated tools for astrophotography, you can’t remove the noise in the background. That noise will prevent you from recording faint stuffs like the Milky Way. This is where stacking can come in to help. Stacking can reduce noise accompanied with light pollution and allow you to record Milky Way. However, as I said, this is not easy. This requires many things to be done correctly. Stacking is just part of it.

Cheers,

Wei-Hao

Insert caption here. If you do not edit this text it will be automatically removed. dc colorado said:

I am no expert, but recently I have been studying this same topic. Caveat emptor!

First, you are discussing image stacking, not focus stacking. Focus stacking is something completely different.

Image stacking is used to counteract sensor noise, which is a feature of every digital camera to some extent. Noise is especially amplified when using higher ISO. In general, it is good to avoid high ISO, but for night sky photos without using a tracker so you may be forced to use higher ISO in order to capture an image with modest shutter speeds. Note that stacking works best when the set of images are all taken with the same framing, so a tripod is assumed. Using an intervalometer is helpful, and many newer camera models have them built-in. An intervalometer is not a requirement, but you will need to be careful to keep the camera still and framed consistently from shot to shot.

Image stacking can be accomplished with PS or other softwares with more or less degrees of automation. Specialty softwares for stacking star images are probably the most helpful for Milky Way images. It seems Deep Sky Stacker is a popular example that is free.

The number of images to stack is a matter of practicality and diminishing returns. The noise elimination provided by stacking is essentially a statistical averaging technique. Anywhere between 10-30 images should do 90%-98% of the job. Using 120 images is probably overkill-- capturing 120 images takes a long time, it might tax the processing time on your PC, and possibly burn up storage for very little extra benefit.

Many youtubers out there describe the general process, including use of processing softwares. Check out Milky Way Mike and Nebula Photos and AstroBackyard -- they all provide helpful basic tutorials and some advanced tutorials as well.

Stacking is not going to counteract the effects of light pollution. Supposedly there are lens filters that will counteract light pollution but I don't know how well they actually work. I would not expect much from them. Light pollution can be eliminated to some extent using gradient filters in PS. One of the tutorials at Nebula Photos describes the process in detail.

As you know, nothing is better than shooting under a clear, moonless night away from light pollution. But you can get pleasing images with some light pollution, just don't let your expectations run wild!

Click to expand...

Generally, the above is correct, but there are a few minor points that should be cleared up…

1) Stacking addresses more than just sensor noise. It also addresses shot noise. The light itself—both from the Milky Way and from light pollution—has a noise component, and stacking addresses that. In fact, for an image taken in the city, shot noise from light pollution is probably the bulk of the noise.

2) The amount of benefit from stacking is not, strictly speaking, dependent on the number of images taken. The better correlation is to the integration time—the total amount of exposure time. For example, a stack of 4 thirty second shots would likely have slightly less noise than a stack of 120 one second shots because the read noise in the stack of four would be lower. Other noise benefits—thermal noise and shot noise—would be identical. The reason 120 shots were used is likely that there was no tracker, so longer exposures would have blurred. In any event, fewer but longer exposures are generally better if you can manage it, and you do get diminishing returns as you expose longer and longer.

3) You actually can counteract light pollution with stacking to a great extent, but it’s not ideal. First, light pollution itself is simply subtracted out of images. It’s not as straightforward as one would like because light pollution amounts are often uneven across an image, but you can generally just subtract it out, then apply a gradient correction to remove the uneven remainder. It’s not perfect, but it can be pretty good. What is left is the noise contribution from light pollution, and that can’t be subtracted. However, that noise IS addressed from stacking. As you average more and more frames together, the noise grows less slowly than the signal, so your noise component is proportionately smaller, even the light pollution component. If you get enough integration time you can make up for an awful lot of light pollution. The problem is the “long enough” part. For example, I have roughly magnitude 3.2 skies here at home (in a city). That is, the faintest stars I can see are magnitude 3.2 or so. From a reasonably dark location, I would be able to see perhaps magnitude 6.2 stars. It’s a logarithmic scale with a base equal to the fifth root of 100 (historical reasons—ask the ancient Greeks). That means for every minute I image in the country, I’d need to image for about 16 minutes here in the city to get the same result. You can do it for a Milky Way shot, but it becomes less practical with fainter objects like distant galaxies. 10 hours of exposure from the country is equivalent to six and two thirds entire DAYS of exposure time from the city. You might manage ten hours of exposure on a given subject over a couple nights, but how in the heck would you take six days of exposure? Hence, the desire for dark skies. Still, you’d be surprised what is possible with good technique and good equipment under even the most light polluted skies as long as you pick the right target.

Here is a shot from Oakland, CA—horrible light pollution in a metropolitan area with around 7 million people—taken on a full moon night this week… Three hours of stacked one minute images.

This is an extreme example....It's not the Milky Way but shows you what stacking can do. I used to live in a large urban downtown with skies so bright it was difficult to see any stars...


On the left is a single 40 sec. image shot using my camera tracker so that the stars don't trail. On the right is the result of stacking 250 images like the example and then processed.

