Planetary Astrophotography

There are other objects to photograph other than galaxies and nebulae, also known as deep sky objects (DSO’s). Solar system objects including the planets, sun and moon are also favorite targets for astrophotographers, especially when you can tease out detail, such as the bands of Jupiter or rings of Saturn. Planetary imaging is not quite the same as DSO imaging, though. Getting decent photos of planets requires different gear than DSO’s. Galaxies, nebulae, the sun and the moon are relatively large in comparison to planets. But because planets appear small in telescopes, additional factors must be considered in order to get decent images .

Magnification will be necessary, which brings its own challenges. Typical DSO’s can be photographed with mid-range focal lengths. Although some detail can be seen if you just zoom into an image taken with a large sensor, some resolution will be lost, and the image will not be good. You really want the image of the planet to natively fill as much of the sensor as possible without blowing it up. Proper magnification can be achieved in a few ways. You can use a telescope that natively has a long focal length. I recommend at least 2000 mm focal length. Or you can attach a Barlow lens to a telescope with a shorter focal length, which multiplies the focal length of the telescope. For example, if your telescope has a native focal length of 1000 mm, attaching a 3x Barlow lens will result in a focal length of 3000 mm, which will be long enough to get decent magnification. Aperture is also important.

Magnification comes with its own issues. Spreading the optical path over a larger area results in a dimmer image, which is not necessarily bad since the major planets are pretty bright. Also, the image will quickly rotate out of the small field of view without excellent tracking. Luckily, guiding the telescope mount will probably not be necessary, since planetary imaging requires very short exposures.

Another challenge of planetary imaging is atmospheric turbulence, which varies quite a bit depending on location. Even in areas with low turbulence (also called good “seeing”), there is always some atmospheric movement. The key is to capture the planet in those moments when the air is still. This is why some of the best planetary images are taken with video cameras. It is possible to take a several-minute-long video at 90 – 150 frames per second (fps), which will result in 1000 or more frames that can be filtered into only those frames that capture those moments of stillness. If you can get 100 still images out of the 1000 total frames and stack them together with software, the resulting image can be excellent and reveal much detail on the planet’s surface. It is also possible to achieve this with a single exposure camera by taking many very short exposures, sorting only the best images, and similarly stacking them.

Which telescopes are best for planetary imaging?

All things being equal, larger apertures are capable of resolving more detail. Some would argue that catadioptric telescopes are the best telescopes for planetary imaging since the folded light paths result in natively long focal lengths, and they come with large apertures of greater than 8 inches. Theoretically, large refractors should be the best for planetary imaging because they don’t have secondary mirrors to obstruct the light path (secondary mirrors can reduce surface detail resolution). The problem is that large aperture refractors are very long and heavy and, more importantly, very expensive.

Which cameras are best for planetary imaging?

Let’s say you already have your telescope with a long focal length and decent size aperture. How do you choose a camera to best match the telescope? If you already have a modern DSLR or mirrorless digital camera, it is possible to capture really good images, especially if you take videos and sort the frames as described above. There are many T-rings and other adapters that are designed for connecting the camera to a telescope.

If you don’t have a camera, there are a few things to consider first. What are the seeing conditions at your chosen location? In other words are the atmospheric conditions relatively stable, resulting in good “seeing”? Or is the air pretty turbulent most of the time, resulting in poor “seeing”? Generally speaking poor seeing requires lower focal ratios. The other consideration is the pixel size of the camera. In general, poor seeing requires larger pixels.

I would emphasize that although these are generalizations, I wouldn’t overthink the importance of getting an exact pixel size match. You can get decent images regardless of pixel size. In my opinion seeing conditions and focal ratio are a bit more important. It makes sense that if seeing conditions are bad, the greater magnification from larger focal ratios is going to result in progressively poorer images as you go longer on the focal length.

One camera I recommend for planetary photography is the QHY 5-III 462C. It has a relatively small sensor at 1920 x 1080 pixels sized at 2.9µ. Because it has a small sensor, focal lengths of 2000mm or more will frame nicely without further magnification. Its video full-frame rate is a speedy 135fps, and it has USB 3.0 for fast data transfer. It is a color camera, so you will lose a little effective resolution because of the way color sensors work. But you won’t have to worry about using color filters. The small pixels are ideal for good or average seeing, but in video mode, you can still capture those rare moments of good seeing in poorer conditions. It has a standard ST-4 guide port and a 1.25 inch telescope connection.

How Jupiter would look in the field-of-view of the QHY-5-III 462C with a focal length telescope of 2500mm

Another camera that I highly recommend for planetary imaging is the ZWO ASI178MC. As I have mentioned in other posts, ZWO makes some fantastic and affordable gear for astrophotography. This camera has a higher resolution than the QHY 5-III 462C, coming in at 3096 x 2080 pixels sized at 2.4µ. The field of view is slightly larger than the QHY camera, making objects appear smaller. But the higher resolution makes it possible to zoom and crop an image without washing it out. As you would expect with a higher resolution sensor, frame rates at full resolution and USB 3.0 are slower at 60 fps, but increase to 235 fps at 640 x 480. It also comes with a standard ST-4 guiding port and cable and a 1.25-inch nose piece to attach to the telescope. It is slightly heavier at 120g vs 88g for the QHY. It also comes in a monochrome version that would require color filters to achieve a color image.

How Jupiter would look in the field-of-view of the ZWO ASI 178MC camera with a 2500mm focal length telescope.

Overall you can’t go wrong with either camera, since both manufacturers produce very high-quality gear. You probable get a bit more bang for your buck with the ZWO ASI camera, availability may ultimately make your decision for you.

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