An Introduction to Deep-Sky Astrophotography
There is nothing like taking your first deep-sky astrophotography image. You’re taking pictures of objects that lie thousands or millions of light-years away. Galaxies, nebulae, and star-forming regions are out there, and there’s an endless supply of new targets to capture.
The best part is that you can see way more in an image than you ever could with your eye. Even if visual astronomy is your thing, how can you not enjoy seeing the object in incredible detail, especially when you’ve captured it yourself?
When you photograph a deep-sky object, you connect to it in a strange new way. You’ll remember what it felt like to see the first image appear on the screen and the intricate details of the night you captured it.
For this reason, everyone should try deep-sky astrophotography, and it’s not as complicated as you think.
How I Got Started in Astrophotography
In the summer of 2010, I bought the biggest telescope I could afford: a very small Dobsonian telescope, the Orion SkyQuest XT4.5. I placed my compact digital camera up to the eyepiece, pointed it at one of the brightest deep-sky objects in the night sky, and pressed the shutter button.
In a 15-second exposure, I could see a faint purple glow from the Orion Nebula, and I knew I was hooked. It was out of focus, noisy, and a whopping 2MP in size, and I was thrilled.
This type of telescope is known as a Dobsonian reflector, a visual telescope that uses mirrors to reflect an image into the eye. It is the most affordable telescope type in terms of dollar-per-aperture, which means you get the most ‘bang for your buck’, at least in terms of visual performance.
I should note that this is not the type of telescope you want to get into astrophotography. While it performed admirably from a visual observing perspective, it was not meant for attaching a camera or for long-exposure astrophotography.
There was no computer control or tracking; I moved the telescope around and looked through it. That’s it. This is how I learned the basics of the night sky and fell in love with being outside at night.
That first summer with that Orion SkyQuest XT4.5, I saw Jupiter, Saturn, and the Andromeda Galaxy for the first time. While I love visual astronomy, it’s a different experience than astrophotography.
Visual astronomy is all about the ‘in-the-moment’ experience, while astrophotography is about documenting and revealing the object in incredible detail. It lets you share the experience in a wonderful way.
Capturing Deep-Sky Objects
Capturing a deep-sky object forces you to invest some serious time in your project. As you frame the object, it becomes more ‘real’ in the sky — like it was waiting for you to find it.
When you collect your images, revealing more and more detail, you can’t help but become even more fascinated by what you’re looking at. What starts as a faint smudge turns into clearly defined spiral arms of a galaxy as the camera collects more light.
There are so many incredible targets in the night sky to photograph, from sprawling hydrogen emission nebulae that cover several degrees of the sky to small galaxies that need higher magnification and hours of exposure time to be revealed.
Of course, the capturing stage is just one part of the process. This is the ‘outside’ portion of the astrophotography experience, and I enjoy this stage as much as the image processing (inside) stage.
To plan my dark sky trips, I look at the light pollution map to find a rental property with a low number on the Bortle Scale (Bortle 4 and under is ideal).
Choosing Your Equipment
Now comes the fun part. Choose your equipment and build a deep-sky astrophotography rig to capture the types of photos you’re most interested in.
When selecting a new camera, telescope, and/or mount, it’s important to consider your primary interests. For example, the type of setup needed to photograph planets differs from one capable of large nebulae projects.
The setups shown above share one key feature: their ability to track the night sky. This allows astrophotographers to freeze galaxies and nebulae in their place and take incredibly detailed portraits of a sky that ‘moves.’
Accurate tracking is essential for long-exposure deep-sky astrophotography. Before I dig into each specific piece of gear I use for deep-sky astrophotography, have a look at my current astrophotography equipment.
DSLR/Mirrorless Cameras
If you’re serious about astrophotography, you’ll need a capable camera. While it’s possible to take some pretty cool shots with a smartphone, you’ll want to get a DSLR or mirrorless camera (I’m just going to call both of them DSLRs).