Jared Willson said:

Insert caption here. If you do not edit this text it will be automatically removed. dc colorado said:

I am no expert, but recently I have been studying this same topic. Caveat emptor!

First, you are discussing image stacking, not focus stacking. Focus stacking is something completely different.

Image stacking is used to counteract sensor noise, which is a feature of every digital camera to some extent. Noise is especially amplified when using higher ISO. In general, it is good to avoid high ISO, but for night sky photos without using a tracker so you may be forced to use higher ISO in order to capture an image with modest shutter speeds. Note that stacking works best when the set of images are all taken with the same framing, so a tripod is assumed. Using an intervalometer is helpful, and many newer camera models have them built-in. An intervalometer is not a requirement, but you will need to be careful to keep the camera still and framed consistently from shot to shot.

Image stacking can be accomplished with PS or other softwares with more or less degrees of automation. Specialty softwares for stacking star images are probably the most helpful for Milky Way images. It seems Deep Sky Stacker is a popular example that is free.

The number of images to stack is a matter of practicality and diminishing returns. The noise elimination provided by stacking is essentially a statistical averaging technique. Anywhere between 10-30 images should do 90%-98% of the job. Using 120 images is probably overkill-- capturing 120 images takes a long time, it might tax the processing time on your PC, and possibly burn up storage for very little extra benefit.

Many youtubers out there describe the general process, including use of processing softwares. Check out Milky Way Mike and Nebula Photos and AstroBackyard -- they all provide helpful basic tutorials and some advanced tutorials as well.

Stacking is not going to counteract the effects of light pollution. Supposedly there are lens filters that will counteract light pollution but I don't know how well they actually work. I would not expect much from them. Light pollution can be eliminated to some extent using gradient filters in PS. One of the tutorials at Nebula Photos describes the process in detail.

As you know, nothing is better than shooting under a clear, moonless night away from light pollution. But you can get pleasing images with some light pollution, just don't let your expectations run wild!

Click to expand...

Generally, the above is correct, but there are a few minor points that should be cleared up…

1) Stacking addresses more than just sensor noise. It also addresses shot noise. The light itself—both from the Milky Way and from light pollution—has a noise component, and stacking addresses that. In fact, for an image taken in the city, shot noise from light pollution is probably the bulk of the noise.

2) The amount of benefit from stacking is not, strictly speaking, dependent on the number of images taken. The better correlation is to the integration time—the total amount of exposure time. For example, a stack of 4 thirty second shots would likely have slightly less noise than a stack of 120 one second shots because the read noise in the stack of four would be lower. Other noise benefits—thermal noise and shot noise—would be identical. The reason 120 shots were used is likely that there was no tracker, so longer exposures would have blurred. In any event, fewer but longer exposures are generally better if you can manage it, and you do get diminishing returns as you expose longer and longer.

3) You actually can counteract light pollution with stacking to a great extent, but it’s not ideal. First, light pollution itself is simply subtracted out of images. It’s not as straightforward as one would like because light pollution amounts are often uneven across an image, but you can generally just subtract it out, then apply a gradient correction to remove the uneven remainder. It’s not perfect, but it can be pretty good. What is left is the noise contribution from light pollution, and that can’t be subtracted. However, that noise IS addressed from stacking. As you average more and more frames together, the noise grows less slowly than the signal, so your noise component is proportionately smaller, even the light pollution component. If you get enough integration time you can make up for an awful lot of light pollution. The problem is the “long enough” part. For example, I have roughly magnitude 3.2 skies here at home (in a city). That is, the faintest stars I can see are magnitude 3.2 or so. From a reasonably dark location, I would be able to see perhaps magnitude 6.2 stars. It’s a logarithmic scale with a base equal to the fifth root of 100 (historical reasons—ask the ancient Greeks). That means for every minute I image in the country, I’d need to image for about 16 minutes here in the city to get the same result. You can do it for a Milky Way shot, but it becomes less practical with fainter objects like distant galaxies. 10 hours of exposure from the country is equivalent to six and two thirds entire DAYS of exposure time from the city. You might manage ten hours of exposure on a given subject over a couple nights, but how in the heck would you take six days of exposure? Hence, the desire for dark skies. Still, you’d be surprised what is possible with good technique and good equipment under even the most light polluted skies as long as you pick the right target.

Here is a shot from Oakland, CA—horrible light pollution in a metropolitan area with around 7 million people—taken on a full moon night this week… Three hours of stacked one minute images.

Click to expand...

M13? How did you resolve it to the core? You must have used a 10" telescope or larger.....



--
In the councils of government, we must guard against the acquisition of unwarranted influence, whether sought or unsought, by the military industrial complex. The potential for the disastrous rise of misplaced power exists and will persist. We must never let the weight of this combination endanger our liberties or democratic processes. We should take nothing for granted. Only an alert and knowledgeable citizenry can compel the proper meshing of the huge industrial and military machinery of defense with our peaceful methods and goals, so that security and liberty may prosper together.
-Dwight D. Eisenhower, 1961

Thanks to everyone for their input on this.

There is much to process and review here.

I think all the feedback has given me encouragement to try this out.

