A key decision you’ll need to make is whether you want to image using a DSLR Camera, or a dedicated astronomy camera. There’s a good chance you already have a DSLR at home so lets start there.

Using a DSLR can be a very easy way to start your astro-imaging journey. All you need are a few adapters to connect the camera to your telescope and you’re good to go. You can produce some absolutely incredible shots using a DSLR, as many have done so before. There are however, a few downsides and it does get quiet complicated so I’ll do my best to simplify this:

Hydrogen alpha – This is a wavelength of light that many, many emission nebula broadcast in. It’s a wonderful deep vibrant red – see the image above of IC1396 – The Elephant’s Trunk Nebula. Unfortunately, because it’s so close to infrared, many IR filters in DSLR’s also block out hydrogen-alpha light. This means that when you try to image many nebula targets, you’ll be left disappointed. The target will barely be visible even after capturing hours of data, because the IR filter is stopping those wavelengths from reaching the sensor. To get around this, many astrophotography’s get their DSLR “Astro-modified”. This means removing the IR sensor, allowing the wonderful Ha light to reach the sensor. The downside is that it voids your warranty and the camera can no longer be used for daytime photography without re-adding an IR filter, usually to the front of your lens.

Calibration Frames

Calibration frames are absolutely vital in getting the best out of your data. However they are temperature sensitive and they must be taken at the same temperature as your light frames. Find out more below:

Dark Frames

When you take long exposure images, the sensor begins to heat up and generate thermal noise. Because the signals from the target your imaging are very faint, we’re often left with a poor signal-noise ratio. Lots of noise, not a lot of signal. What if there was a way we could separate the signal from the noise? We can! It’s achieved using Dark Frames.

Dark frames capture only the thermal noise, not the signal from your target. We do this by keeping the lens cap on the telescope and capturing exposures the same length as your light frames. What this does is capture the noise, and only the noise. By ensuring your dark frames are the same length as your light frames, you’re ensuring that the noise profile you capture is the same as what’s in your light frames. This gives the stacking software two bits of information: Images with noise and signal, and images with only noise. This allows the software to differentiate between noise and signal, and it can then remove the noise, whilst keeping the signal from your target. 

The key here, is the temperature and exposure time. We can easily take the darks at the same exposure time as the lights – but we can’t control the temperature. And seeing as we’re trying to remove thermal noise, this becomes a problem. If the dark frames aren’t taken at same temperature as the light frames, you’ll get different thermal noise profiles and will end up with very poor results. This can somewhat be mitigated by sacrificing time from your light frames to capture the darks whilst the outside temperature is stable. However, if the temperature outdoors is fluctuating, this won’t work. 

 Additionally, even when capturing your light frames, the temperature will probably change. I’ve started imaging at 10 Pm at 9 degrees and finished at 2 am at -1 degrees. That’s a big change. This means I’ll need dark frames for that entire temperature range – which is almost impossible.

Dedicated Astronomy Cameras with Active Cooling

A lot of dedicated cameras come with built in coolers called Thermo-Electric Coolers. They don’t use gasses or liquids. They use something called the Peltier effect which uses different materials in an electrical circuit to rapidly heat up one side of the system, whilst cooling the other side. They can often reach temperatures 35 degrees below ambient, which means on a typical spring evening around 10 degrees, you cool your sensor down to a potential -25 Celsius. This is great, because now you can set the temperature of your camera and more importantly, keep it there! This means you can exactly match your light frames and dark frames using exposure time AND temperature. This vastly increases the quality of your dark frames, making noise reduction significantly more effective. 

Additionally, you no longer have to sacrifice your imaging time by trying to take darks on the same night (to match the temperature). Because you can now control your temperature, you can take your darks whenever you want, and from indoors. Make use of rainy/cloudy nights, set your camera to a fixed temperature, put the lens cap on, and shoot away. Take exposures lengths that you commonly use – I do 1,2,3,4,5 minutes at -10, – 15 and – 20 Celsius.