Foreword by Josh: This is a guest post written by Andrea Minoia, and we’ve been blown away by the depth and quality of his writing. This is by no means a quick and easy read (at nearly 4,000 words), but what it is, is a hugely detailed guide to deep sky astrophotography, covering absolutely everything you might possibly need to know to get started. Take notes, bookmark the page, and dive in…
“There are more things in heaven and earth, Horatio/ Than are dreamt of in your philosophy.” – Hamlet (1.5.167-8)
Astrophotography is a challenging and technical game. To keep things interesting and as easy as possible, this article will discuss the photographic part here, from setting up your camera to shooting the picture. We will discuss editing techniques in a following article.
If you are a confirmed astrophotographer, this article will help you refresh your knowledge. And if you’re a beginner? You’re about to embark on a fascinating journey among the stars.
This article will give you the basic principles you need. It will put you on the path to getting the best results with equipment you most likely already own and with few essential upgrades.
The DSLR Astrophotographer
People often believe that astrophotography requires the use of huge telescopes and dedicated, cooled, astro cameras. That’s true if you want to do planetary work, but we will not go into that in this article.
For most beginners, DSLR cameras with normal and telephoto lenses, are enough to start you off.
If you’re not careful, you can soon end up dealing with monsters like this. Astrophotography is quite an addiction.(Credit: Warren Landis )
For the casual astrophotographer and beginner, a simple setup with their equipment on a fixed tripod is enough. You can learn the basics while being able to capture interesting images.
Another advantage of this is that it’s hassle and maintenance free. You can concentrate on learning the basics of astrophotography without worrying too much about gear.
It’s All About Size And Brightness
When it comes to astrophotography, the concept of distance is not important. Both the Moon and Andromeda galaxy are at “infinite” distance for practical purposes.
The Andromeda galaxy is 2.5 million light years away from us and has a radius of 110,000 light years. For comparison, our Sun is a mere 8 light-minutes away from us.
We cannot grasp the sheer enormity of those numbers. But if you were using a telescope to photograph it, you will have to do a mosaic composition to image the whole galaxy.
To get a photo of Andromeda and its surroundings, a 400mm, 300mm or even 200mm telephoto lens will do the job.
The Andromeda galaxy (also known as M31) with its satellite galaxies M110 and M32. This was photographed with a 50 years old, fully manual, 200mm f/4 telephoto lens on m43 camera (400mm EFL).
What really matters in astrophotography is the apparent dimension, or angular size (in degrees), and its brightness, or apparent magnitude. The smaller it is, the brighter the object is in the sky.
For comparison, our sun has apparent magnitude of about -27.
Andromeda has apparent magnitude of 3.44 and apparent size of 3ºx1º.
The full moon has apparent magnitude of -12.6. This is much brighter than Andromeda, but its angular size is only 1/6 of that of Andromeda.
Unfortunately, most of Andromeda’s size is made up of faint dust. This is difficult to capture, particularly when dealing with heavily polluted skies.
The bright “star” near the centre is Andromeda. This is how it appears in a raw image taken from a moderately light polluted location.
Targets For DSLR Astrophotographers
Even if you’re not photographing planets, there is no shortage of targets for astrophotography. Between the Moon, the Sun, constellations, stars and the countless deep sky objects you can keep yourself busy for many years to come.
The Moon and the Sun
The Moon and the Sun can be photographed with a long telephoto lens and even without a tripod if you have image stabilisation.
Warning! DO NOT attempt to observe the Sun or photograph it without using a properly designed filter: they are cheap and will save your eyes and your gear.
This article deals with Moon and Sun photography in detail.
Stars and Star Clusters
Bright individual stars such as Sirius and Betelgeuse are nice target if you have a long telephoto lens and teleconverter.
Some star pairs are famous. Pollux and Castor or Procyon and Gomeisa are some, where one star is orange (“cold”) and the other blue (“hot”).
