There are basically four different kinds of digital cameras.
Digital Single Lens Reflex Cameras
DSLRs have removable lenses with a wide variety of focal lengths from fisheyes to super telephotos. They offer complete creative control over aperture, shutter speeds and ISO. They have large digital sensors. They offer an extensive line of accessories such as remote release times and external flashes. DSLR cameras can be used to shoot all kinds of astronomical objects including the Sun, Moon, planets, stars, clusters, nebulae and galaxies. Although these cameras usually have a long-wavelength filter in front of the sensor to make the camera's color balance more similar to human vision, which is not that sensitive to long red wavelengths, they can be used to shoot most deep-sky objects, but don't record the red light of hydrogen-alpha emission nebula very well. It's not that they can't be used at all for red nebulae, but you will probably want to use a special filter for these nebulae and you will need longer total exposures. If you want to get really serious about long-exposure deep-sky astrophotography of red emission nebulae, a DSLR can be modified by removing the long-wavelength filter that blocks most of the red hydrogen alpha light, making the camera much more sensitive to this light. Removing this filter alters the color balance when the camera is used for daytime photography, but it can be corrected by using another filter and a custom white balance. Modifying a camera by removing the long-wavelength filter also voids the warranty. Canon did make the EOS 20Da and 60Da DSLR camera bodies specifically for astrophotography. It could take photos of all types of astronomical objects as well as emission nebula, and it could also be used for normal daytime photography with excellent results just like a normal DSLR. The 20Da was discontinued and Canon replaced it with the 60Da in the spring of 2012. The nice thing about the Canon 60Da is that it can be used in 640x480 Movie Crop Mode at 60 frames per second for high-resolution planetary photography. A digital single-lens reflex (DSLR) camera works the same way as a regular film SLR camera except it uses either a CCD (Charge Coupled Device) or CMOS (Complimentary Metal-Oxide Semiconductor) sensor to detect light instead of film. The big advantages of DSLR and SLR cameras is that they have a removable lens and allow long exposures. The ability to remove the lens allows use of a wide range of lenses of different focal lengths from fisheye and ultra-wide-angle lenses to super telephotos. It also allows the camera body to be attached to a telescope so that the scope takes the place of the camera lens. The focal length is then simply the focal length of the telescope. These cameras also allow long shutter speeds with a special "bulb" shutter speed setting. The term "bulb" comes from the earliest days of film photography when shutters were opened and held open for long exposures with an air bulb.
What distinguishes an SLR type of camera is the reflex mirror that is located between the lens and the shutter. This mirror intercepts the light from the lens and sends it to a focusing screen. It is then sent through a pentaprism or mirrors which correct it for inversion and presents the scene to the photographer correctly oriented through the viewfinder eyepiece. This arrangement allows the photographer to focus and frame the subject by looking directly through the lens or telescope. When the shutter button is pressed, the mirror flips up out of the way, the shutter opens and light hits the sensor or film. Digital snapshot cameras, whose lenses cannot be removed, do not have a reflex mirror. In these cameras the Live View image is displayed in real time on the LCD on the back of the camera for focusing and composition purposes. In this case the digital sensor makes a series of exposures in short succession and displays them very rapidly. Many of these cameras can also make movies in this manner. Most of the latest generation of DSLR cameras now have Live View and also record movies. Early generations of DSLR cameras did not display the image in real time on the LCD on the back of the camera. The latest generations of DSLR cameras made since 2008 (with the exception of the Nikon D3000 and D60) can flip the mirror up, open the shutter, and read out the sensor in real time, displaying a "Live View" image on the LCD on the back of the camera, or sending it to a computer. This can greatly aid in focusing the camera for astrophotography.
Astronomical CCD Cameras
These cameras are specially cooled to reduce noise, a concept we will discuss in chapter 3, and are specifically made for scientific and imaging purposes by manufacturers such as Santa Barbara Instrument Group (SBIG), Starlight Express, Apogee Instruments, Finger Lakes Instruments and Atik Cameras. They are made to be used through a telescope, but adapters can be purchased that allow use with camera lenses for wide angle shots. For a long time these cameras were black and white only, with color images constructed by shooting individual images through red, green and blue filters and combining these exposures into a color picture. Now many cooled astronomical cameras can take color images in one shot, just like a DSLR.
Digital Snapshot Cameras
Point and shoot DSC cameras have non-removable lenses and limited maximum exposure times. Because they have very small sensors with small pixels, they are usually noisy and not well suited for long exposures of faint objects. These cameras are generally used by viewing the scene or subject with a small LCD display on the back of the camera in real time, which is used for framing and autofocus confirmation. Some have optical viewfinders that you can look through to frame the scene and save battery life. Unfortunately, you won't be able to do much deep-sky astrophotography with a simple point and shoot digital snapshot camera because you can't remove the lens, or take exposures longer than 15 to 30 seconds and because these cameras usually have high noise in long exposures under faint light conditions.
Webcams
Webcams are inexpensive video cameras with color CCD or CMOS chips that are made to stream video over the internet and world-wide web. They have very small chips with tiny high-resolution pixels that shoot continuous digital video. Higher-end video cameras have also been adapted and optimized for astronomical planetary work although they cost more than a simple webcam. These cameras are excellent for high-resolution planetary imaging. Individual frames of the video can be analyzed and the sharpest frames selected and separated from the video. These good frames are then stacked together to produce excellent images of the planets in the form of a single still picture.
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