Beginner’s Guide to Deep Sky Astrophotography
Have you ever wondered how everyday people use their DSLR camera to photograph deep sky objects in space?
If you have ever looked through a telescope, you probably already know that the vibrant colors and contrast you see in the images do not appear that way in the eyepiece. This is because a camera sensor can record much more light that our eyes can.
If you are interested in taking your own photos of the astonishing deep sky objects in the night sky such as the Orion Nebula and the Andromeda galaxy, this beginner deep sky astrophotography guide will get you on your way.
The Orion Nebula captured using a DSLR camera and telescope.
“Deep sky” (often called “deep space”) means photographing objects like nebulae, galaxies, and star clusters. Some of these objects are faint or underwhelming through the eyepiece, yet explode with color and detail when photographed.
The core aspects of this particular form of astrophotography include understanding the camera settings required to capture long exposure images, how to achieve (and maintain) sharp focus, and everyone’s favorite, image processing.
As you can imagine (or may have experienced), a hobby like this involves a specific and potentially overwhelming set of equipment.
A basic deep sky astrophotography setup with a DSLR camera and telescope.
My personal imaging gear has evolved and expanded over the past 8 years, and the learning process never ends. Small milestones and gradual progress along the way are a huge motivator to continue pushing forward. Don’t think that you can just skip over a number of steps by buying top of the line equipment early on.
Because I’ve got news for you. It never ends.
Your shiny new toy loses it luster quickly, and you’re left with the actual enjoyment of the process to fall back on. That is how you sustain your enjoyment of this hobby long term.
No matter what it looks like from the outside looking in, deep sky astrophotography is not about what gear you have. It’s about the journey you take on the way to achieving the image you’ve always wanted to capture.
The Andromeda Galaxy. Canon Rebel Xsi DSLR camera and 80mm telescope.
A Beginner’s Guide to Deep-Sky Astrophotography
Photography at night requires a basic understanding of your camera’s ISO and aperture and shutter speed settings. Essentially, you need to understand how to predictably control the amount of light that reaches the sensor.
Modern day DSLR cameras offer an incredible way to capture the night sky. They are well equipped to maximize the amount of light collected in a single exposure. Settings like ISO, shutter speed, and aperture (f-stop) control how much data you can collect on your subject at once.
If you’ve heard the term “modified for astrophotography” before, it simply means that the standard internal IR cut filter has been removed, or replaced with one that allows more of the red signal (primarily emitted by emission nebulae) to reach the sensor.
Related post: Choosing a camera for astrophotography
There are professional services available that do this, or you can brave this task on your own. Fair warning, the first time I modified my Canon Xsi, and put it all back together, it didn’t turn back on.
Before attempting to modify your existing DSLR camera to improve your astro-images, I’d explore the impressive capabilities of a stock camera first. The photo of the Andromeda Galaxy above was taken using an entry-level (used) Canon EOS Rebel Xsi.
All of these images were captured using a camera lens, not a telescope.
Recommended Camera Settings
Seeing as we chose a hobby that involves taking pictures in the dark, we can’t use the same camera settings that daytime, terrestrial photographers do. Most of the time, we want to let in as much light (or signal) to the sensor as possible.
Traditional photography settings that work well in daytime situations just don’t cut it for astrophotography. For example, if you try to take a photo of the night sky in a single shot using ISO 100, @F/4, for a less than a second – you might as well keep the lens cap on.
Typical DSLR astrophotography camera settings use a much higher ISO setting, usually north of ISO 800 or much more. The exposure length for a single astrophotography image can be anywhere from 15-seconds to 5-minutes when using a DSLR.
If the focal ratio (the f-stop, or aperture) of your lens or telescope is closer to the F/2 range, you’ll have many more options when it comes to settings and the way you approach your target. A “faster” optical system means you’ll be able to collect more signal in a single shot.
Unlike a dedicated astronomy camera, a DSLR lets you to control camera settings without computer control.
To tap into the full control of your DSLR, you’ll of course want to use manual mode. It’s easy to make a mistake in this mode, as the camera is no longer assisting you in any way in terms of recommended settings. However, digital cameras are great for taking test shots and experimenting.
Benchmark Camera Settings for Astrophotography
- Camera Mode: Raw
- Focus Mode: Manual
- File Type: RAW
- White Balance: Daylight or Auto
- Exposure Length: 30-seconds *
- ISO: 800-1600
- F-Stop: F/4 or faster
* this will depend on the focal length of your camera lens/telescope, and whether you are tracking the movement of the sky or not. Some people like to use the 500 rule as a guide.
