There are two types of video cameras for astrophotography. The first is planetary cameras, used for imaging solar system objects at high frame rates. The other are deep-sky video cameras, which integrate longer exposures to give a "real time" view of deep sky objects.
Planetary Video Cameras
Above: Typical planetary imaging camera, the Celestron Skyris series.
Planetary imaging cameras capture images at video rates, usually 30 frames per second or faster. This is the ideal way to take images of solar system objects. Planetary images are taken at high magnification because the planets appear so small. The atmosphere is constantly in motion, usually blurring the image of a planet. But every now and then you will get a good moment with a sharp image. So the best way to capture a good image of a planet is by taking hundreds or thousands of frames and picking out the best ones. There is software to do this automatically, and to then stack the sharpest frames together to bring out more detail and reduce noise.
The newest planetary camera use high-speed USB 3.0 ports for fast downloads of high resolution images. The cameras themselves are usually tiny and fit into a telescope just like an eyepiece. Depending on the focal length of the telescope and the size of the camera pixels, a Barlow lens is sometimes used to increase the magnification.
Deep-Sky Video Cameras
As cameras become more sensitive and faster optical systems become more readily available, the idea of real-time deep-sky imaging becomes possible. Real-time imaging opens up a world of possibilities: sharing the view with many people at once, extending the capabilities of your scope, and even overcoming disabilities are all options with these imaging systems. However, in many cases the same effect can be achieved by using a standard deep-sky CCD camera and short exposures, with even better results.
Why Real-Time Imaging?
Below are some examples of systems that showcase the value of real-time imaging.
At public observing events with large crowds of people, it can take a great deal of time to share the view of even one object with everyone. If 100 people have to stand in line to look through an eyepiece, each viewing for just 30 seconds, nearly an hour is required. If you can show images of deep-sky objects in real time to groups looking at monitors, it is possible to see more during an observing session. Also, anyone who has tried to show people a less-than-distinct lunar feature (such as the Apollo 11 landing site) knows how difficult it can be to describe its exact location. Pointing it out on a screen simplifies matters greatly.
Extending Observing Capabilities
When imaging the night sky no longer requires hours but just seconds, imaging is suddenly more like observing. With a telescope and a highly sensitive video camera, it is possible to see more detail in a celestial object on the monitor than in the eyepiece, especially in light-polluted areas. Stars fainter than 16th magnitude are visible in an 8" scope from a moderately dark site, compared to 14th magnitude at best visually (and probably more like 12th magnitude with average eyesight and observing conditions).
Real-time imaging also offers a solution to the problem of sharing the view through a telescope with observers who are in wheelchairs. Anyone who has tried to share the view with wheelchair-bound stargazers knows that it can be difficult to get the eyepiece into a comfortable observing position, and it may well be impossible to reach the eyepiece of some telescope designs (such as large Dobsonians or long refractors) in certain parts of the sky. A specific example comes to mind. The Flandrau Science Center in Tucson, AZ uses a 16" Cassegrain telescope for public viewing. This instrument is located in a dome atop a flight of stars, restricting its use for people in wheelchairs. The solution was to mount an 8" SCT piggyback on the 16" so that it viewed the same object. A low-light video camera attached to a HyperStar lens and connected to a monitor downstairs allows real-time imaging of deep-sky objects in every bit as much detail as is visible through the much larger telescope. Another example is that of sharing the view with observers with poor eyesight. In fact, at Starizona we have set up a real-time imaging system that allowed legally blind individuals to view objects on a monitor that were invisible to them at the eyepiece, allowing them to share in a stargazing experience they otherwise would have been unable to participate in.
Real-Time Imaging Cameras
There are several types of video systems that work for astronomical imaging.
The least expensive are low-light security cameras. These are sold without lenses so they can be attached to a telescope in the same fashion as an eyepiece. These cameras often do not have gain controls and so lack any adjustments, but they are thus extremely easy to use. They output to a standard video connection (such as a TV would have) and can be input to a computer using a video capture card. For deep-sky imaging of anything other than the brightest subjects, these systems require very fast optical systems, preferably of moderate to large aperture (8" or greater). See below for more details on the appropriate telescope systems.
Above: Stacking video system, the Mallincam.
The ultimate in real-time imaging are video cameras that allows multiple frames to be automatically stacked, such as the popular Mallincam. No computer is required for this setup, although it is possible to use one with a video capture card. Otherwise data is simply output to a monitor. Integrated hardware allows multiple images to be combined before being output. In the case of the Mallincam, up to 256 frames can be stacked. The level of detail that can be seen, especially in a fast optical system, is incredible. The spiral arms of the Whirlpool Galaxy are easily visible in an 8" scope from a light-polluted location. The views of deep-sky objects with an 8" scope under 4th magnitude skies are equivalent to those seen visually through a 14" scope from a dark observing site, without the need to lug around a big scope.
Telescopes for Real-Time Imaging
Real-time video systems depend on the sensitivity of the camera being used and the speed of the optical system employed. A faster (smaller focal ratio), larger aperture system will give better results in terms of detail in deep-sky targets. For stacking video systems like the Mallincam, smaller, slower scopes can still be used as the camera will integrate multiple exposures. Still, fast scopes yield the best results. For the low-light, non-stacking video cameras, speed is even more important for deep-sky imaging, and greater aperture will allow fainter stars to be imaged.
Fast Newtonian scopes offer large aperture and reasonably fast focal ratios for reasonable prices. An 8" or 10" f/4 Newtonian would work well with a stacking system. Smaller refractors are not as well suited, as they have small apertures (usually 4" or less) and slower focal ratios (usually f/5 to 5/7).
The popular Schmidt-Cassegrain telescopes (SCTs) offer the most intriguing possibilities. With inherently slow systems (usually f/10), focal reducers can be used to reduce the focal ratio to f/6.3, f.5 or even f/3.3, which is well suited to real-time imaging. The ultimate solution is the HyperStar system, which allows very fast imaging at large apertures. Celestron's 6" to 14" scopes can be used at speeds of f/2, yielding incredibly detailed real-time deep-sky images!