Procyon (bottom right) is the eighth-brightest star in the night sky. The Yellow star is Gomeisa. Both stars are in the Canis Minor Constellation.
Clusters of stars are nice too and the most famous in the Northern sky is the open cluster of the Pleiades (or M45). This is a region of bright, hot (hence the blue color) and young stars soaked in dust clouds. This gives the Pleiades the classic blue nebulosity.
This cluster is so bright (apparent magnitude 1.6) and large (apparent size of about 2º) you can easily spot it from a large city.
Constellations And Star Fields
With a wide angle lens you can photograph constellations or the Milky Way shining over a landscape. I discussed this previously in this article.
In this image I have sketched in red the full Orion Constellation. If you consider the image was taken with a fisheye lens, you can have a good idea of how big this constellation is.
You can also photograph constellations as part of wide star fields. The aim here is to capture the many nebulae and dust clouds that are part of the constellation.
If you are in the Northern hemisphere, one of the best targets to photograph is the Orion Constellation. Its shape is so unique that you can easily see it high in the winter sky. This is thanks to the three stars in line forming the belt of Orion.
This is a very complex and rich region, with plenty of bright nebulae. There’s the famous Orion Nebula (M42), Flame Nebula, Horsehead Nebula, Running Man Nebula and Barnard’s Loop. It also contains the bright and colourful stars of Betelgeuse (red) and Rigel (blue).
And on the right of Rigel there is the darker Witch Head Nebula, while on the left of Betelgeuse there is the Rosette Nebula.
My first serious attempt photographing the Orion Constellation. All the main stars are visible as well as M42 and the Flame and Horsehead nebulae.
With a 300mm lens on a full frame camera, you can close in on the belt and sword of the Orion constellation. Like this you’ll get a family portrait of the Orion, Running Man, Flame and Horsehead Nebulae.
Other great targets are Auriga, which contains the Flaming Star Nebula, and the dense region of Deneb (here below). You can see the bright North America and Pelican Nebulae in the Cygnus Constellation.
Finally, we have the star field containing the Pleiades and the California Nebula.
There are many galaxies you can photograph. The easiest one is Andromeda and its M110 and M32 satellite galaxies.
Andromeda photographed with a 200mm lens on m43 camera (400mm EFL).
The Pinwheel Galaxy and Triangulum Galaxy are bright enough to be photographed without a telescope. They will be rather small in the field of view of a telephoto lens on a DSLR camera though.
Nebulae are the most colourful and photogenic subjects up there. You can find bright emission nebulae everywhere in the sky.
Notable examples are:
1. California Nebula. This nebula is bright and large enough to be an easy target even in moderately light polluted skies.
2. Flame Nebula and Horsehead Nebula. These nebulae are near the leftmost star in the Orion Belt. Here I was able to photograph them in a moderately polluted sky.
The Great Orion Nebula (M42). This is without any doubt the queen of the Nebulae. It is huge, it is bright and you can see it with the naked eye from a city. It is located in the sword of Orion and it appears as a coloured star to the unaided eye. But point your camera at it and you will be amazed.
M42 photographed few km away from Brussels city centre, the largest city in Belgium.
Without a doubt, M42 should be your first target. You can easily photograph it with some details even on a fixed tripod or from the city.
A famous catalog of objects (both easy and less easy to photograph) is the Messier Catalog. This includes 110 deep sky objects, such as star clusters, nebulae and galaxies. You can use this for inspiration.
The Messier Catalog (photo by Michael A. Phillips, CC BY 4.0)
Some of the objects we’ve described here are not visible in the southern hemisphere (but there are other great objects such as Megellanic clouds).
Also, many of these targets are seasonal. Orion, for example, is not visible during the summer and so best photographed in the winter.
I recommend using a software like Stellarium to check your target’s visibility in the sky.
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How to Do Astrophotography: Basic Rules
Now that you have an idea about what you can photograph, it is time to discuss how to do it.
