Mastering astrophotography with your Dobsonian telescope

Many believe that deep-sky astrophotography with a Goto Dobsonian is either impossible or too difficult to achieve good results. But this belief is being steadily disproven by a dedicated group of astrophotographers who are pushing the boundaries of what a Dob can do. If you learn to capture deep-sky targets with your Dobsonian, you’re not just taking great photos - you’re proving to the world that a Goto Dobsonian can indeed be a formidable imaging rig. And here’s the best part: it might not be as hard as you think.

This guide is specifically designed for owners of Goto Dobs, but the advice on Alt/Az astrophotography can also apply to other Alt/Az computerized telescopes, including Goto SCTs and some entry-level Alt/Az Goto scopes from brands like Celestron, Sky-Watcher, and Orion. These scopes often share similar controllers from Synta, making the techniques applicable across a range of equipment.

One of the biggest challenges in starting with a Dobsonian for astrophotography is that most deep-sky imaging guides are tailored to equatorial mounts, often paired with a small refractor or camera/lens setup. These setups are popular for good reasons - they’re reliable, well-documented, have a large user community, and widely recommended for beginners. But if you already own a Goto Dobsonian and you’re proficient in using it, you might already have a great platform for astrophotography. If you know your Dob well, you’re likely halfway to capturing stunning images of the night sky.

Mastering astrophotography with Your Dobsonian telescope with Steven Miller

This guide is not intended to serve as a comprehensive introduction to astrophotography. For that, there are numerous resources available, including websites, videos, books, and forums. Some books that I’ve found particularly helpful in my journey include *The Deep Sky Imaging Primer* by Charles Bracken, *Beginner’s Guide to Astrophotography* by Jerry Lodriguss, and *The Astrophotography Manual* by Chris Woodhouse. These resources provide a solid foundation, though they may not specifically address imaging with a Dobsonian. That’s why I decided to create this guide - to help fill the gaps and share insights specific to using a Goto Dobsonian for deep-sky astrophotography.

First astrophotos using planetary cameras

Top 5 things you'll need for Dobsonian astrophotography

The key to successful deep-sky imaging with a Goto Dobsonian lies in five essential steps, made possible by recent technological advances:

1. Use a modern CMOS camera with low read noise, operating at high gain or ISO.
2. Capture very short exposures of 4-10 seconds.
3. For longer sessions, automate the telescope with NINA software to keep the target centered.
4. Process the images on a fast computer with ample storage, letting stacking software work for hours if needed.
5. Utilize fast stacking software like SIRIL to manage and process hundreds to thousands of exposures efficiently.

The rest of the guide delves into the details, covering equipment choices, challenges, and optimization strategies.

What has made Deep Sky imaging on a Goto Dobsonian possible?

Deep sky imaging on a Goto Dobsonian has become possible due to significant advancements in technology over the past decade. Previously, successful deep sky astrophotography required high-quality motorized equatorial mounts, long exposure times, and complex guiding systems to avoid issues like field rotation and star trails. However, modern low-read noise CMOS sensors now allow for much shorter exposures, which mitigate many of the challenges associated with Alt/Az mounts, such as unguided tracking issues and sensitivity to environmental factors like wind. Additionally, faster stacking software, such as SIRIL and PixInsight, along with improved control software such as NINA, has made it feasible to automate tracking and stack numerous short exposures efficiently, even with the inherent limitations of a Dobsonian mount.

These advancements have also made astrophotography more accessible for beginners using Goto Dobsonians. With the combination of large apertures, short exposure times, and advanced software, it is now possible to capture satisfactory deep sky images from suburban locations with less optimal conditions. Newer image processing tools also allow for the effective removal of light pollution and the use of dual-band filters, making high-quality astrophotography in light polluted skies achievable without the need for expensive narrowband filters in filter wheels to be used with mono cameras. While there are still challenges, such as handling large data volumes and mitigating field rotation, these technological improvements have transformed Goto Dobsonians into viable options for deep sky imaging, particularly for those looking to capture small to medium-sized targets.

How short can my exposures be before Camera Read Noise becomes a big factor?

For two decades, astrophotography relied on long exposure times to mitigate camera read noise, which was a significant issue with early CCD cameras. The need to reduce read noise drove the development of precise, expensive equipment, such as large equatorial mounts and separate guiding systems, to support these lengthy exposures. With exposure times often ranging from 10 to 20 minutes, the goal was to ensure that the signal from the target was strong enough to overshadow the fixed noise introduced by the camera each time an image was read.

Recent advancements in camera technology, particularly with modern CMOS sensors, have dramatically reduced read noise, allowing for much shorter exposure times. According to Dr. Robin Glover's research, with the proper camera settings, exposure times can now be reduced to just a few seconds without compromising image quality. This shift means that even in challenging conditions, such as suburban light pollution, high-quality images can be captured with much shorter exposures, making astrophotography more accessible and less dependent on expensive, specialized equipment.