It’s not like the film days anymore. You can shoot, delete, and shoot again with one adjustment made to monitor the results. These experiments may eat up some time early on, but once you identify the sweet spot, you’ll remember it moving forward.
You’ll also have a much better understanding of the functionality and mechanics of your camera and why you’re using the settings you are and not blindly following a recipe of best practices.
The Carina Nebula. Image created using 30-second images @ ISO 6400.
The 3 Challenges
A “fast” camera lens or telescope essentially lets more light through to the sensor at once, and when shooting objects in the dark, that’s a great option to have.
So you might be thinking to yourself, okay, I’ll shoot a 30-second exposure, at ISO 6400 @ F/2.8. That should pack a punch in the signal department. Well, there’s a catch – to ALL 3 of those promising night sky settings. A higher ISO setting will result in more “noise” in the image.
A fast f-ratio will make it difficult to focus and present challenges with bright stars. And perhaps most importantly, a 30-second exposure will show elongated, trailing stars if you’re not accurately tracking the apparent movement of the night sky. The longer the focal length of your camera lens or telescope is, the more critical this becomes.
These are 3 reminders of why not everyone captures and shares incredible images of the night sky. If it were easy, everyone would do it, and I for one am happy that this isn’t the case.
Luckily, there are some relatively straightforward steps we can take to avoid the common challenges night sky photographers face. And when you keep them in mind, shots like this is actually quite achievable using basic gear.
The Cygnus region of the Milky Way and the “Great Rift”. DSLR and 14mm Camera Lens.
The photo shown here did not use a telescope to capture the scene, even though several deep sky objects can be seen in the image. Instead a wide-angle camera lens (Rokinon 14mm F/2.8) was used to capture a captivating area of the night sky that includes the constellation Cygnus, and the Summer Triangle.
This is where concepts like the signal-to-noise ratio come into play.
All this means, is that by increasing signal (light), we can improve the ratio between the data collected on our subject, and the artifacts resulting from a hot camera sensor.
Thermal and digital readout noise is pronounced in long exposure images using a high ISO, but we can offset these problems by stacking (integration) several exposures together.
Noise, and other annoying artifacts can be removed in the pre-processing stages through calibration.
Why You Need to Stack and Calibrate Images
Here is a look at why we stack images. The image on the left a single 2-minute exposure, with plenty of noise, and poor signal. The image on the right includes 70 x 2-minute exposures (non-linear) for a stronger signal and less noise.
It is not possible to pull that kind of detail out of a single exposure. By improving the signal to noise ratio, you have more flexibility and creative options for processing.
You are not limited to the restraints of a single, noisy image with a weak signal to noise ratio.
I won’t go deep into calibration frames in this article, because I have covered these items in previous posts. But I will say that one method involves taking pictures with the lens cap on (dark frames), and the other with a white t-shirt stretched over the objective of the telescope (flat frames).
When you are ready to take your passion (and magnification) further, you can upgrade from a small camera lens and attach a telescope or telephoto camera lens to your DSLR, and use nearly identical camera settings for an up- close view.
Tracking the Night Sky
It’s the tracking that makes all the difference. As you’re aware, the blue marble we can home is spinning on its axis, and at a good clip I might add.
This is great news for landscape photographers that are interested shooting quick exposures of the dramatic changing light of a sunrise or sunset. But for those of us that are up at 2 am trying to photograph the Andromeda Galaxy, this apparent movement of the sky is a real challenge. (It’s not moving, we are!)
A Sky-Watcher EQ6-R Pro Computerized Equatorial Telescope Mount.
We need to freeze these objects in their tracks, because our cameras need time to soak in hours of light onto the sensor. Think about that, we have actually chosen to photograph objects that are dim, require precise focus, and are moving.
Luckily, this obstacle is easily surmountable thanks to the ingenious design of the German equatorial telescope mount (often called a “GEM”).
As long as the mount is aligned with the polar axis of the Earth on which it rotates, the optical instrument riding on top can accurately “track” the moving sky. This movement is slow enough to only realize after an extended period of time, and is called “sidereal” rate in the telescope mount world.