An astrophotography session is not quick. Even experienced astrophotographers can still take several minutes (or more) to properly set up their gear, find and frame the target and get the star in focus.
And then you have to wait to get tens (or hundreds) of exposures of the same target before you move on.
It is a waiting game, but you can get busy doing star trails with another camera or stargazing.
Here is a 10 steps checklist for you to ensure you will go home with the best data to process:
- Put your gear on a stable tripod;
- Frame your target;
- Focus on stars;
- Freeze those stars;
- Shoot in RAW, forget JPG.
- Use the lowest ISO that is invariant for our camera;
- Use the widest usable aperture;
- Set the lowest usable shutter speed;
- Take as many images of your target as you can;
- Take dark frames (optional).
Let’s now discuss some of those steps in more details.
Use A Stable Tripod
Your gear must be as stable as possible. Your tripod must be rock solid and quick to dump vibrations.
You can use either a good tripod for landscape photography or a field tripod for astrophotography. These are usually heavier and more expensive, but more stable.
You can also hang some weight to the tripod, to further stabilise it.
A child bucket filled with stones hanging with a bungee cord to the tripod is an easy solution to stabilise it.
Frame Your Target
How do you frame a target you can barely see (if you can see it at all) when you have to use a telephoto lens with a narrow field of view?
There are three ways:
- Buy a (rather expensive) computerised GoTo mount which has memorised the coordinates of many deep sky objects. Once you set it up properly, it will point the camera at the desired target;
- Trial and error: try your best to blindly frame your target. Exhausting at best;
- Buy a cheap red dot star pointer and hot shoe adapter. This is is the perfect choice for beginners.
A red dot star pointer is usually shipped with any telescope. You can purchase it with an adapter to mount it on the camera hot shoe.
The idea is simple. A laser is sent on the screen of the pointer so that a red dot becomes visible and can be superposed to the unmagnified view of the sky.
The red dot on the finder’s screen and how the finder is mounted on the camera.
If you aligned the pointer properly, the dot superposed to a target should be more or less centred in the frame.
In the image below, I used my micro four thirds camera with a 300mm lens to frame the moon.
The lens gives me a field of view of only 3.3ºx2.5º, which makes it difficult to find and frame the bright moon.
By superposing the red dot to the moon, I can find and centre it in the frame in no time.
Just put your pointer on the camera hot shoe, and switch it on. Then, use the knobs to align it to your gear from time to time.
If you cannot see your target and you have no GoTo mounts, you have to become fluent in the star hopping technique.
Nothing keeps you from aiming your camera at a random direction in the sky. This is, in fact, how I took my first astro image, the one that opens this article.
If you want to know what you have photographed, you can upload your final image on Astrometry for free. This process is called plate solving and will return you your image with annotations for all identified objects.
It turned out I had photographed the Deneb/Sadr region in the Cygnus Constellation. The North America Nebula is visible near the star S7Cyg, below Deneb.
This image is made out of 30 images taken on a fixed tripod using my old Olympus Zuiko OM 50 f/1.4 on OM-D EM 10 m43 camera.
How To Focus On Stars
Even worst than framing an invisible target is focusing on stars. There is nothing more frustrating than finding all your images are out of focus after a night spent in subzero temperatures.
This was supposed to be the Auriga Constellation.
To focus on the stars, forget your autofocus. Also don’t trust the infinite mark on your lens. Instead, use your live view at high magnification and frame a bright star.
The more you focus in, the smaller the star will become and more of the faintest stars will begin to come into view. Turn your focusing ring slowly back and forth to find the sweet spot.
The problem is that every time you touch the camera, the image will blur due to camera shake. This makes the whole process a bit random.
You can buy a Bahtinov mask, but I like DIY solutions when possible. I found that placing a kitchen sieve on the front of my lens will create diffraction spikes around the stars when the focus is good.