A DSLR is an interchangeable lens camera body that allows you to control every camera setting (e.g., aperture, ISO, and exposure). These settings can affect the amount of light your camera sensor collects, which is really important for astrophotography.
Many automatic camera features were designed for regular daytime photography, so tapping into the custom settings can help you take better pictures at night.
For example, you can set the camera to take a 30-second exposure of a starry sky using a low f-stop and record more starlight than your eyes can see.
I started with an entry-level Canon Rebel DSLR and a used wide-angle lens. This camera made me fall in love with collecting light in the dark. Once I realized how much was up there, I couldn’t stop.
You can use your DSLR camera with lenses or a telescope. Lenses are great for creative shots like nightscapes or wide areas of the night sky, while telescopes are best when you want to pull those targets in for a closer look.
Attaching a camera to a telescope is simple. You will need a T-ring adapter to thread the camera onto the telescope focuser. Then, you must determine the correct backspacing between the telescope and the camera.
Dedicated Astronomy Cameras
While a DSLR is a great all-purpose astrophotography camera, many people eventually make the leap to a dedicated astronomy camera. These cameras look like metal cylinders with vents on the side.
Inside, you’ll find a CMOS camera sensor like the ones used in a DSLR, except this sensor is modified for astro-imaging and cooled. Cooling is important for taking long exposure images with less noise, and the modification to the sensor allows for recording more of the reds (i.e., Hydrogen Alpha or Ha) found in many nebulae.
The catch is that they are a little trickier to control for a beginner. Unlike a DSLR, there is nothing to actually control the camera on the body itself. They require dedicated software and hardware to run. You’ll need to run an application on your laptop computer, mini PC, or Wi-Fi controller via your tablet or smartphone.
These tools give you a lot more control, but they also add complexity to your overall setup. I recommend starting with DSLRs before stepping up to an astronomy camera.
You can also get an astro-modified version of a DSLR camera. Several third-party services will do this for you but don’t get hung up on modification.
You do not need an Astronomy-Modified DSLR or dedicated astronomy camera to enjoy astrophotography, but when you feel like you need one, you’ll know. Specific projects, especially those involving emission nebulae (like the California Nebula), can benefit from it.
Choosing a Telescope for Astrophotography
The most common question I get asked is, ‘What telescope should I buy?’ I’ve had the same answer for over a decade — a wide-field, apochromatic refractor.
Some early telescope challenges are focusing, finding targets, and understanding which projects fit your optics well. Certain telescope types can make this process harder than it should be due to their design and will significantly impact your progress early on.
You’ll want to consider these specs:
Aperture: the diameter of the telescope lens/mirror. A bigger aperture generally means a brighter, deeper view.
Focal Ratio: the ‘light gathering ability’. The lower the number, the better. An F/4 telescope is considered ‘fast,’ and F/10 is considered slow.
Focal Length: the magnification of the ‘scope. 250mm is considered a ‘wide’ or a ‘short’ focal length. 1000mm+ is considered a ‘narrow’ or ‘long’ focal length.
A common misconception is that a large telescope with a high magnification is needed to take great astrophotos. But a little APO is one of the best options for deep-sky imaging.
For example:
- A SCT (Schmidt-Cassegrain Telescope) is a popular choice, but this ‘slow’ telescope makes finding your target and focusing your camera more difficult. Add an ultra-long focal length (2000mm), and you’re ‘lost in space’.
- A Newtonian Reflector is a good choice, but it can be tricky to balance and align optically and is substantially larger than a compact refractor.
- An Apochromatic Refractor (a triplet or Petzval lens design) is best suited for color correction, an essential trait for astro-imaging. There are lots of great options in this category, from 50mm in diameter up to 185.
One of my favorite beginner telescopes is the William Optics RedCat 51 (aperture of 51mm, focal length of 250mm, and a focal ratio of F/4.9).