The act of aligning the axis of the telescope mount with the North Celestial Pole (NCP) here in the northern hemisphere is called polar alignment. There are many useful tools, both hardware and software, to help you accomplish a quick and easy polar alignment.
With an accurate polar alignment from your latitude, a well balanced payload, and a telescope mount that can slew to any object available in the sky above, you got yourself the key to heavens door.
In the northern hemisphere, we are lucky enough to have a pole star, Polaris, to help us identify the position of the NCP. In the southern hemisphere, you’ll need to use Sigma Octans to guide you, as there are no bright stars close to the south celestial pole.
Devices such as the QHY PoleMaster (shown below) use a small camera and dedicated software to aid in the polar alignment process.
A QHY PoleMaster electronic polar scope for accurate polar alignment.
Alignment accuracy is an important aspect of this system, as objects are often so dim that you cannot see or find them visually. It’s not until you’ve taken a long exposure image that you actually start to reveal the DSO – and even then it may be very faint.
For this reason, a rock-solid star alignment routine is critical. This is the process of calibrating your telescope mount so it knows exactly where everything is.
Typically this is done by selecting and confirming a number of alignment stars, or taking pictures of a star field and using software to calculate where its pointed. Again, the higher the magnification of your lens or telescope, the more critical pointing accuracy becomes.
Let’s say your equatorial telescope mount is perfectly polar aligned, and tracking your target smoothly, now what?
Using a Camera Lens vs. a Telescope
If you’re using a camera lens, the way it’s attached to the DSLR camera body is a non-issue. Camera lenses lock on to the body at the right distance to achieve perfect focus, and allow you to control the f-stop of the lens. The trickiest part about using a camera lens for deep sky astrophotography is often how you actually mount it to the tracker.
If the lens isn’t too heavy, you can simply mount the camera and lens to a ball head, without having too much of an issue balancing the rig. However, as the lenses get heavier, finding a even balance in the payload becomes tricky.
Lightweight mid-range telephoto lenses like the Rokinon 135mm F/2 are a great way to get your feet wet.
But what about a refractor telescope with a focuser draw tube, a diagonal, and an eyepiece at the end? To connect a DSLR camera, you need a t-ring (one made for your specific camera) and a t-adapter.
This is called prime-focus astrophotography, and it uses the native focal length of the telescope. There are no additional eyepieces to magnify the image between the camera body and telescope. Whatever the fixed magnification of your telescope is, that’s what you get.
A DSLR camera attached to a small telescope with a 250mm focal length (RedCat 51).
In the case of my Sky-Watcher Esprit 100, that’s a rather wide 550mm focal length, which is considered to be a wide field of view in the world of deep sky astrophotography.
In the refractor realm, it’s often best to invest in a specialized adapter that flattens the imaging field and/or reduce the focal length of the telescope. These are called field flatteners or “field correctors” and can be very specific to a particular optical design.
A typical DSLR astrophotography setup for deep sky imaging will have the DSLR camera body attached to the field flattener (or flattener/reducer) via the t-ring, and the barrel of the flattener fastened to the telescope focus tube.
It’s important that the spacing between the camera sensor inside of the body and the optical element of the flattener/reducer to is correct. With the camera attached appropriately, you can then slew to your target of choice and begin running an imaging sequence.
Attaching the camera to a telescope using a T-ring and a 2″ field flattener.
Automation and Running a Sequence of Images
Because you’ll want to capture as many images as possible, it doesn’t make sense to stand there next to your telescope activating the shutter one by one. Depending on the location of your target, you may spend upwards of 6 hours or more collecting images on the same target.
To automate this process, and remove the possibility of creating any shake or unwanted movement in the camera or telescope, you can use a remote shutter release cable.
These simple and affordable devices allow you to set a series of exposures to fire off on their own. For example, you can program the remote to take 50 shots, with a 10-second delay between each one.
The manual camera settings you’ve input will remain, and the remote gets to work by flipping the mirror up and recording the data in 50 separate instances. When the sequence is finished, you can review your bounty and remove any frames that suffered from periodic error of the mount, airplane trails, or someone kicking your tripod with their foot on their way to the snack bar.
A DSLR camera and telescope tracking an object in the night sky.
Like all forms of photography, achieving a sharp focus on your subject is extremely important. You may experience the moment every amateur astrophotographers faces at one point early on where they review their nights work up close on the computer, and realize they were more than a hair off throughout their entire session.