I found this method very reliable and fast. If you have difficulties to see the spikes on your live view, try to do your best to focus and take a test shot. When you see you have sharp diffraction spikes, your focus is good.
You can also mark the true infinite on your lens with a marker, to make the whole process even faster in the future. Use a bit of gaffer tape to lock down your focus. That way you will not move it accidentally when, for example, you put a dew strip around your lens.
After having focused the image with my kitchen sieve, I put the dew strip on the lens and accidentally moved the lens out of focus.
Freezing The Stars
In astrophotography, except for star trails, you want to have stars like points of light. But stars move across the sky at, in average, about 15º/ hour because of Earth’s rotation.
That does not seem much when we stargaze. But consider that a 100mm lens on a full frame camera gives you a field of view of only about 20º x 15º.
This is how the sky will look after a 3 minute-long exposure.
There are only two ways to freeze those stars:
- Take very short exposures on a fix tripod;
- Use an equatorial mount to rotate the camera around the celestial pole to counteract Earth’s rotation.
Astrophotography With a Fixed Tripod
On a fixed tripod you are forced to take very short exposures. A rule of thumb, also known as 500 rule, is the following:
ET= 500 /(FL*CF)
ET is the longest exposure time (in seconds) before stars trails can be clearly see. FL is the focal length and CF is the crop factor of your camera sensor.
My m43 camera has crop factor 2 and with a 50mm lens I could expose for maximum 5 seconds before I start noticing the star trails. Rules like the 400-500-600 are qualitative and I was able to expose the Orion Nebula for 3 seconds on a 150mm lens with decent results.
This image is the result of aligning and stacking 50 exposures. In the 2 minutes that it took me to do so, this is how much the nebula moved in the field of view.
It is obvious you cannot expose the sky for much longer than the value calculated with the 500 rule.
Astrophotography With Equatorial Mount
An equatorial mount is a motorised head that, once aligned to the celestial pole will rotate the camera to counteract the Earth’s rotation. This will negate the apparent motion of the star in the sky.
Here is how much the Orion Nebula moves in the field of view of a 270mm lens (EFL 540mm) in a session of about 45 minutes.
The tracking is not perfect but I could expose each image for 40 seconds without getting star trails.
My gear was mounted on the Skywatcher Star Adventurer, which is great for DSLR work. It also comes with some interesting functions for photographers, such as automated time lapse and pano capabilities.
As you can see, an equatorial mount is by far the most important piece of equipment. It’s the first one you should buy if you are serious about astrophotography.
If you don’t plan to use large telescopes and extremely long focal lengths, you can buy a lightweight, portable and cheaper mount for photographic tripods. Examples include my Skywatcher Star Adventurer, iOptron Skyguider, etc.
ISO And ISO Invariance
In the ISO invariant regime, the noise you will introduce by brightening up the image in post-processing by 1 EV is comparable to that introduced in the same image by shooting at a double ISO value in the first place.
So in the invariant regime, there is no “noise” advantage to shoot at higher ISO. And you will reduce your sensor’s performance by reducing the dynamic range you can capture.
You should find online which ISO values your camera is ISO invariant for. And you should use the lower ISO in this regime to maximise the dynamic range the sensor can capture without noise penalty.
Don’t worry if the histogram is crammed to the right side (like in the picture below). The important thing to remember is not to clip the highlights.
Capture As Much Data As You Can
It should also be clear that rising the ISO will not let you collect more data. To do that, you have to expose longer and/or use a wider aperture.
We want to stack many images to improve the signal to noise ratio in the final image. As such, your exposure should be long enough to collect a good amount of data. But not so long that you will clip the highlights, get oval-shaped star and trails or run into tracking issues.
Poor polar alignment, unstable gear and mount periodic errors can ruin your exposures.
If you do not track the sky with an equatorial mount, use the 500 rule and take as many photos as you can. If you can track, take a test shot of 30″/60″ and see how it looks like.