Does this make the Goto Dob the new best place to start out with astrophotography?

For many, a small widefield refractor or camera/lens on a small equatorial mount will be a better option due to their portability, wide level of support, and some aspects being easier to use. While recent advancements in cameras, accessories, and software have made astrophotography possible on Goto Dobsonians, making them viable for beginners, the challenges associated with larger scopes and long focal lengths still make them more difficult to use for some. However, if you're considering a Goto Dobsonian for its other benefits and have an interest in astrophotography, it can certainly handle basic planetary, lunar, and deep sky imaging reasonably well.

What new problems or downsides will I experience using my Dob?

Using a Goto Dobsonian for deep sky imaging comes with several challenges that require careful consideration and mitigation. One of the biggest issues is the sheer volume of data generated by short exposures, leading to long stacking times and significant demands on computer resources. This can be addressed by using a powerful computer with ample RAM, high-performance SSDs, and by optimizing camera settings to avoid unnecessarily high resolutions that add to the data load.

Another challenge is field rotation, which can limit exposure lengths and complicate the stacking process by producing larger final images. To mitigate this, it's essential to choose the right part of the sky to minimize rotation, crop images to reduce the impact, and avoid shooting at higher resolutions than necessary. Additionally, using a Dobsonian telescope, which is a Newtonian design, introduces optical defects like coma, particularly at the edges of the frame. This can be corrected with a coma corrector or by using a smaller sensor camera to avoid the worst affected areas.

Dobsonian telescopes also have limitations in camera and accessory choices due to their design, particularly with limited backfocus, making it harder to attach certain cameras or accessories. The narrow field of view inherent to larger Dobs can also be a drawback, making it more difficult to capture wide scenes, though this can be partially mitigated by using focal reducers, larger sensors, or shooting mosaics for larger targets.

Finally, despite their large apertures, Dobs may not produce as sharp overall images as smaller telescopes due to the effects of atmospheric seeing, imperfect tracking, and optical imperfections. While Dobs excel at resolving details in small targets, they often produce softer images when viewed as a whole. Moreover, the size and weight of Dobs make them less portable and can introduce practical challenges in managing cables and equipment during imaging sessions. Despite these challenges, with the right approach and tools, Goto Dobs can still be effective for deep sky imaging.

Is bigger better? Are there any advantages to imaging with a big Alt/Az Dob?

Yes, bigger can be better when it comes to imaging with a large Alt/Az Dobsonian, as these telescopes offer several advantages. Goto Dobsonians are user-friendly, with straightforward setup and alignment, making them accessible for beginners. They are versatile, providing bright and detailed views of both deep sky objects and planets, and are particularly well-suited for "lucky imaging" of planets and the moon. Their large apertures make them effective for quickly capturing images of small to medium-sized deep sky targets, thanks to their fast f-ratios. With the addition of a focal reducer, a Dobsonian can achieve even faster imaging speeds, making it a productive choice for astrophotography.

However, while large Dobs excel at capturing smaller targets quickly, they have a narrower field of view, which can make imaging large targets more challenging. A large sensor camera can help maximize the light captured, but for particularly large objects like the Andromeda Galaxy or the Rosette Nebula, mosaics may be necessary. Even with these challenges, a large Dobsonian can still be highly effective, especially on nights with good seeing conditions, where its large aperture allows for more detailed images of small deep sky targets. Overall, while not without its trade-offs, a big Alt/Az Dobsonian offers significant advantages for deep sky imaging, particularly for those focused on smaller targets.

Entry into astrophotography on a Goto Dobsonian

Starting astrophotography with a Goto Dobsonian depends on various factors, including your prior experience with astrophotography, interest in both planetary and deep sky imaging, proficiency with your telescope, and the darkness of your observing location. While this guide focuses on Goto Dobs, much of the advice may apply to other Alt/Az Goto systems, particularly those compatible with NINA. Given the diversity of computerized telescopes on the market, it's challenging to provide specific recommendations for each setup. Instead, this document outlines the author's journey and key elements to success, while also directing readers to external resources for more comprehensive guidance.

Goto Dob operation basics

To be successful in astrophotography with a Goto Dobsonian, several key fundamentals are essential, many of which may not be initially apparent. Basic collimation is crucial, and while the telescope comes with a collimation cap, upgrading to a more precise tool like a 2" Cheshire Collimator can significantly improve star quality. Proper alignment is also vital; this includes ensuring accurate date, time, and location settings, leveling the telescope, and carefully centering alignment stars using slow slewing modes to remove mechanical backlash. Enhancing pointing accuracy by using the Pointing Accuracy Enhancement (PAE) feature, particularly for specific sky regions, can improve tracking, though sometimes a simple restart and different alignment stars may be necessary. Additionally, balancing the telescope is important to prevent drift, especially when adding accessories that affect its weight distribution. While there is a wide range of advice on alignment with Synscan Alt/Az mounts, following these steps can help achieve better tracking and imaging results.