If nothing at all, I am sure I will learn a few things along the way!

OutsideTheMatrix said:

Jared Willson said:

Insert caption here. If you do not edit this text it will be automatically removed. dc colorado said:

I am no expert, but recently I have been studying this same topic. Caveat emptor!

First, you are discussing image stacking, not focus stacking. Focus stacking is something completely different.

Image stacking is used to counteract sensor noise, which is a feature of every digital camera to some extent. Noise is especially amplified when using higher ISO. In general, it is good to avoid high ISO, but for night sky photos without using a tracker so you may be forced to use higher ISO in order to capture an image with modest shutter speeds. Note that stacking works best when the set of images are all taken with the same framing, so a tripod is assumed. Using an intervalometer is helpful, and many newer camera models have them built-in. An intervalometer is not a requirement, but you will need to be careful to keep the camera still and framed consistently from shot to shot.

Image stacking can be accomplished with PS or other softwares with more or less degrees of automation. Specialty softwares for stacking star images are probably the most helpful for Milky Way images. It seems Deep Sky Stacker is a popular example that is free.

The number of images to stack is a matter of practicality and diminishing returns. The noise elimination provided by stacking is essentially a statistical averaging technique. Anywhere between 10-30 images should do 90%-98% of the job. Using 120 images is probably overkill-- capturing 120 images takes a long time, it might tax the processing time on your PC, and possibly burn up storage for very little extra benefit.

Many youtubers out there describe the general process, including use of processing softwares. Check out Milky Way Mike and Nebula Photos and AstroBackyard -- they all provide helpful basic tutorials and some advanced tutorials as well.

Stacking is not going to counteract the effects of light pollution. Supposedly there are lens filters that will counteract light pollution but I don't know how well they actually work. I would not expect much from them. Light pollution can be eliminated to some extent using gradient filters in PS. One of the tutorials at Nebula Photos describes the process in detail.

As you know, nothing is better than shooting under a clear, moonless night away from light pollution. But you can get pleasing images with some light pollution, just don't let your expectations run wild!

Click to expand...

Generally, the above is correct, but there are a few minor points that should be cleared up…

1) Stacking addresses more than just sensor noise. It also addresses shot noise. The light itself—both from the Milky Way and from light pollution—has a noise component, and stacking addresses that. In fact, for an image taken in the city, shot noise from light pollution is probably the bulk of the noise.

2) The amount of benefit from stacking is not, strictly speaking, dependent on the number of images taken. The better correlation is to the integration time—the total amount of exposure time. For example, a stack of 4 thirty second shots would likely have slightly less noise than a stack of 120 one second shots because the read noise in the stack of four would be lower. Other noise benefits—thermal noise and shot noise—would be identical. The reason 120 shots were used is likely that there was no tracker, so longer exposures would have blurred. In any event, fewer but longer exposures are generally better if you can manage it, and you do get diminishing returns as you expose longer and longer.

3) You actually can counteract light pollution with stacking to a great extent, but it’s not ideal. First, light pollution itself is simply subtracted out of images. It’s not as straightforward as one would like because light pollution amounts are often uneven across an image, but you can generally just subtract it out, then apply a gradient correction to remove the uneven remainder. It’s not perfect, but it can be pretty good. What is left is the noise contribution from light pollution, and that can’t be subtracted. However, that noise IS addressed from stacking. As you average more and more frames together, the noise grows less slowly than the signal, so your noise component is proportionately smaller, even the light pollution component. If you get enough integration time you can make up for an awful lot of light pollution. The problem is the “long enough” part. For example, I have roughly magnitude 3.2 skies here at home (in a city). That is, the faintest stars I can see are magnitude 3.2 or so. From a reasonably dark location, I would be able to see perhaps magnitude 6.2 stars. It’s a logarithmic scale with a base equal to the fifth root of 100 (historical reasons—ask the ancient Greeks). That means for every minute I image in the country, I’d need to image for about 16 minutes here in the city to get the same result. You can do it for a Milky Way shot, but it becomes less practical with fainter objects like distant galaxies. 10 hours of exposure from the country is equivalent to six and two thirds entire DAYS of exposure time from the city. You might manage ten hours of exposure on a given subject over a couple nights, but how in the heck would you take six days of exposure? Hence, the desire for dark skies. Still, you’d be surprised what is possible with good technique and good equipment under even the most light polluted skies as long as you pick the right target.

Here is a shot from Oakland, CA—horrible light pollution in a metropolitan area with around 7 million people—taken on a full moon night this week… Three hours of stacked one minute images.

Click to expand...

M13? How did you resolve it to the core? You must have used a 10" telescope or larger.....

Click to expand...

305mm telescope (just over twelve inches in diameter). As you surmised, it is M13. I had to shorten the exposures to one minute to avoid saturating the core, but once I did that I was able to resolve everything OK. I was hoping for a night of better seeing--this yielded about 2.3" FWHM diameter stars after stacking.

- Jared

Jared Willson said:

OutsideTheMatrix said:

Jared Willson said:

Insert caption here. If you do not edit this text it will be automatically removed. dc colorado said:

I am no expert, but recently I have been studying this same topic. Caveat emptor!