Unfortunately there is no single setting that will fit all the situations. And, again, take as many images you can.
Before heading home, if you have time, take a few extra images with the lens cap on. These are dark frames and will be useful in post to improve the quality of your final image.
It is important that you take them at the end of your session, directly on location. They need to be taken at the same temperature as your images.
Photographic Equipment For Astrophotography
As I said, you need a DSLR or mirrorless camera, but very good high ISO capabilities are not really important. What you need is some ISO invariant behaviour. Also, your camera must allow you to shoot in RAW format.
An entry level model such as the Canon 1300D is usually good enough to start with.
Mirrorless M43 cameras such the Olympus OM-D EM-5 Mk ii are interesting because of their 2x crop factor. This allows you to close in on targets with shorter, lighter and cheaper telephoto lenses than those required with full frame cameras.
Finally, keep in mind that your camera will be pointing at the sky. A camera with an articulated LCD screen is preferable. You will be able to operate it more easily and comfortably.
You don’t need to have the latest pro-graded lens to do astrophotography. You will not need autofocus or in-lens/in-camera stabilisation.
I actually use old manual lenses from the film era.
What you need is your lenses to not have bad coma or too much chromatic aberration at relatively wide apertures.
This is where modern ED lenses and APO refractors using low dispersion glass often have an edge over many old legacy lenses.
A great way to check if you can use your lens in astrophotography and how well it will perform, is to search it on Astrobin, a great free site and image database you can even contribute to.
Finally, you should go for lenses that perform well with apertures in the range of f/2.8 to f/5.6 for deep sky astrophotography. Slower lenses will require longer exposures and even more images to be stacked.
Typical focal lengths for full frame camera are:
- 50mm for wide star field images;
- 135mm for closing in on small constellation or narrower star fields;
- longer telephoto for nebulae and galaxies close ups.
- Teleconverters can be used to extend your reach, but the will cost you some light. They are a good solution for Moon and Sun photography and for the brightest targets such the Orion Nebula and the Pleiades.
If you want to see how big a target will be in the field of view of your camera-lens system, you can use the free software Stellarium. Just insert the info for camera sensor and lens (telescope).
A word of advice. Don’t get stuck with filling the frame at all costs. Very long focal lengths and high magnifications are challenging. Cropping your image is perfectly acceptable in astrophotography.
Must Have Accessories
If you don’t have the luxury of doing astrophotography from your garden and have to backpack like me, you’ll be out in the field for a while. You’ll need to carry your gear and things to make your astro trips more comfortable. I prefer to use backpacks similar to those for mountaineers, such as those in the F-stop mountain series.
This is a short list of the essential accessories you should have with you:
- Remote shutter with intervalometer;
- Sturdy tripod;
- A good camera backpack with some room for your stuff;
- A lightweight equatorial mount with accessories is the first investment to consider to up the game. A classic compass is more reliable than the one on your smartphone and it is essential for a good polar alignment of the mount;
- UHC and LPR filters (Baader, Hoya, etc.) to boost contrast in the images and fight light pollution;
- USB dew strip heater to prevent your lens to fog in cold and humid conditions. A powerbank to power it;
- Lens hood;
- Spare batteries and good memory card with high capacity;
- Food, water/tea/coffee, camping chair, warm clothes and something to do while you wait;
- Headlamps with red light to see what you are doing without spoiling your night adaptation;
- Tea/kitchen sieve to help you focusing on stars;
- Smartphone with astronomy apps to help you to get familiar with the night sky.
We are at the end of this long article. There’s still things for you to research on your own how to find a dark sky and how to fight light pollution, for instance. But this guide will help you get started.
You’ll also be able to join the fantastic world of astrophotography with fewer headaches than I got when I started imaging the night sky.
Stay tuned for the next article on how to edit your data. In the meantime, grab your gear and get out there and play with the night sky.
A note from Josh, ExpertPhotography's Photographer-In-Chief:
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