Planetary and lunar astrophotography

I'm not going to teach planetary astrophotography as there are many resources available that apply well to an Alt/Az Goto Dobsonian, unlike those for deep sky imaging. However, planetary and lunar astrophotography can be a good starting point for those interested in deep sky imaging because it's generally easier. Planets are easy to locate, don't require dark skies, and tracking accuracy is less critical. You can capture high-quality images in a short time, with simpler software and processing than deep sky work. The downsides are that the best planets aren't always visible, seeing conditions are important, and planetary imaging equipment may differ from what is used for deep sky. Solar imaging is also an option but is more advanced and involves safety concerns, such as the need for a proper solar filter and careful handling to avoid damaging equipment. For those interested in starting with planetary imaging, I recommend watching a presentation by renowned planetary imager Damian Peach.

Planetary camera selection

The primary focus when starting planetary astrophotography should be on camera choice, especially if you're considering a camera that can double as a beginner deep sky camera. If you're not particularly interested in planetary imaging and already have a deep sky camera, there's no need to invest in a planetary-oriented camera. However, if you don't have a dedicated astro camera and want to start with planetary and lunar imaging, it's advisable to choose a planetary camera that can also serve for deep sky imaging. DSLRs are generally not ideal for planetary work due to their slower frame rates, though they can work for lunar imaging with some limitations. The best camera choice depends on various factors, including your telescope, seeing conditions, and your plans for deep sky imaging. While new cameras are frequently released, the recommendation is to consider types that can effectively bridge both planetary and deep sky imaging, keeping in mind that these dual-purpose cameras tend to be more expensive.

Considerations for a planetary camera

When choosing a camera for planetary imaging with a Dobsonian, it's crucial to match the camera's pixel size with your telescope's focal length and available Barlow lenses. Ideally, you want to achieve an imaging scale of 0.1-0.2 arcseconds per pixel, with 0.1 being preferable for very good seeing conditions. This involves calculating the pixel scale using the formula: pixel scale = 206 * pixel size (in microns) / focal length (in mm). For example, a 12” F4.9 Dob with a 2x Barlow and a camera with a 2.9µm pixel size would result in 0.2 arcseconds per pixel, suitable for average seeing. Using a 3x Barlow would improve this to 0.13 arcseconds per pixel for better seeing conditions. Additionally, sensor size plays a significant role in camera choice, affecting both cost and performance. Larger sensors are advantageous for planetary imaging as they make it easier to locate and track planets, adjust focus, and capture wider views, including Jupiter's moons or the Moon and Sun. They also offer better potential for entry-level deep sky imaging. Therefore, while tiny sensor cameras may be cheaper, the trade-offs in functionality make larger sensors a better investment, especially if you plan to use the camera for deep sky imaging as well.

A few planetary camera options with some deep sky considerations

Instead of recommending a specific camera, I discuss various "types" from ZWO that are generally good considerations for planetary imaging, highlighting their trade-offs. For example, the ASI224MC is affordable but has a small sensor, making it challenging to keep planets centered and less versatile for other uses. The ASI678MC offers a slightly larger sensor and higher resolution, making it a better option for planetary imaging with a Dobsonian. The ASI585MC is a good all-around choice, with a larger sensor suitable for both planetary and entry-level deep sky imaging. The ASI183MC, with its higher resolution and larger sensor, is useful for capturing more of the Moon and Sun but has some limitations for deep sky use. The ASI533MC and ASI294MC are more expensive but offer better crossover potential for both planetary and deep sky imaging, with larger sensors and lower read noise, making them versatile options for beginners. Lastly, the ASI071MC Pro is primarily a deep sky camera with some planetary capability but requires serious Barlow lenses due to its large pixel size, making it less ideal for those primarily focused on planetary imaging. The choice ultimately depends on balancing cost, sensor size, and intended use.

Entry into limited deep sky astrophotography with your Goto Dob

When starting with deep sky astrophotography, if you have a DSLR or Mirrorless camera, you can start with that as long as you can come into focus on your Dob.  Mirrorless cameras can almost certainly come into focus, but due to have far the camera sensor is mounted inside of standard DSLRs, many Dobs cannot bring a standard DSLR into focus without some additional accessories or a modification to the telescope.