First, you are discussing image stacking, not focus stacking. Focus stacking is something completely different.

Image stacking is used to counteract sensor noise, which is a feature of every digital camera to some extent. Noise is especially amplified when using higher ISO. In general, it is good to avoid high ISO, but for night sky photos without using a tracker so you may be forced to use higher ISO in order to capture an image with modest shutter speeds. Note that stacking works best when the set of images are all taken with the same framing, so a tripod is assumed. Using an intervalometer is helpful, and many newer camera models have them built-in. An intervalometer is not a requirement, but you will need to be careful to keep the camera still and framed consistently from shot to shot.

Image stacking can be accomplished with PS or other softwares with more or less degrees of automation. Specialty softwares for stacking star images are probably the most helpful for Milky Way images. It seems Deep Sky Stacker is a popular example that is free.

The number of images to stack is a matter of practicality and diminishing returns. The noise elimination provided by stacking is essentially a statistical averaging technique. Anywhere between 10-30 images should do 90%-98% of the job. Using 120 images is probably overkill-- capturing 120 images takes a long time, it might tax the processing time on your PC, and possibly burn up storage for very little extra benefit.

Many youtubers out there describe the general process, including use of processing softwares. Check out Milky Way Mike and Nebula Photos and AstroBackyard -- they all provide helpful basic tutorials and some advanced tutorials as well.

Stacking is not going to counteract the effects of light pollution. Supposedly there are lens filters that will counteract light pollution but I don't know how well they actually work. I would not expect much from them. Light pollution can be eliminated to some extent using gradient filters in PS. One of the tutorials at Nebula Photos describes the process in detail.

As you know, nothing is better than shooting under a clear, moonless night away from light pollution. But you can get pleasing images with some light pollution, just don't let your expectations run wild!

Click to expand...

Generally, the above is correct, but there are a few minor points that should be cleared up…

1) Stacking addresses more than just sensor noise. It also addresses shot noise. The light itself—both from the Milky Way and from light pollution—has a noise component, and stacking addresses that. In fact, for an image taken in the city, shot noise from light pollution is probably the bulk of the noise.

2) The amount of benefit from stacking is not, strictly speaking, dependent on the number of images taken. The better correlation is to the integration time—the total amount of exposure time. For example, a stack of 4 thirty second shots would likely have slightly less noise than a stack of 120 one second shots because the read noise in the stack of four would be lower. Other noise benefits—thermal noise and shot noise—would be identical. The reason 120 shots were used is likely that there was no tracker, so longer exposures would have blurred. In any event, fewer but longer exposures are generally better if you can manage it, and you do get diminishing returns as you expose longer and longer.

3) You actually can counteract light pollution with stacking to a great extent, but it’s not ideal. First, light pollution itself is simply subtracted out of images. It’s not as straightforward as one would like because light pollution amounts are often uneven across an image, but you can generally just subtract it out, then apply a gradient correction to remove the uneven remainder. It’s not perfect, but it can be pretty good. What is left is the noise contribution from light pollution, and that can’t be subtracted. However, that noise IS addressed from stacking. As you average more and more frames together, the noise grows less slowly than the signal, so your noise component is proportionately smaller, even the light pollution component. If you get enough integration time you can make up for an awful lot of light pollution. The problem is the “long enough” part. For example, I have roughly magnitude 3.2 skies here at home (in a city). That is, the faintest stars I can see are magnitude 3.2 or so. From a reasonably dark location, I would be able to see perhaps magnitude 6.2 stars. It’s a logarithmic scale with a base equal to the fifth root of 100 (historical reasons—ask the ancient Greeks). That means for every minute I image in the country, I’d need to image for about 16 minutes here in the city to get the same result. You can do it for a Milky Way shot, but it becomes less practical with fainter objects like distant galaxies. 10 hours of exposure from the country is equivalent to six and two thirds entire DAYS of exposure time from the city. You might manage ten hours of exposure on a given subject over a couple nights, but how in the heck would you take six days of exposure? Hence, the desire for dark skies. Still, you’d be surprised what is possible with good technique and good equipment under even the most light polluted skies as long as you pick the right target.

Here is a shot from Oakland, CA—horrible light pollution in a metropolitan area with around 7 million people—taken on a full moon night this week… Three hours of stacked one minute images.

Click to expand...

M13? How did you resolve it to the core? You must have used a 10" telescope or larger.....

Click to expand...

305mm telescope (just over twelve inches in diameter). As you surmised, it is M13. I had to shorten the exposures to one minute to avoid saturating the core, but once I did that I was able to resolve everything OK. I was hoping for a night of better seeing--this yielded about 2.3" FWHM diameter stars after stacking.

- Jared

Click to expand...

It's spectacular! I use a 3D sim called Space Engine to fly through globular clusters (and even galaxies.) It made me wonder if there is any way we can take 3D photographs of these objects? What if we use the earth's orbit as a baseline and take a picture of it in the summer and then again in the winter? I wonder if the difference in the position of the earth along its orbit would be enough to create a 3D effect?

