What is a Planetary Camera?
Planetary cameras are cameras designed specifically to capture objects within the solar system. The term "planetary camera" is a catch-all phrase used to describe small sensor, high frame rate cameras that are designed to photograph the planets, the moon, and the sun, among other objects. These are different than cooled astronomy cameras used for deep sky, which feature cooling, large sensors, and generally slow frame rates. Because planetary cameras feature small sensors and don't need to be cooled, they are often much smaller as a result. To understand how to get the best results out of a planetary camera and which one is right for you, it's important you understand astronomical seeing and lucky imaging first.
What is astronomical seeing?
When capturing planetary, lunar, or solar images, it's best to do so during periods of good astronomical seeing. Astronomical seeing is the reason why stars appear to twinkle. This is caused by turbulent air in the Earth's atmosphere, and it constantly affects planetary views and images. When observing through a telescope, the turbulent effects of poor astronomical seeing are amplified. When using a small sensor planetary camera on a telescope, the turbulent effect is amplified even further. During periods of poor seeing, this can cause the planets to appear as very blurry blobs, even when in focus on the best telescopes. During periods of excellent seeing, however, the planets will appear much more stable and detailed — all without changing any equipment.
Astronomical seeing conditions can vary from night to night, hour to hour, minute to minute, second to second, and even between fractions of a second, so it can be difficult to predict when good seeing occurs. Some weather forecasts, like Astrospheric and MeteoBlue, offer astronomical seeing predictions, but these are only so accurate. Your location can also impact astronomical seeing greatly. For example, some of the best seeing conditions are usually found at high elevations close to oceans, which is why sites like Mauna Kea in Hawaii are often chosen for the world's premier observatories. It's also why some telescopes, like Hubble, are in space — so they can avoid the effects of Earth's atmosphere entirely. So how do planetary imagers capture such sharp images of the planets?
What is lucky imaging?
Lucky imaging is a technique used to capture the planets, moon, and sun in extremely high detail. Lucky imaging works by capturing hundreds of frames per second using software like FireCapture, and then using separate software to select only the sharpest frames out of thousands. Lucky imaging is incredibly effective at capturing sharp, detailed images of planets. Even during periods of average seeing conditions, there will occasionally be a split second of excellent seeing. By capturing high frame rate video for minutes at a time, you're more than likely to capture a few moments where the planet appeared very sharp. You can then use software
What are Cassegrains best used for?
Generally speaking, Cassegrains are best used for observing and imaging targets at long focal lengths, but it depends on the type of Cassegrain:
Schmidt Cassegrain Telescopes are best used for just about any target. When paired with other accessories like focal reducers or Barlow lenses, SCTs are some of the most versatile telescopes out there. You can use them for observing and imaging solar system objects like the planets and the moon, as well as deep sky objects like galaxies and nebulae. The SCT is a jack of all trades, with probably its biggest strong suit being planetary imaging.
Ritchey Chretién Telescopes are some of the most well-corrected telescopes consumers can buy, as their design allows for a flat field across a large image sensor. They are best used for long focal length deep sky imaging, which tends to lend well for research. However, RCs are ill-advised for planetary images because their optical design lowers contrast dramatically.
Maksutov Cassegrain Telescopes make for excellent planetary viewing and imaging, and can provide even better views of the planets than SCTs due to slightly longer focal ratios. However, unlike SCTs, they do not pair well with reducers and therefore aren't nearly as versatile for other applications like deep sky imaging.
Dall Kirkham Telescopes are a more expensive telescope design best used for planetary viewing or imaging, and are often used for research. Please note that there are other modified variants of Dall Kirkham telescopes (see below), which are different in that they feature corrective optics for professional imaging with large sensor cameras.
Other Cassegrain Telescopes exist as well, such as the Classical Cassegrain, the Corrected Dall Kirkham (CDK), the Riccardi Dall Kirkham (RiDK), Riccardi-Honders, Harmer-Wynne, and more are usually reserved for research purposes.