Stars are quite unforgiving when not in focus, and tend to grow the farther out you are. Luckily, the bright ones are present a great point of reference to use when setting up.
The brightest named stars will shine through to the live view display on your DSLR camera. Modern DSLR cameras allow you to zoom by 10X on a star with a live video display. This allows you to focus on it in real time, making subtle adjustments and taking test images if necessary.
Once your happy with the sharpness of the star and it’s as tight as possible, you can position the camera and telescope on your target with the pre-focused settings applied.
As the temperature drops overnight, you may need to refocus again. Reviewing your images by scrolling (while zoomed-in) is a great way to review a gradual loss of focus right on the camera display. The good news is, you won’t be far off, and only a slight tweak is needed to sharpen those stars to a point again.
Beginners often start with a small star tracker for wide angle astrophotography images. Entry-level camera mounts such as the iOptron SkyTracker Pro are a great place to start. This opens the doors to long exposure photos that include deep sky objects such as galaxies and nebulae.
Above all else – a star tracker or equatorial telescope mount is the single most important investment you’ll make in this hobby. It gives you the power to let the exposure time do the heavy lifting. It opens up new options in terms of the camera settings you’re able to use and the focal lengths that you shoot at.
A Camera and wide-angle lens mounted to the iOptron SkyTracker Pro.
Light pollution filters can make a huge impact on your individual sub-exposures. A broadband light pollution filter such as the Optolong L-Pro helps collect longer images without blowing out the histogram.
Any sort of filtering will disrupt the natural balance of the of colors in your image (particularly, the stars), but I find a mild filter like the one shown below to be extremely handy from the city.
How do you know what exposure length to use? Early on, the answer to this is likely as long as your telescope mount or tracker will let you. For example, it’s better to shoot for 2-minutes at ISO 400 than to shoot for 30-seconds at ISO 6400.
The ISO 400 image will have a better signal to noise ratio, because you put the exposure time to work, not the sensors amplification of the electronic signal (which produces more noise). Use a fast F/2.8 lens, and you’ve got even more attractive options to choose from. Now you could roughly pull in the same amount of signal you would in a 4-minute exposure at F/4.
Can you see how combining a tracking mount with a fast lens or telescope is advantageous?
As for the “best” exposure possible, look to the histogram. This is a visual representation of your data and a useful point of reference. Aim to have the mountain of data in the middle or slightly to the right of the histogram.
The key is to avoid clipping data on either end as this indicates unrecoverable details in either the shadows or the highlights of your image.
This data will be stretched extensively after the fact anyway, so the important thing is that you can see that the individual image data fits safely in this range.
You’ll need to be realistic about the types of images you can expect to collect from your location. The current moon phase, the amount of light pollution, your elevation, and any obstructions will dictate the target you choose.
The Lagoon Nebula in Sagittarius. 17 x 5 minutes @ ISO 1600.
Practical Tips You can Try Tonight
Remember to consider your location and time of year when selecting targets. For example, what altitude will your subject reach, and when? The target you select from a dark sky site will likely be very different than one that you would choose from a light polluted backyard.
If you’re lucky enough to be somewhere dark (Bortle Scale Class 4 or below) take advantage of your surroundings by shooting something in broadband “true” color. That means collecting precious broadband color data that just isn’t possible from my backyard. I’ll never shoot using narrowband filters from somewhere like Cherry Springs, because I can do that almost any clear night at home.
There is no substitute for dark skies, and never is this more evident then when attempting to collect full color images using a DSLR. The goal is capture natural and vibrant star colors in these areas while you’re there, as this is nearly impossible to achieve from the city.
Because DSLR astrophotography can be done without a laptop computer, and all of the extra accessories and devices that accompany a cooled astronomy camera, it’s often more enjoyable.
With a portable tracking mount like the like SkyTracker Pro, you can mount your DSLR camera and lens to frame up massive deep sky targets such as Rho Ophiuchi.
A small remote shutter release cable is all you need to collect incredible data that can be stacked and processed into an unforgettable image. No need to plug into anything, just take the entire rig and go.
What a setup like this lacks in complexity (and perhaps proficiency), it makes up for in freedom, creativity and the pure enjoyment of astrophotography without all of the headaches.
This is why I will always shoot deep sky with a DSLR camera, and why I believe it’s the perfect entry point into the incredible addicti – I mean hobby, we call astrophotography.