FWIW approaching a globular cluster while flying through the galaxy looks like approaching a very large swarm of bees lol.



--
In the councils of government, we must guard against the acquisition of unwarranted influence, whether sought or unsought, by the military industrial complex. The potential for the disastrous rise of misplaced power exists and will persist. We must never let the weight of this combination endanger our liberties or democratic processes. We should take nothing for granted. Only an alert and knowledgeable citizenry can compel the proper meshing of the huge industrial and military machinery of defense with our peaceful methods and goals, so that security and liberty may prosper together.
-Dwight D. Eisenhower, 1961

OutsideTheMatrix said:

Jared Willson said:

OutsideTheMatrix said:

Jared Willson said:

Insert caption here. If you do not edit this text it will be automatically removed. dc colorado said:

I am no expert, but recently I have been studying this same topic. Caveat emptor!

First, you are discussing image stacking, not focus stacking. Focus stacking is something completely different.

Image stacking is used to counteract sensor noise, which is a feature of every digital camera to some extent. Noise is especially amplified when using higher ISO. In general, it is good to avoid high ISO, but for night sky photos without using a tracker so you may be forced to use higher ISO in order to capture an image with modest shutter speeds. Note that stacking works best when the set of images are all taken with the same framing, so a tripod is assumed. Using an intervalometer is helpful, and many newer camera models have them built-in. An intervalometer is not a requirement, but you will need to be careful to keep the camera still and framed consistently from shot to shot.

Image stacking can be accomplished with PS or other softwares with more or less degrees of automation. Specialty softwares for stacking star images are probably the most helpful for Milky Way images. It seems Deep Sky Stacker is a popular example that is free.

The number of images to stack is a matter of practicality and diminishing returns. The noise elimination provided by stacking is essentially a statistical averaging technique. Anywhere between 10-30 images should do 90%-98% of the job. Using 120 images is probably overkill-- capturing 120 images takes a long time, it might tax the processing time on your PC, and possibly burn up storage for very little extra benefit.

Many youtubers out there describe the general process, including use of processing softwares. Check out Milky Way Mike and Nebula Photos and AstroBackyard -- they all provide helpful basic tutorials and some advanced tutorials as well.

Stacking is not going to counteract the effects of light pollution. Supposedly there are lens filters that will counteract light pollution but I don't know how well they actually work. I would not expect much from them. Light pollution can be eliminated to some extent using gradient filters in PS. One of the tutorials at Nebula Photos describes the process in detail.

As you know, nothing is better than shooting under a clear, moonless night away from light pollution. But you can get pleasing images with some light pollution, just don't let your expectations run wild!

Click to expand...

Generally, the above is correct, but there are a few minor points that should be cleared up…

1) Stacking addresses more than just sensor noise. It also addresses shot noise. The light itself—both from the Milky Way and from light pollution—has a noise component, and stacking addresses that. In fact, for an image taken in the city, shot noise from light pollution is probably the bulk of the noise.

2) The amount of benefit from stacking is not, strictly speaking, dependent on the number of images taken. The better correlation is to the integration time—the total amount of exposure time. For example, a stack of 4 thirty second shots would likely have slightly less noise than a stack of 120 one second shots because the read noise in the stack of four would be lower. Other noise benefits—thermal noise and shot noise—would be identical. The reason 120 shots were used is likely that there was no tracker, so longer exposures would have blurred. In any event, fewer but longer exposures are generally better if you can manage it, and you do get diminishing returns as you expose longer and longer.

3) You actually can counteract light pollution with stacking to a great extent, but it’s not ideal. First, light pollution itself is simply subtracted out of images. It’s not as straightforward as one would like because light pollution amounts are often uneven across an image, but you can generally just subtract it out, then apply a gradient correction to remove the uneven remainder. It’s not perfect, but it can be pretty good. What is left is the noise contribution from light pollution, and that can’t be subtracted. However, that noise IS addressed from stacking. As you average more and more frames together, the noise grows less slowly than the signal, so your noise component is proportionately smaller, even the light pollution component. If you get enough integration time you can make up for an awful lot of light pollution. The problem is the “long enough” part. For example, I have roughly magnitude 3.2 skies here at home (in a city). That is, the faintest stars I can see are magnitude 3.2 or so. From a reasonably dark location, I would be able to see perhaps magnitude 6.2 stars. It’s a logarithmic scale with a base equal to the fifth root of 100 (historical reasons—ask the ancient Greeks). That means for every minute I image in the country, I’d need to image for about 16 minutes here in the city to get the same result. You can do it for a Milky Way shot, but it becomes less practical with fainter objects like distant galaxies. 10 hours of exposure from the country is equivalent to six and two thirds entire DAYS of exposure time from the city. You might manage ten hours of exposure on a given subject over a couple nights, but how in the heck would you take six days of exposure? Hence, the desire for dark skies. Still, you’d be surprised what is possible with good technique and good equipment under even the most light polluted skies as long as you pick the right target.

Here is a shot from Oakland, CA—horrible light pollution in a metropolitan area with around 7 million people—taken on a full moon night this week… Three hours of stacked one minute images.

