What is it?
The term “Deep Sky Obects” (in short DSOs) refers to all the celestial bodies that are outside our Solar System, such as stars and star clusters, galaxies, and nebulas. DSOs astrophotography is the art of collecting these Lonely Photons and turn them into beautiful, colourful images.
Contrary to planetary imaging, DSOs astrophotography is severely affected by light pollution. Even though light pollution filters are getting more and more effective, nothing can replace a good dark sky!
The Bortle scale is a numeric measurement (1 to 9) of the night sky’s brightness due to the effects of light pollution; using a Bortle map is a useful tool to find the darkest sky in your area.
Most DSOs are seasonal and the best moment to observe or image them is when they are at their highest in the sky, as far as possible from the interference of atmosphere and light pollution.
Planning an observation in advance using a tool like “Stellarium” can help get the best results and avoid a lot of frustration!
What do you need?
You’d be surprised to know that the most important thing in DSO astrophotography is not the telescope! In fact, the main difference between planetary imaging and DSO astrophotography is that the latter heavily depends on an advanced EQ mount, beefy enough to support all your gear while maintaining a high level of precision. Here’s what’s needed for DSO AP:
The mount. DSO astrophotography works best with long exposure photography; these are single frames of long duration (1 to 5 min on average). In this range of time, even the slightest vibration or tracking inaccuracy will result in elongated stars, or even star trails, and blurred images.
A good quality EQ mount, such as one of the Sky-Watcher EQ or NEQ series, then becomes vital to load all the gear while also tracking the objects in the sky with accuracy. It must be remembered, when choosing an EQ mount, that is suggested to use only 60-70% of the mount’s maximum payload capacity.
Autoguiding. Even the best mounts, however, have some flaws and if you aim at perfection autoguiding is then essential. This works with a guide scope mounted on top of the main telescope and a camera, such as the ZWO ASI120MM, attached to it and connected to the mount with a cable.
Using a software called PHD2, the camera locks onto a star in proximity of the main target and communicates with the mount to keep that star, and therefore the main target, precisely in the field of view.
The telescope. Finally, it’s time to choose the telescope. Unfortunately, when it comes to DSOs there is not a one-size-fits-all solution: with the exception of Andromeda, galaxies are quite far away and need telescope with longer focal lengths, such as SCT or large-aperture reflectors. Large refractors work too but are generally quite long, heavy, and expensive.
Want to buy a telescope? Check out this guide: A complete guide to buying your first telescope
Nebulas, on the other hand, are closer to Earth and appear larger, thus requiring a telescope with a wide field of view, which is normally obtained with fast, short focal lengths.
In this case apochromatic refractors dominate the field! There are plenty of options on the market, but as a rule of thumb, refractors with 3 (triplets) or more glass elements provide the sharpest results. William Optics, Sky-Watcher and Explore Scientific are some of the most widely used brands.
The camera. When collecting hours and hours of data on a target using long exposures, the camera can get quite hot and the accumulated heat can affect the frames’ quality, or even damage the camera.
For this reason, many DSOs astrocameras come with a cooling system behind the sensor, which allows to keep a constant temperature throughout the night. Some models have an active cooling, and the temperature can be chosen via the capturing software. Altair Astro, ZWO and QHY produce the most common cooled astrocameras.
DSLR cameras are also widely used for DSO astrophotography, with some advantages such as portability (no need to bring a laptop around!), and their very wide field of view, which is great for capturing very large nebulas. Canon seems to rule the scene here but Nikon is always there fighting.
Computer. Stacking hundreds of DSO frames takes way more effort and time than stacking planetary videos. Software such PixInsight or Astro Pixel Processor can run for several hours before providing you with the stacked image. Therefore, in my opinion, a powerful desktop computer with no less than 16GB RAM, good graphic components, and A LOT of disk space, is a must have.
More gear. For refractors, especially those with two glass elements, a field flattener is necessary to counteract the field curvature of the optical system and improve edge sharpness. Some flatteners are also focal reducers, which make the scope faster and increase the field of view by reducing the scope’s focal length.
Filters can be real game changers in DSOs AP, particularly when shooting from light polluted areas such as cities. Light pollution filters are getting better every day and Optolong, IDAS and Baader, to name some, offer very good and reliable products. When shooting with mono cameras RGB and some narrowband filters lie HA, Sii and Oiii are needed to achieve the best results.
Those who have to take their telescope on a ride to their favourite locations will also need a power tank to power up all the gear. Some also use smart devices such as the ZWO ASIAIR Pro to control remotely with a phone or tablet all the gear, including the mount, the camera and the guide camera, the dew heater and many other electronic devices.
Finally, a Bahtinov mask is necessary to reach spot on focus whereas an electronic auto-focuser will keep the scope in focus as the temperature changes through the night.
An electronic filter wheel is useful when shooting in mono with different filters. Both the electronic focuser and filter wheel can be controlled remotely with the ZWO ASIAIR Pro.
How does it work?
As said, DSOs astrophotography works better with long exposures, which allow to capture even the faintest details of these distant objects. Acquisition software like SharpCap, N.I.N.A. or APT (there are plenty more!) go even further and present some interesting tools that help with polar alignment, calibration frames and more.
It is generally advised to collect several hours of data on each target to reduce the noise generated by the camera and enhance the level of details. Also, sometimes combining different set of data (ie Narrowband with broadband) can lead to interesting results.
Once acquired enough hours of data is time for stacking; there are several programs to do that but, personally, I find that PixInsight and APP do a great job. Although not mandatory, it is recommended to always use calibration frames, such as darks and flats, to subtract camera noise and lens’ imperfections from the final image.
Once stacked, the final frame is a dark, linear image that needs post-processing before it can be turned into a beautiful astrophoto. Here PixInsight has an advantage as it allows to do everything in one software with terrific results, although Photoshop or GIMP are valid alternatives.
Regardless of the software, some of the basic techniques behind the magic include stretching the histogram to bring out colours and details, playing with curves to increase contrast, sharpening and colour balancing. StarNet++ (now included in PixInsight) removes all the stars from a picture, allowing for more aggressive histogram stretching, whereas Topaz DenoiZe does a great job in removing noise and increase sharpness.
When it comes to DSO post-processing, there are so many ways and different techniques to obtain those Hubble-style pictures you see on the web or on dedicated forums.
YouTube in this context is the best school for all the tutorials available. Time, on the other hand, helps everyone find their own style and flow, bearing in mind that astrophotography is an endless learning curve.