Photographing stars and the Milky Way

General information

Photographing stars and the Milky Way is a wonderful challenge. Of course, you can take a "point-and-shoot" photo with your cell phone or camera. And there will undoubtedly be "something" in your photo. But taking a beautiful, clear, sharp, and colorful photo of the night sky, and in particular of the jewel that is the Milky Way, will require a little more knowledge, skill, equipment, and perseverance. I'll explain how it can be done.

Tip: if you would like to know more about how to photograph the celestial phenomenon known as the "aurora borealis," please refer to our special section on this topic: "Photographing the aurora borealis." Below, we discuss regular astrophotography.

How do I take a simple astrophotograph?

In fact, anyone with limited resources can take an astrophotograph. Point your camera at the sky and focus. Use the most powerful lens possible and photograph for as long as possible.

However, when viewing your results, you will quickly notice that it is best to use a tripod, as you will have to work with an exposure time of several seconds. And you cannot hold your camera/lens combination still for that long. The minimum equipment required is therefore: camera + tripod.

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The message for beginners is: start very simply and make it more complex later. Start by placing your camera on a tripod, choose a wide-angle lens (you are much less likely to make tracking errors with this – more on that later), choose ISO-800 and a duration of, for example, 15 to 30 seconds. If you choose a beautiful foreground, you're already well on your way to a potentially new hobby: astrophotography. Try to find the constellations you've captured in your photo. You can do this using a planetarium program (more on that below).

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How do I avoid streaky stars?

When you take an astrophotograph, you will see that, in principle, not much will appear with an exposure of a second or so: the captured signal is too low. So you will want to expose for longer. Most cameras go up to exposures of 30 seconds as standard. If you take a 30-second exposure of the stars, you will notice that the stars are not depicted as dots but as streaks. And of course, you don't want that. The reason for this is simple: the Earth rotates once around its axis in 24 hours. If you consider the Earth's surface to be fixed, you could also say that the stars are constantly moving. You won't notice this with short exposures, but you will with longer ones. See our section "Photographing star trails." Sometimes, for aesthetic reasons, you do want trail-shaped stars, but that's a different story. Take a look at that other section page.

Here, we assume that if you want to capture stars and the Milky Way, you want to do so as sharply as possible and with the highest possible level of detail. For long exposures, you will need to use a tracking system. Such a system always consists of a small platform that rotates at the same speed as the celestial sphere, i.e., one revolution per day. You don't "see" the tracking system rotating because it moves so slowly, but it does indeed follow the movement of the stars.

There are various tracking systems available. We will show you two of them here: the first is a tracking mount. This is robust and designed to carry heavy loads such as telescopes or heavy telephoto lenses. An example of this is the renowned Astro-Physics 900 GTO mount. The second is a particularly compact motor that you can place on your regular tripod. An example of this is the Move Shoot Move (abbreviated as MSM). Such a tiny tracking system is lightweight and can easily be placed in your backpack, allowing you to take it with you wherever you go. This makes it ideal for capturing beautiful nightscapes (milky way images) in the mountains or countryside, for example.

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Here is an example of a piece of sky captured with a 20 mm f/1.4 L wide-angle lens. The exposure time was 30 seconds. On the left, you see the original without tracking, on the right with Move Shoot Move activated. In the second row, you see a crop of the upper left corner of the original in the first row.

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Why are the lines not always the same length everywhere?

There are two factors that determine how long the star trails will be in a particular photo:

1) All stars revolve around a single point: this point is the North Star. The North Star is located in the Northern Hemisphere of the Earth, roughly in line with the axis around which the Earth revolves. All stars therefore revolve around the North Star, as it were. The further you move away from the North Star in the sky, the greater the apparent speed of the star. A star that is very far from this central star, for example at the celestial equator, will move faster and therefore make a larger streak on the photo.

2) The longer the focal length of the lens you are using to take the photo, the longer the star streak will be. After all, longer focal lengths magnify the sky, and therefore also the streak. So if you expose for 10 seconds with a wide-angle lens of, say, 10 mm, you will see virtually no star trails. If you do the same with a 400 mm telephoto lens, you will see very clear trails.

That's why it's always better to use a tracking system. Then you never have to ask yourself: will I get rutting or not?

What exposure time should I take to avoid trail formation at the stars?