Click to expand...

M13? How did you resolve it to the core? You must have used a 10" telescope or larger.....

Click to expand...

305mm telescope (just over twelve inches in diameter). As you surmised, it is M13. I had to shorten the exposures to one minute to avoid saturating the core, but once I did that I was able to resolve everything OK. I was hoping for a night of better seeing--this yielded about 2.3" FWHM diameter stars after stacking.

- Jared

Click to expand...

It's spectacular! I use a 3D sim called Space Engine to fly through globular clusters (and even galaxies.) It made me wonder if there is any way we can take 3D photographs of these objects? What if we use the earth's orbit as a baseline and take a picture of it in the summer and then again in the winter? I wonder if the difference in the position of the earth along its orbit would be enough to create a 3D effect?

FWIW approaching a globular cluster while flying through the galaxy looks like approaching a very large swarm of bees lol.

Click to expand...

It's a cool idea to use parallax to figure out relative distances and build a 3D model, but our telescopes simply aren't big enough. The first successful use of parallax (different angle to background stars over a six month time period) to calculate stellar distances was 61 Cygni in 1838. The problem is just that the angles are insanely small. Proxima Centauri has the greatest trigonometric parallax at 0.77".

Basically, with current space-based equipment we can measure parallax out to about 1,500 ly or so. The European Space Agency's Gaia project may be able to do a bit better, but the farther out you go the greater the uncertainty. To get a 3D mapping, you'd need accuracy to about a tenth of a percent. This is a fairly nearby globular cluster with a distance in excess of 20,000 ly, so it's not practical at this time.

We can do parallax to radio objects at much greater distances using multiple radio telescopes, but we don't have a way to do that using optical telescopes over significant baselines.

So, "not yet" is the answer at least for direct measurement. You could probably come up with a simulation if you wanted using spectral classes, some assumptions about intrinsic luminosity and develop a model that would "work". By "work" I mean it would give you an answer, but it would only be correct on average, and could be off by a LOT for any given star. Might be fun, though.

Jared Willson said:

OutsideTheMatrix said:

Jared Willson said:

OutsideTheMatrix said:

Jared Willson said:

Insert caption here. If you do not edit this text it will be automatically removed. dc colorado said:

I am no expert, but recently I have been studying this same topic. Caveat emptor!

First, you are discussing image stacking, not focus stacking. Focus stacking is something completely different.

Image stacking is used to counteract sensor noise, which is a feature of every digital camera to some extent. Noise is especially amplified when using higher ISO. In general, it is good to avoid high ISO, but for night sky photos without using a tracker so you may be forced to use higher ISO in order to capture an image with modest shutter speeds. Note that stacking works best when the set of images are all taken with the same framing, so a tripod is assumed. Using an intervalometer is helpful, and many newer camera models have them built-in. An intervalometer is not a requirement, but you will need to be careful to keep the camera still and framed consistently from shot to shot.

Image stacking can be accomplished with PS or other softwares with more or less degrees of automation. Specialty softwares for stacking star images are probably the most helpful for Milky Way images. It seems Deep Sky Stacker is a popular example that is free.

The number of images to stack is a matter of practicality and diminishing returns. The noise elimination provided by stacking is essentially a statistical averaging technique. Anywhere between 10-30 images should do 90%-98% of the job. Using 120 images is probably overkill-- capturing 120 images takes a long time, it might tax the processing time on your PC, and possibly burn up storage for very little extra benefit.

Many youtubers out there describe the general process, including use of processing softwares. Check out Milky Way Mike and Nebula Photos and AstroBackyard -- they all provide helpful basic tutorials and some advanced tutorials as well.

Stacking is not going to counteract the effects of light pollution. Supposedly there are lens filters that will counteract light pollution but I don't know how well they actually work. I would not expect much from them. Light pollution can be eliminated to some extent using gradient filters in PS. One of the tutorials at Nebula Photos describes the process in detail.

As you know, nothing is better than shooting under a clear, moonless night away from light pollution. But you can get pleasing images with some light pollution, just don't let your expectations run wild!

Click to expand...

Generally, the above is correct, but there are a few minor points that should be cleared up…

1) Stacking addresses more than just sensor noise. It also addresses shot noise. The light itself—both from the Milky Way and from light pollution—has a noise component, and stacking addresses that. In fact, for an image taken in the city, shot noise from light pollution is probably the bulk of the noise.

2) The amount of benefit from stacking is not, strictly speaking, dependent on the number of images taken. The better correlation is to the integration time—the total amount of exposure time. For example, a stack of 4 thirty second shots would likely have slightly less noise than a stack of 120 one second shots because the read noise in the stack of four would be lower. Other noise benefits—thermal noise and shot noise—would be identical. The reason 120 shots were used is likely that there was no tracker, so longer exposures would have blurred. In any event, fewer but longer exposures are generally better if you can manage it, and you do get diminishing returns as you expose longer and longer.