Telescope Mounts for Astrophotography
The most important piece of equipment is your astrophotography mount. Without a reliable tracking platform, you will not be able to enjoy astrophotography.
Because the sky appears to move throughout the night from our vantage point on Earth, we need to offset this motion to take a clear picture. Even a short exposure of 5 seconds will show star trailing. As the focal length increases, this time gets even shorter.
To avoid rotation in the image, astrophotographers use an equatorial telescope mount designed to align its center axis with the celestial pole and spin at the same speed as Earth. This is a simple process, but getting the mount polar aligned and tracking smoothly is a common challenge for beginners.
The mount’s maximum payload capacity, which is how much gear it can hold, is important. It can be anywhere from 11 lbs to 150 lbs.
- Star Trackers: smallest, most travel-friendly portable tracking mounts.
- Observatory-Grade: can weigh up to 100 lbs and are not designed to be set up and torn down on a whim.
- Strain Wave/Harmonic drive: Unlike traditional equatorial mounts, strain wave/harmonic drives don’t require perfect balance to operate efficiently. Small- and medium-sized loads also don’t need a counterweight, which is a huge bonus.
Modern mounts have built-in computer databases that can find, point to, and track objects. They can be controlled from a hand controller that plugs into the mount or from your laptop or smartphone.
The ZWO AM5 (and smaller AM3) strain wave mounts are extremely popular because they can hold a full-size setup yet are the size of a compact star tracker.
Astrophotography Accessories
While the camera, telescope, and mount are the three most important elements of a deep-sky astrophotography setup, some key accessories are also needed to operate the equipment:
- Camera Control Software / Computer
- Light Pollution Filters / Multi-Bandpass Filters
- Guide Scope and Guide Camera for Autoguiding
- Adapters, Focal Reducers, Field Flatteners
- Power Banks and Portable Power Supply
Using a Laptop Computer for Camera Control
While taking pictures of a deep-sky object by pressing the shutter button on your DSLR for each shot is possible, you will quickly realize that standing next to your camera all night isn’t enjoyable.
A better option is to use a device that automatically fires the pictures off in a sequence. Many tools, from a remote shutter release cable or intervalometer to computer software with advanced controls, can help with this.
You can connect your camera and mount to a laptop computer to run whichever software you want. Some popular camera control software tools for deep-sky imaging include:
All of these tools have advanced camera control settings, from adjusting the temperature of the sensor to leveraging star catalogs to help your telescope find objects.
You will need to make sure that all of the necessary software is installed on the machine to run your equipment. This includes everything from the camera control software to the latest camera and mount drivers.
All of your gear and accessories must be plugged into the computer’s USB ports. You’ll want to keep that as organized as possible to avoid anything catching throughout the night.
The ZWO ASIAIR WiFi Controller
The ASIAIR has made my life so much easier, and I feel compelled to tell you how great it is. Since first using the ASIAIR Pro in 2020, I have not used my laptop computer for a night of astrophotography.
It’s a mini Wi-Fi camera controller that is accessed through a dedicated app on your smartphone. The tiny device is mounted to your telescope so all cables can run neatly into it. The app does everything from pointing your telescope to running detailed imaging plans involving multiple targets.
It’s not fair how easy this device has made the entire astrophotography experience. For a complete breakdown of my ASIAIR setup for deep-sky astrophotography, be sure to read My Latest Portable Astrophotography Build.
The one downside of the ASIAIR is that it was developed by a company called ZWO and is only compatible with their ASI-dedicated astronomy cameras and most DSLRs. Fortunately, ZWO makes the most widely used and reliable astronomy cameras on the market.
Astrophotography Filters
Filters are another crucial part of astrophotography; learning which ones are best for specific scenarios is essential.
They come in varying levels of light pollution filtration, from mild (broadband) light pollution filters to strong (narrowband) filters that only allow a very narrow wavelength of light to pass through to the sensor. Both have their place, but filters are generally necessary in the city and are often not used at all in a dark sky location.