So, we ask ourselves the following question: assuming I don't use a tracking mechanism, how many seconds can I expose at a certain focal length without getting trail formation?

If you do use a tracking mechanism, you can in principle expose for hours without any tracking error. In this case, it depends on the mechanical accuracy of the mechanism and the one you have set up. If both are OK, you can in principle photograph for an unlimited amount of time.

In the other case, if you do not have a tracking mechanism, there are two commonly used rules for calculating the maximum exposure time for a specific focal length.

1. The 500 Rule (simplest)
This rule says:

Maximum shutter speed (in seconds) ≈ 500 / focal length (full-frame equivalent)

Example:
Lens = 24 mm on full-frame → 500 / 24 ≈ 20 s
Lens = 50 mm → 500 / 50 = 10 s
If you are using an APS-C or MFT, then you must first multiply the focal length by the crop factor.

This usually roughly prevents stars from becoming stripes. However, higher resolution astrophotographers use other values than that 500, precisely because one works with very small pixels these days:

500-line → for older, lower resolution cameras
300-line → for medium resolution
200-line → for very high resolution (40-60+ MP)

Example: for my Canon EOS R5 40 Mpixel sensor, I obtain these values using the 200 rule:
- 10 mm wide angle: 20 seconds
- 24 mm wide angle: 8 seconds
- 50 mm standard lens: 4 seconds
- 100 mm telephoto lens: 2 seconds

2. The NPF rule (more accurate).

The 500 rule is actually quite coarse. This NPF rule takes into account:
1) focal length
2) aperture
3) pixel size of your sensor

Formula (simplified):

T ≈ 15 × N + 30 × p / f
where:
T = maximum shutter speed (s)
N = aperture value
p = pixel pitch (µm)
f = focal length (mm)

There are apps that calculate this automatically(PhotoPills, PlanIt!).

How can I recognize a good astrophotograph?

When you look at your first astrophotographs (objectively), there is a good chance that you will notice that something is not quite right. Here we discuss some common shortcomings in a good astrophotograph.

Sharpness

A good astrophotograph stands out because the stars, and by extension everything else, are depicted with razor-sharp clarity. Many people don't pay enough attention to focusing. Well, with something like the night sky, which is very dim, this can pose some challenges. So here's a tip: choose one particular bright star, zoom in on it as much as possible, and set your lens to "manual focus" if it has that option. Thenmanually focus on that bright star. Once focused, leave the lens at that focus setting.

Note: during long observation sessions, your lens may cool down (in winter) or heat up (in summer). As a result, when creating time series (i.e., taking several consecutive images—more on this below), the focus may become incorrect over time. You will mainly notice this when working with longer focal lengths. So check the focus from time to time. Don't just assume that it will remain constant throughout the entire observation session.

Lens errors

Lenses are never perfect. I repeat: lenses are never perfect. Even if you have the most expensive lenses in the world, there will always be some kind of flaw in the image (photo) produced by them. We are not going to give a full explanation of all possible flaws here, but in summary it boils down to this: color flaws (chromatic aberration), distortion flaws (barrel distortion, astigmatism = stretched stars, especially visible in the corners of the photo) and vignetting (this can be seen as dark corners and a brighter center of your photo).

In a good astrophotograph, you will see significantly fewer (ideally none) of these errors. So always check your results for the three types of lens errors mentioned above.

So, what can you do to avoid or minimize these lens errors? The following options are available:
1) Choose the best possible lens from a technical point of view: don't just go by what the manufacturer says (even if they explicitly state that a particular lens was made specifically for astrophotography). Read reviews of the lens in question, look at photos taken by other amateur astronomers, ask for advice at a public observatory (in Flanders, you can visit volkssterrenwachten.be), and so on.
2) If you do have lens errors, know that they usually decrease when you use the lens at a smaller aperture. Simply put, if you use an aperture of f/2.8 (with a certain lens) instead of f/1.4, you will get better results at f/2.8. Why? Very simple: with a smaller aperture, you use more of the central parts of the lens, and the outer rays of light are cut off. And that is precisely the weakness of this solution: we don't really want to underutilize our lenses, because we want to capture as many photons as possible on our sensor. But anyway, it's a compromise. If you want razor-sharp photos with few lens errors, and you can't afford expensive lenses, then you can certainly resort to this measure.