3) You actually can counteract light pollution with stacking to a great extent, but it’s not ideal. First, light pollution itself is simply subtracted out of images. It’s not as straightforward as one would like because light pollution amounts are often uneven across an image, but you can generally just subtract it out, then apply a gradient correction to remove the uneven remainder. It’s not perfect, but it can be pretty good. What is left is the noise contribution from light pollution, and that can’t be subtracted. However, that noise IS addressed from stacking. As you average more and more frames together, the noise grows less slowly than the signal, so your noise component is proportionately smaller, even the light pollution component. If you get enough integration time you can make up for an awful lot of light pollution. The problem is the “long enough” part. For example, I have roughly magnitude 3.2 skies here at home (in a city). That is, the faintest stars I can see are magnitude 3.2 or so. From a reasonably dark location, I would be able to see perhaps magnitude 6.2 stars. It’s a logarithmic scale with a base equal to the fifth root of 100 (historical reasons—ask the ancient Greeks). That means for every minute I image in the country, I’d need to image for about 16 minutes here in the city to get the same result. You can do it for a Milky Way shot, but it becomes less practical with fainter objects like distant galaxies. 10 hours of exposure from the country is equivalent to six and two thirds entire DAYS of exposure time from the city. You might manage ten hours of exposure on a given subject over a couple nights, but how in the heck would you take six days of exposure? Hence, the desire for dark skies. Still, you’d be surprised what is possible with good technique and good equipment under even the most light polluted skies as long as you pick the right target.

Here is a shot from Oakland, CA—horrible light pollution in a metropolitan area with around 7 million people—taken on a full moon night this week… Three hours of stacked one minute images.

Click to expand...

M13? How did you resolve it to the core? You must have used a 10" telescope or larger.....

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305mm telescope (just over twelve inches in diameter). As you surmised, it is M13. I had to shorten the exposures to one minute to avoid saturating the core, but once I did that I was able to resolve everything OK. I was hoping for a night of better seeing--this yielded about 2.3" FWHM diameter stars after stacking.

- Jared

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It's spectacular! I use a 3D sim called Space Engine to fly through globular clusters (and even galaxies.) It made me wonder if there is any way we can take 3D photographs of these objects? What if we use the earth's orbit as a baseline and take a picture of it in the summer and then again in the winter? I wonder if the difference in the position of the earth along its orbit would be enough to create a 3D effect?

FWIW approaching a globular cluster while flying through the galaxy looks like approaching a very large swarm of bees lol.

Click to expand...

It's a cool idea to use parallax to figure out relative distances and build a 3D model, but our telescopes simply aren't big enough. The first successful use of parallax (different angle to background stars over a six month time period) to calculate stellar distances was 61 Cygni in 1838. The problem is just that the angles are insanely small. Proxima Centauri has the greatest trigonometric parallax at 0.77".

Basically, with current space-based equipment we can measure parallax out to about 1,500 ly or so. The European Space Agency's Gaia project may be able to do a bit better, but the farther out you go the greater the uncertainty. To get a 3D mapping, you'd need accuracy to about a tenth of a percent. This is a fairly nearby globular cluster with a distance in excess of 20,000 ly, so it's not practical at this time.

We can do parallax to radio objects at much greater distances using multiple radio telescopes, but we don't have a way to do that using optical telescopes over significant baselines.

So, "not yet" is the answer at least for direct measurement. You could probably come up with a simulation if you wanted using spectral classes, some assumptions about intrinsic luminosity and develop a model that would "work". By "work" I mean it would give you an answer, but it would only be correct on average, and could be off by a LOT for any given star. Might be fun, though.

Click to expand...

Thanks! I'm actually using the Gaia add on for Space Engine (DR2), the only issue with it is that I'm not sure the spectral classifications for the stars is 100% correct. At some point it also comes down to computing resources, I don't use the whole catalog of course (1 billion stars!), right now I'm just using the stars down to 13th mag. It does seem to be an approximation like you said, but not completely accurate. I just had a nice 12 hour 'trip' to the center of the galaxy and back out the other side though


NASA has done a nice image of the center of the galaxy, looks like this is where all the action is



--
In the councils of government, we must guard against the acquisition of unwarranted influence, whether sought or unsought, by the military industrial complex. The potential for the disastrous rise of misplaced power exists and will persist. We must never let the weight of this combination endanger our liberties or democratic processes. We should take nothing for granted. Only an alert and knowledgeable citizenry can compel the proper meshing of the huge industrial and military machinery of defense with our peaceful methods and goals, so that security and liberty may prosper together.
-Dwight D. Eisenhower, 1961

Hi! This would be photo stacking (not focus stacking) and you need software capable of identifying and aligning the stars. It can also be done by aligning images manually, but you woud not do manual alignment with 120 images.

There are various tools that can be used, one of the good ones to start from is Deep Sky Stacker, which is free. Stacking astrophotos is a very popular technique amongs astro photographers.

Most photos of nigh sky (especially the bright, colorful nebulas) are result of many hours of exposures taken piece (like seconds or at best minutes) at a time and stacked together. In light poluted area sky glow filter such as L'Pro will be helpful.

Agnies said:

Hi! This would be photo stacking (not focus stacking) and you need software capable of identifying and aligning the stars. It can also be done by aligning images manually, but you woud not do manual alignment with 120 images.