While light pollution filters can help reduce a lot of the unwanted glow from the city in your images, they can also dull out the subtle, natural colors of the stars and potentially your object.
From a light-polluted area, I recommend a dual-band filter like the Optolong L-eXtreme for emission nebulae and supernova remnants. For broadband targets like star clusters, galaxies, and reflection nebulae — you’ll want to use something milder like the Optolong L-Pro.
Astrophotographers usually list the filters used for a particular project, which is one of the best ways to decide which one to use for your intended target.
Guide Scope and Guide Camera
A guide scope and guide camera are used for autoguiding. This is a process that uses a small secondary camera and guide scope (a smaller telescope) to help improve the tracking accuracy by sending small corrections to your telescope mount.
The guide scope provides an adequate field of (in-focus) stars so the autoguiding camera can find an appropriate candidate for calibration and guiding. The cameras used for autoguiding have a much smaller sensor than you’d find in your primary imaging camera.
Whether you’re shooting with a DSLR or a dedicated astronomy camera, capturing longer exposures means that more light (or signal) can be recorded in a single shot.
Dew Heater Bands to Prevent Moisture and Frost
Dew heater bands are wrapped around your telescope and guide scope to keep them from moisture throughout the night. They can be the difference between capturing hours of exposure time on a target and coming up empty-handed.
Dew prevention is an essential precaution for long-exposure imaging sessions. Dew heaters are a cheap and reliable option that can be powered as part of your overall setup.
Research and Planning
As with most things, a little planning and preparation can go a long way. You’ll have plenty of cloudy nights to research new equipment, decide on your next project, and look for, for example, images of the object you have in mind.
Stellarium is one of my favorite tools for planning an astrophotography session. You can import a custom landscape (such as your backyard) to see where a particular deep-sky object will be in the sky from your vantage point.
You can also see important things like the moon’s location and phase, meteor showers, ISS passes, and much more.
Every astrophotography project starts in Stellarium, and I highly recommend the desktop and mobile versions.
You can increase your chances of success by choosing an appropriate target for your setup. It should be an object that fits your telescope in terms of size and resolution, and that can be imaged well from your imaging location.
Certain targets will need dark skies and a moonless night to do justice. Galaxies, reflection nebulae, and dark nebulae are in the category. Other targets, like emission nebulae, can be captured from almost anywhere, even during a full moon with the right filter.
The Importance of Overall Exposure Time
All of your deep-sky projects will involve taking many images of your target.
For example, a typical deep-sky project will include 3 hours or more of total exposure time. Rather than a single 3-hour exposure (which would not be pretty), the image is built up over time using shorter exposures, for example, 60 exposures at 3 minutes.
The image stacking process is a key part of astrophotography and how we capture images with incredible detail. Not only does the stacking process increase the amount of overall light collected, but it also reduces the things we don’t want — like noise. This is known as the ‘Signal-to-noise Ratio.’
The difference between a single 3-minute exposure and a stack of 60 exposures is dramatic. For those willing to put in the time, it is not uncommon for a project to reach 20 hours of total exposure time.
In general, the more time you spend on the image, the better. It will be more enjoyable to process, and your results will be better, too.
Reviewing Your Images and Next Steps
The ‘art’ of processing an astrophoto is a lengthy process. You may enjoy this element of astrophotography even more than the image capture stages. Running your camera and telescope is the outside portion, and image processing is the inside portion.
Because you don’t get a good look at the images you take outside in the moment, it can be exciting to review your pictures on the computer finally. You can simplify the image processing side into two stages: image stacking and image processing.
Image storage is also something to consider. With single image (RAW) files ranging from 50MB to 100MB, a hard drive will fill up very quickly after a few imaging sessions.
I advise organizing your image data by date, target, and equipment used. This is especially important for long-term projects combining image data over multiple nights.