As an aside, you might think that it is always best to work with the smallest possible aperture. After all, who wouldn't want to minimize lens errors? Well, this reasoning is incorrect! There is another effect that comes into play. Suppose you go to f-values in the order of magnitude of f/16, f/20, f/22... and so on, then you will notice that the "resolving power"(also called resolution, the degree of detail) will decrease. This is due to the physical phenomenon of "diffraction." We will not go into this further here. For most lenses, you will achieve optimal sharpness, resolving power, and minimal lens errors between one or two stops above the maximum aperture of your lens and a maximum of f/5.6 to f/8. If you go higher, which you almost never do in normal astrophotography because you want to capture as much light as possible, the quality of the image will decrease. Sometimes this is very subtle, but it is still there. Be aware of this. This can be important for landscape photographers. Suppose you want to take a photo of the Milky Way with a beautiful, detailed foreground, then all of this can play a role. So you photograph the Milky Way using the techniques discussed here and you photograph the foreground separately at a more normal exposure. Afterwards, you combine the two photos.

Noise formation

To take an astrophotograph, you need not only a lens (on a tripod), but also a sensor. We will not go into too much detail here about the various types of sensors that exist (CCDs, CMOS, back-illuminated sensors or not, etc.). They all have one thing in common: no matter how good they may be, they always generate some noise. Noise is actually a signal that does not come from the light source (star, galaxy, etc.) but just comes out of nowhere due to limitations of the sensor.

To be completely accurate, I should also mention that noise does not originate solely from the sensor. For example, not all photons from a star arrive at your sensor at the same rate. So there can be some variation. And that is also noise.

So once again, the question arises: what on earth can you do about it? Well, you are not completely powerless here either:

1) Choose the best possible sensor. Some sensors generate more noise than others. So here too: read reviews, consult with other amateur astronomers, look at their results. Usually, the more expensive the camera, the less noise is captured.

2) Don't immediately choose a sensor with the highest number of pixels. Some people are obsessed with wanting to work with the highest possible resolution. Be aware that high resolution means working with very small pixels. And the smaller the pixel, the fewer photons can be stored in it. This means that fewer photons can be stored for the same amount of noise. In other words, the signal-to-noise ratio, which you actually want to be as high as possible, will be worse than when working with large pixels. In summary: a camera with, for example, 24 megapixels will score better in terms of signal-to-noise ratio than, for example, a 40-megapixel camera (assuming that all other parameters of the sensor are the same, of course).

3) Try to choose longer exposure times: the longer the exposure, the more signal gain you will get. However, you must be careful not to overexpose your photo. This can happen quickly when you use longer exposure times here, where there is a lot of light pollution. However, when you are in really dark locations, such as the deep Ardennes or the Morvan, you can easily go for exposure times of 2 minutes or more (instead of 15 seconds here – just to give an example).

3) Choose the lowest possible ISO setting. Be aware that your camera always captures the same amount of photons, regardless of the ISO setting you choose. All you are doing when you choose a higher ISO setting is increasing the electronic amplification of your camera. Be aware that this also amplifies the noise. Of course, you will have no choice but to set a slightly higher ISO value (otherwise the signal will be too weak), but be aware that this will also increase the noise. Normally, the noise will increase faster than the gain you get from a stronger signal. So it becomes a compromise. In the first cameras, it was best to limit your ISO to around ISO-800, but the new cameras can easily handle ISO-3200 or even ISO-6400 (or sometimes even higher).

4) The final technique you can use to reduce the noise level in your recordings is to opt for time-lapse photography. This means that you take several photographs of the same scene in the sky. You may be thinking, "But then you'll end up with, say, 10 copies of the same noisy result, right?" That's true, of course, but that's where the laws of statistics come into play. By statistically processing all the recordings, the signal-to-noise ratio will improve: all the photos are added together (we "stack" the photo —that's what we call this process) and the average is calculated. Due to the nature of noise, it will be strongly averaged out. Noise is random in nature, and the average of "randomness" is... zero! And that's what we want. So the more shots you take of the same scene, the better the "stacked" photo will be.

