How to shoot DSO Astrophotography
There are different trackers on the market. They have an electric motor that makes it possible to shoot for an extended time without getting star trailing in the images. They are intended to be used with DSLR-cameras (or mirror-less digital cameras) with lenses up to 200 or maybe 300 mm focal length.
Although some models are possible to guide, most use them unguided. They are great for wide-field astrophotography as well as starscapes, and you can take impressive images of larger DSOs such as Andromeda Galaxy M31, Orion Nebulae M42, or the Pleiades M45.
The next step is to use a mount. The most common model is called German Equatorial Mount and has two axis – the right ascension (equatorial) and a declination axis. The equatorial axis has a sidereal clock drive, which rotates around its axis every 23 hours and 56 minutes, which corresponds to the rotation of the Earth.
With an equatorial mount you first have to do a polar alignment; the declination axis has to point to the north celestial pole. When the polar alignment is done, the mount could follow a celestial object making it possible to shoot very long exposures with focal lengths ranging from about 400 mm to 2000 mm and even beyond.
In astrophotography, the mount is one of the essential parts.
There are different kinds of telescope designs—reflectors such as Newton and Schmidt-Cassegrain and refractors. I will focus on refractors, which is widely used for wide-field astrophotography.
A refractor is a lens design, not that different from a camera lens, but they are constructed with a focuser for long distances. A typical refractor has a front lens diameter ranging from about 70 mm to 150 mm. The price tends to grove almost exponentially as a function of the front lens diameter, so larger telescopes are costly. For astrophoto of small DSOs, many therefore use reflector telescopes, which are a lot cheaper than large refractors.
It’s possible to use a DSLR or mirrorless camera. However, there are specific Astro-modified cameras without an IR-filter. Nikon D810a and Canon EOS Ra are two such models. It’s also possible to remove the IR-filter in a regular DSLR. I have done it myself in a Canon EOS 60D, a model that is quite easy to modify.
Without the IR-filter, the camera sensor is able to collect a lot more of the wavelengths that emit from many Deep Sky Objects.
There are also dedicated Astrocameras. They have no display and look almost like a can. They are built around both CCD and CMOS sensor technology (CMOS has become the most common nowadays in consumer models), and they could be either color (OSC – one-shot color) or mono. An OSC is almost like a regular DSLR – you get the color in one shot. With a mono camera, you have to shoot through different filters and then in postproduction, combine the files to get color images.
Dedicated Astro-cameras is often possible to chill. A chilled sensor reduces noise in the images, which could be very important, especially in hot climates.
There are pros and cons with every model.
I wrote that it’s possible to shoot exposures with a motorized equatorial mount. That is a truth with modification. The accuracy of a mount could is limited in the majority of consumer models. Therefore it’s common to use a smaller guide telescope with a small digital guide camera that locks on a target star and sends signals to the axis motors in the mount so they could compensate during the exposure. It could be necessary when using longer focal lengths and exposure time from around 2 minutes and beyond. Without guiding, there is a risk that you get elongated or “egg shaped” stars.
First, you have to find a dark site. The darker, the better! Nowadays, there is a lot of light pollution in almost every corner of the Earth. However, here in Swedish Lapland, west of Jokkmokk, there is a large area with almost no light pollution at all with skies graded as Bortle I and II. It’s probably the largest dark site in Europe that is also quite easily accessible, and therefore it’s a little bit like an astrophotographers heaven. The downside is that it lies north of the Arctic Circle, and the climate could be harsh and demanding. Both for you and your gear!
But when the conditions are at the top, it’s world-class!
When you have found your spot, find the polar star, Alpha Polaris. At a latitude around 66-67°, it’s high in the sky. Next, you have to do a polar alignment of your mount. Nowadays, it’s relatively easy to do with digital technology.
When it’s dark, I use to slew the telescope to a bright star, often Capella, and make a focus adjustment. For that, I use a Bahtinov mask that creates diffraction spikes that help you to focus.
When you have the correct focus, slew to your object. Nowadays, there is a technique that is called plate-solving that helps the mount to automatically slew to an object.
When it comes to exposure, it’s all about collecting light. Today it’s most common to take many exposures and then use stacking techniques in post production to combine them. A regular exposure time on objects such as M31 or Horsehead Nebulae is around 3-5 minutes. Total integration time could be something like an hour up to many hours, depending on your motivation and the quality of the night sky and some other factors.
To manage noise and artefacts, it’s common to take additional calibration frames: Dark, Flat, and Bias frames, all that have a different purpose.
Dark Frames – I use to take around 20-30. You shoot with the lens cap on the telescope, at the same settings as when you shoot the object (often called Light frames), at the same temperature. This will give you images of the sensors’ thermal noise.
Flat Frames – is exposures at an evenly lit surface (an iPad with a white screen for an example). This gives you images that show vignetting, dust, and other artifacts. I use to take 10 to 30 flats or something like that.
Bias Frames – is exposures with the fastest exposure time and the lens cap on. It gives information about unwanted signals from the sensor electronics such as dark currents.
Together with the Light Frames that you exposed on the object, these calibration frames help to create a master file with a lot of light information and very little noise that you can use further in postproduction to create the final image.