DSLR vs Dedicated Astro Cam

If you skipped the planetary imaging step or purchased a dedicated planetary camera with a sensor too small for deep sky work, then you need a camera. If you do not have a DSLR that can function you can still consider DSLRs (or Mirrorless cameras) as they offer the advantage of being versatile with the use of camera lenses, but they come with some disadvantages like lower quality in hot weather and the need for modifications to effectively capture the H-alpha light present in many nebula. Dedicated Astrocams, while more expensive, provide better support for astrophotography, with features like cooling, more sensitivity H-alpha light, better integration with software, and ease of handling. 

Deep sky cameras: Bigger is much better

Through my experience with various cameras, I've found that larger sensors are better for deep sky imaging with a Dobsonian. Larger cameras capture more of the sky, making it easier to frame targets and compensate for tracking errors. However, larger sensors also come with challenges like increased cost and greater sensitivity to imaging issues like coma and vignetting. Despite these challenges, the benefits of a larger sensor often outweigh the drawbacks.

Deep sky pixels: Bigger pixels are better while total megapixel resolution isn’t very important

For deep sky imaging with a Dobsonian, I find that larger pixels are preferable to higher megapixel counts. Larger pixels are more forgiving of issues like tracking errors and atmospheric seeing, which are common with long focal lengths. While smaller pixels can provide better resolution, they often don’t offer a significant advantage due to the limitations imposed by atmospheric seeing and the Dobsonian's long focal length.

Using electronically assisted astronomy as a deep sky starting point

Electronically Assisted Astronomy (EAA) serves as a great entry point into deep sky astrophotography. With EAA, I can get immediate feedback by viewing enhanced images in real time on a screen, making it ideal for social gatherings and quick sessions. The process is similar to traditional astrophotography but with less complexity, allowing for a more user-friendly experience and faster learning curve. EAA is particularly useful in light-polluted areas where direct viewing through the eyepiece might not be as effective.

Taking your first true deep sky shot

When I'm ready to take my first true deep sky shot, I focus on preparation. I start with a well-aligned telescope and select a bright, easily recognizable target. I ensure the target is within the camera's field of view and take multiple short exposures to minimize issues like field rotation and tracking errors. After capturing the main images, I also take bias frames for calibration during post-processing. This careful approach helps ensure that the first deep sky shot is successful and satisfying.

Exposure time, what should i start with given my equipment and light pollution?

When determining your exposure time for deep sky imaging with your Dobsonian, I typically start with short exposures, usually between 4-8 seconds, depending on factors like tracking accuracy and light pollution. Longer exposures might cause issues with field rotation and tracking, while shorter exposures might lead to too much data to process and increased camera read noise. For most situations, this range is a good starting point, allowing you to capture quality images without overwhelming your system.

So what target to start with?

For my first deep sky target, I choose something bright and easily visible, like M27 (Dumbbell Nebula), M57 (Ring Nebula), or M13 (Hercules Globular Cluster). These targets are relatively small, fit well within the field of view, and have high surface brightness, making them easier to capture in light-polluted skies. Selecting a target in a star-rich field is also beneficial as it aids in locating and focusing, especially if you’re using a small sensor camera.

Planning your shot with Stellarium

Before beginning an imaging session, I always use Stellarium to plan my shot. This free software helps me determine if the target will fit within my field of view based on my telescope and camera setup. I use it to see when the object will rise and set, ensuring I have enough unobstructed viewing time. By visualizing how the target will appear in the frame, I can better prepare for the imaging session and avoid surprises.

To read the full 150-page guide, filled with detailed insights and practical tips for deep sky imaging with a Goto Dobsonian, click below.

Comments

jroper

This article is focused on astrophotography with GoTo Dobsonians, and that's fine, but I feel that it should at least mention the alternative for Dobsonian astrophotography, which is to use an equatorial platform. These are very cheap and simple to make yourself (a non GoTo dob + platform is much cheaper than a GoTo dob), it's a platform that rotates in a conical fashion, allowing tracking of the sky with no field rotation, which allows much longer exposures, 30+ seconds is not unreasonable. There are also a few companies that manufacture and sell them for significantly cheaper than an equivalent equatorial mount. The industry is young, but people are starting to get very good results, that rival those that can be obtained with an equatorial mount, especially when it comes to using them with much larger homemade dobs. A number of us in the Dobsonian Astrophotography Facebook group that you link to are using this kind of setup. I've attached a photo of my platform in action.

8/21/2024 12:49:50 AM

rharris

That's a great point! Including a mention of equatorial platforms for longer exposures without field rotation is particularly noteworthy, especially given the impressive results that enthusiasts like yourself are achieving with them. Thanks for sharing the photo of your setup as well!

8/21/2024 11:28:31 AM

Smiller

That's a good observation and my full guide and the video both mention the equatorial platform as an option for non-motorized Dobs. It would probably be a good addition to at least mention them here too.

8/22/2024

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