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5) Sometimes you will see that your photo is peppered with bright, colored pixels (image points). We call these hot pixels. We can also consider this a kind of "fixed noise." It is caused by errors in the sensor itself. There is not much you can do about this. High-end cameras do have an option to remove these hot pixels. To do this, after each exposure, the camera will take a so-called dark exposure (a "dark frame") of the same duration as the actual exposure. No light is allowed in during this dark exposure: it is as if the lens cap were on the lens. In principle, the camera should not register anything because no light is coming in. However, the hot pixels will appear on such a dark frame. When you subtract this dark frame from your exposure, these hot pixels will disappear from your exposure. The disadvantage of this method is that you are only processing real photons for half the time. The other 50% of the time, the camera is busy taking dark frames.
Avid astrophotographers will carry out this process themselves: they make sure they have one good dark frame. Afterwards, they take all the normal photos. They then subtract the dark frame themselves. This way, you can spend almost 100% of your time collecting real photons. It makes a huge difference in terms of productivity. We don't have that many cloudless nights, so it's important to work as efficiently as possible when there are no clouds. 😉

Colors

A good astrophotograph also stands out thanks to the fact that everything is depicted correctly in terms of color. A few points require extra attention here.

1) A common problem with astrophotography is that the background is not perfectly black . This is usually due to light pollution. The photo will often have an orange-reddish tinge, usually caused by the yellow sodium lamps that used to be installed as street lighting. Gradually, these are now being replaced by whiter LED lights. These lamps still cause light pollution, however. It is therefore best to find a truly dark location, far away from civilization. Such places can be the countryside (far from cities), forest areas (the Ardennes, for example), or mountainous areas (where there is usually a lower population density). The darker the sky, the more beautiful the black background will be. The entire photo will look better.

2) When you use a regular camera to capture the Milky Way, you will notice that certain nebulous structures are not prominently displayed in your photo. This is because these are areas that emit a lot of H alpha radiation. The wavelength of this radiation is blocked in a regular camera because they have a UV/IR filter in front of the sensor. The pink-red H-alpha radiation has difficulty passing through this filter. This is why many nebulae do not show up well in photos taken with conventional cameras. The solution? Use a real CCD camera or a standard camera with the UV/IR filter removed. There are certain companies that specialize in the latter. Some camera manufacturers sometimes release a special version of their normal cameras for astrophotography purposes. These are models that do allow this H alpha light to pass through. I used to use the special Canon EOS 20Da version (note the lowercase "a" that indicates the astrophotography model). Nowadays, you can get the Canon EOS 6Da Mark II model . You can buy it at this Belgian astroshop, for example.

How can I recognize a good Milky Way photo?

Of course, first and foremost, all the requirements described in the previous section must be met. That seems obvious to me. However, a good photo of the Milky Way will also show at least the following:
1) brownish and dark dust lanes
2) the presence of large globular star clusters
3) large, extensive H-alpha regions (reddish in color)
4) in wide-angle shots: other nearby galaxies such as M31 Andromeda Nebula or, if you are in the Southern Hemisphere of the Earth, the Large and/or Small Magellanic Cloud. These are so-called companions of our own Milky Way.

If these elements are beautifully and sharply depicted in your photo, then you have a good Milky Way photo!

Tip: taking a photo of the Milky Way will always be easier when it is high in the sky. For us, that is in the summer. Take advantage of its high position, despite the fact that the nights are less dark then. Wait until twilight has completely disappeared and the night sky is properly dark.

A number of photographers around the world specialize in Milky Way photography. They create what are known as "nightscapes": essentially, these are nighttime landscape photos in which the Milky Way plays a prominent role. It is particularly beautiful when an ultra-wide angle lens is used, sometimes by stitching photos together, to capture the Milky Way from horizon to horizon in a single shot. Something attractive is invariably placed in the foreground: a lake, rocks in a nature park, mountains, a windmill, you name it. Sometimes these foreground objects are slightly illuminated (using an LED loop lamp, for example). Note that all these images are almost always so-called composite images: one (possibly stacked) image for the Milky Way and another (sometimes also stacked) image for the foreground.

Want to try it yourself? Take a look at some examples on Cosmic Captures or in Google Images. Impressive, isn't it?

Where can I find the best place to photograph stars and the Milky Way?

If you want to take the best astrophotographs, it is best to look for a truly dark location. These have become rather scarce in our region. There are so-called "light pollution maps." As an example, I will show you a general map for Western Europe. The blue zones are areas where there is virtually no light pollution. Think, for example, of the southern part of France, Brittany, etc. The first large blue zone for us here in French and West Flanders is the Morvan in France.

