Planetary imaging

Planetary imaging is the branch of astrophotography that aims at capturing the Lonely Photons that travel from objects within our Solar System. It represents for many the entry point in astrophotography…we have all started with the Moon after all, haven’t we?

What is it?

Planetary imaging aims at capturing objects in our Solar System such as planets, the Moon or the Sun. This can be done from virtually anywhere for these objects are so close and bright that light pollution doesn’t have any impact on this kind of photography.

However, a crucial role in planetary imaging is played by what astronomers refer to as “seeing”. This is basically the effect of fast-changing, turbulent airflows in Earth’s atmosphere that can make an image look blurred or distorted. When seeing is unstable it can vary rapidly, even within the same minute, inevitably affecting the final image. Fortunately, this is where a technique known as “Lucky imaging” comes in (more on this later).

Another critical aspect in planetary imaging is planning. Planets can get closer or farther away from Earth as they follow their orbits around the Sun thus the best time to image them is when they are at opposition (the closest approach to Earth). Also, just like Earth the other planets also spin on their axis meaning that some features, such as Jupiter’s Great Red Spot, are not always visible from our position. Planning an observation in advance using a tool like “Stellarium” can help get the best results and avoid a lot of frustration!

Jupiter with the Great Red Spot in evidence

What do you need?

Generally speaking, planetary imaging requires less (and less expensive) gear compared to DSOs AP. A standard Alt-AZ mount will do the job just fine and planetary cameras don’t need cooling so they’re generally less expensive and easy to use. Here’s what you need for planetary imaging.

The telescope. While it’s possible to do AP without a telescope, I am a proud amateur astronomer so I can’t imagine doing it without one. First thing for me then is obviously the telescope. As the planets are quite small, two things become fundamental when choosing a telescope for planetary imaging: focal length and aperture. Longer focal lengths (>2000mm) help capture more details on the planets’ surface whereas larger apertures (>200mm) resolve higher resolutions.

The telescopes that are more suited for planetary imaging are the SCT (Schmidt-Cassegrain Telescopes) although great results can be achieved also with Maksutov Cassegrains and Newtonian or Dobsonians with large apertures. Celestron, Meade, Sky-Watcher are some of the most common brands.

The camera. The second thing to consider is the camera that will have to capture those pretty Lonely Photons. Different types of cameras can be used, with different results. Smartphone cameras are the simplest solution but produce lower-quality images compared to dedicated astrocameras, such as ZWO or QHY, or Digital single-lens reflex (DSLR) cameras, such as Canon or Nikon.

Laptop or computer. Another necessary piece of equipment is a laptop or computer to run all sort of software needed in AP: from data capturing to stacking, denoising or post-processing, there’s a software for anything! A good, fast laptop with at least 16 GB of RAM, good graphic components and A LOT OF space is essential.

More gear. Other helpful gear includes Barlow lenses, which multiplies the magnification and extends the scope’s focal length, UV/IR cut filters, to cut out unnecessary light that would otherwise affect image quality, and an atmospheric distortion corrector (ADC), useful to image targets that are low in the sky, where the effects of the atmosphere are more visible. For mono cameras, RGB filters are necessary too.

How does it work?

Celestron Evolution 8″ EdgeHD SCT paired with ZWO ADC, Televue Powermate x2.5, and ZWOASI 224MC-Pro.

In planetary imaging the best results are achieved through “Lucky imaging”, a technique used by most astronomers to balance the effect of bad seeing. This works with free software such as SharpCap or Firecapture that record videos rather than taking single shots, thus capturing hundreds of frames per second (fps).

The videos are then run in dedicated stacking software, such as Autostakkert or Registax, which analyse them and select only the best, sharpest frames, rejecting the rest. This, of course, increases the probability of capturing good frames.

Once this step is done, what we’re left with is a final stacked image which probably needs some adjustments. The free software Registax offers a “wavelet” functionality that does magic in pushing out surface details whereas for additional touches software like PixInsight, Photoshop or GIMP help get the best out of your images.