There are various tools that can be used, one of the good ones to start from is Deep Sky Stacker, which is free. Stacking astrophotos is a very popular technique amongs astro photographers.

Most photos of nigh sky (especially the bright, colorful nebulas) are result of many hours of exposures taken piece (like seconds or at best minutes) at a time and stacked together. In light poluted area sky glow filter such as L'Pro will be helpful.

Click to expand...

I think Jay has left the building. (The thread is now over a year old and he got quite a few helpful responses.)

DSS will certainly work. However, I usually recommend Sequator (Windows) or Starry Landscape Stacker or Starry Sky Stacker (Mac) to beginners. Like DSS, Sequator is free. The Mac programs are not free but they are not expensive.

One big reason I recommend Sequator or one of the two “Starry” programs is that they handle untracked images with wide angle lenses much better than DSS. The problem is that geometric distortion in the wide-field images causes stacking in DSS to fail outside of the central region of the image. These kinds of images are the most likely ones for beginners to attempt. And some people never go beyond them, producing outstanding nightscapes, for example.

The other nice things about Sequator (can’t speak to the other two programs I mentioned) is that the interface is very simple and it stacks very quickly.

Alen K said:

Agnies said:

Hi! This would be photo stacking (not focus stacking) and you need software capable of identifying and aligning the stars. It can also be done by aligning images manually, but you woud not do manual alignment with 120 images.

There are various tools that can be used, one of the good ones to start from is Deep Sky Stacker, which is free. Stacking astrophotos is a very popular technique amongs astro photographers.

Most photos of nigh sky (especially the bright, colorful nebulas) are result of many hours of exposures taken piece (like seconds or at best minutes) at a time and stacked together. In light poluted area sky glow filter such as L'Pro will be helpful.

Click to expand...

I think Jay has left the building. (The thread is now over a year old and he got quite a few helpful responses.)

DSS will certainly work. However, I usually recommend Sequator (Windows) or Starry Landscape Stacker or Starry Sky Stacker (Mac) to beginners. Like DSS, Sequator is free. The Mac programs are not free but they are not expensive.

One big reason I recommend Sequator or one of the two “Starry” programs is that they handle untracked images with wide angle lenses much better than DSS. The problem is that geometric distortion in the wide-field images causes stacking in DSS to fail outside of the central region of the image. These kinds of images are the most likely ones for beginners to attempt. And some people never go beyond them, producing outstanding nightscapes, for example.

The other nice things about Sequator (can’t speak to the other two programs I mentioned) is that the interface is very simple and it stacks very quickly.

Click to expand...

I love Sequator, it is also much quicker than DSS. I have used both. And it handles light pollution much better too-- it even removes it! The other two programs you mentioned are MAC only correct?

OutsideTheMatrix said:

Alen K said:

Agnies said:

Hi! This would be photo stacking (not focus stacking) and you need software capable of identifying and aligning the stars. It can also be done by aligning images manually, but you woud not do manual alignment with 120 images.

There are various tools that can be used, one of the good ones to start from is Deep Sky Stacker, which is free. Stacking astrophotos is a very popular technique amongs astro photographers.

Most photos of nigh sky (especially the bright, colorful nebulas) are result of many hours of exposures taken piece (like seconds or at best minutes) at a time and stacked together. In light poluted area sky glow filter such as L'Pro will be helpful.

Click to expand...

I think Jay has left the building. (The thread is now over a year old and he got quite a few helpful responses.)

DSS will certainly work. However, I usually recommend Sequator (Windows) or Starry Landscape Stacker or Starry Sky Stacker (Mac) to beginners. Like DSS, Sequator is free. The Mac programs are not free but they are not expensive.

One big reason I recommend Sequator or one of the two “Starry” programs is that they handle untracked images with wide angle lenses much better than DSS. The problem is that geometric distortion in the wide-field images causes stacking in DSS to fail outside of the central region of the image. These kinds of images are the most likely ones for beginners to attempt. And some people never go beyond them, producing outstanding nightscapes, for example.

The other nice things about Sequator (can’t speak to the other two programs I mentioned) is that the interface is very simple and it stacks very quickly.

Click to expand...

I love Sequator, it is also much quicker than DSS. I have used both. And it handles light pollution much better too-- it even removes it! The other two programs you mentioned are MAC only correct?

Click to expand...

Correct. Mac only. I have never used them but online reviews, tutorials and comments indicate they are very capable stacking programs. Starry Landscape Stacker, like Sequator, can stack foreground independent of the sky for nightscapes. Starry Sky Stacker can’t do that and furthermore, according to the developer, requires the camera to be mounted on a tracker. (I thought it could handle untracked images as well, but apparently not.) Hence, it is more like DeepSkyStacker.

https://sites.google.com/site/starrylandscapestacker/home

https://sites.google.com/site/starryskystacker/home

I don’t know why the developer couldn’t write one program to do everything. Sequator, which can do both tracked and untracked, shows that it is possible. I have used Sequator to stack everything from nightscapes to deep-sky telescope images.

https://sites.google.com/view/sequator/