There is also a small area in the French Ardennes. In 2005, we found a pleasant gite in the hamlet of Talma in Grandpré (in the French Ardennes). It is called Gîte Chez la Grand-Mère (Google Maps). It can accommodate up to 12 people. Since we discovered this gite, hundreds of amateur astronomers have followed in our footsteps to astrophotography camp there. It can be very humid at times. We have also been there when we could do virtually nothing due to persistent... fog. Dew formation is a constant concern. But hey, it's still pretty dark there.

Did you know that... there are associations that actively combat light pollution, inform people about it, etc., in most countries? In Belgium, this is the Light Pollution Working Group of the Astronomy Association: Website – Facebook. Internationally, there is the Dark Sky organization: Website – Facebook.

Is astrophotography in a group worthwhile?

Photographing in a group has a number of clear advantages: you can always turn to your fellow travelers for advice and assistance, sometimes even for a quick fix in terms of equipment; if it's cloudy, you always have company; your partners may come along; you always learn something from the others, and they learn something from you, and so on.

A big advantage of organizing a camp is that you have set aside time to take a good astrophotograph. After all, it takes some effort to take a beautiful photo. Together, you decide which dark sky area you will visit, how many people will go, for how long, under what conditions, and so on. Most of the astro camps I have been to usually resulted in beautiful astrophotographs. And: the costs are shared.

You have to make sure you set clear boundaries. You don't want people turning on lights unnecessarily while you're observing, bumping into your setup, making noise while you're sleeping, constantly distracting you while you're working, and so on. So, the participants need to get along well. That's not always easy. Tensions are naturally more likely to arise when it's cloudy for days on end. 😉 It's also best to make clear financial agreements, because "good agreements make good friends."

All in all , it's definitely an advantage to go out together, that's for sure.

How can I prevent condensation from forming on my lens?

It can cool down considerably during the evening and night. This is even more so when it is a cloudless night: clouds form a blanket that prevents the temperature from dropping as quickly. Because the temperature drops significantly and there is often a lot of water vapor in the atmosphere, the water vapor will first and foremost transform into very small water droplets on the surfaces that are the coolest. So: surfaces such as metal objects that cool down particularly quickly. Glass surfaces (lenses) can also cool down significantly, causing them to become covered with a very fine layer of water droplets: dew. When you are creating time series, you will see that the contrast within the photos will begin to decrease as the dew forms. Eventually, the images may even become completely unusable.

How can you avoid this? Simple: by keeping your lens warm. There are special heating strips available that you can wrap around the lens. Of course, you don't want to overheat the lens, just warm it enough to prevent condensation. Take a look at Astroshop to see what these heating strips look like and how much they cost. They are definitely useful for the avid astrophotographer.

Tip: In the past, these heating strips typically ran on a car battery. Nowadays, there are also USB-C models that you can connect to a power bank. That's very handy.

How can I find a specific nebula or globular cluster in the sky?

Amateur astronomers, at least the practical ones, can easily find their way around the sky. I am not one of them. So what do you do? Well, in the past, you would have been in trouble, but fortunately that is no longer the case. 😉

There are now various programs and websites that show exactly what the starry sky looks like for any place in the world and at any time. We call this planetarium software. You can also use these programs to indicate which object you are looking for: e.g. M42 the Orion Nebula, the Horsehead Nebula, the Veil Nebula, etc. So use a planetarium program to find your way among the constellations in general and celestial objects in particular. Below are various links to such planetarium software/websites.

Did you know that... top.vlaanderen immediately generates a link to Stellarium and In The Sky top.vlaanderen all the sites discussed, which now number more than 300, so you can immediately see which starry sky you will see there. Very practical. No more searching and a lot of time saved.

Internet information from and about photographing stars and the Milky Way

Wikipedia information on:
– Astrophotography (EN - FR - AND)
– the Milky Way (EN - FR - AND)
– Light pollution (EN - FR - AND)

YouTube about:
- Nightscapes Milky Way

Light pollution maps:
- Dark Sky Map

Interesting websites:
Royal Dutch Society for Meteorology and Astronomy (Royal Dutch Sailing Association): website
Astronomy Association (VVS): website
French Astronomy Association (AFA): website
Flemish Public Observatories: website

Interesting planetarium programs:
- Stellarium
- In the Sky

Examples of astrophotography

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