Image Stacking
Image stacking is one of the key concepts of astrophotography. Whether a wide-angle picture of the Milky Way or a close-up of the Andromeda Galaxy, image stacking reduces camera noise and increases signal (or light).
In short, take as many images of your subject as you can. Then, an image stacking tool will be used to combine them. All astrophotos start out with humble beginnings, but by ‘stretching’ the data in the image processing stage, you can see what’s really in there. How far you want to take, it is up to you.
When you feel like you have collected enough photons, you get to review the pictures you’ve taken and decide which ones will be included in your master image. A combination (or ‘stack’) of all the best image frames into one glorious master file.
The Importance of Calibration Frames
Calibration frames are additional frames taken to help reduce noise, improve the signal-to-noise ratio, and remove gradients and dust spots.
- Light frames: The individual images you take of your object. More detail can be revealed during processing the more light frames you capture. Shoot a minimum of 2–3 hours worth of exposure.
- Flat Frames: Removes harsh gradients and dust spots. With the camera connected to the scope/lens, shoot at the same ISO/Gain as your light frames. Capture a minimum of 15–20 frames.
- Dark Frames: Improves the signal-to-noise ratio. Shoot at the same exposure lengths with the same ambient temperature as your light frames with the lens cap on. Aim for 15–20 dark frames.
- Bias Frames: Reduces fixed-pattern noise. With the lens cap on, turn the mode to Manual and set the exposure length to Bulb Mode. Shoot 15–20 frames.
The image stacking and calibration of the images are all done simultaneously, using one of the many excellent (often free) image stacking tools like DeepSkyStacker or Siril. While it may seem like a lot of extra work, these steps can make or break your final image. After a bit of practice, these steps will become second nature.
Image Processing
It’s usually a shock for beginners to learn how much stretching is done to a deep-sky astrophoto. The unstretched ‘linear’ data is very dim and looks nothing like the images of galaxies and nebulae you see online.
Some objects pack a punch ‘straight out of the camera’ simply because they are so bright. Most other objects, though, will need their brightness brought WAY up to be thoroughly enjoyed. A healthy amount of saturation (the intensity of color) and sharpening are also key steps to an image-processing routine.
There are more tools for image processing than ever. Most astrophotographers will switch between 1 or 2 tools depending on what they want to do to the image.
Some are powerful but difficult to learn, others are user-friendly but limited in astrophotography-specific functions, and some are free, while others require a subscription.
- Adobe Photoshop: If you have dabbled in photography or graphic design, you have likely already encountered Photoshop. It’s a fantastic tool for deep-sky astrophotography, though it requires a paid subscription.
- PixInsight: if you’re ready to invest some serious time into learning the most powerful astro-processing tool on the market, this is your best bet. It’s not known to be user-friendly and requires a one-time purchase.
- Siril: This one is a refreshing balance between Adobe Photoshop and PixInsight that is completely free.
No matter which one you choose, YouTube is your best friend. During any stage of the process, you will find helpful tutorial videos to walk you through it.
While written material is nice, it can quickly become outdated as the software changes. Some of my favorite astrophotography image processing tutorials are by Lukomatico and Paulyman Astro.
Conclusion
The nature of astrophotography means you’ll be ‘chasing’ moving objects in the sky before they’re gone. When getting started, there is a sense of urgency to get everything working quickly.
A surefire way to get frustrated with astrophotography is to expect everything to work on the first try. Expect to fail for a while, and celebrate the small victories of learning ‘what NOT to do’.
For the control freak, triple-checker, and did your homework for months crowd, this hobby WILL HUMBLE YOU. And it’s the challenge that makes it so special because if it were easy, everyone would do it.
But us? We’re different, and I think that’s worth celebrating. It’s worth going on these long trips to dark sky spots and feeling connected to someone you just met. Because, at the end of the day, we’re all just a bunch of nerds geeking out about space.
