The aim of an astrophoto setup is to focus photons from your target through the optics and collect them on the sensor of the camera while compensating for the relative movement of the Earth.
Optics: 60mm and 85mm (f/5 to f/20) refractors. Mounts: EQ6 (20kg load), Teegul Sky-Patrol III. Cameras: CMOS sensors (7D: 400-700nm, m6). Guider: standalone autoguider (SynGuider). Filters: Ha 4nm narrowband, solar, light pollution
Optical designs in astrophotography are of two kinds: reflectors that use mirrors and refractors that use lenses. I use refractors.
Deep sky photography implies long exposure times because targets are very dim and large (up to Mag 14 and several degrees). This asks for short focal lengths (300-600mm), a fast scope (up to f/6) and precise tracking (up to 2-3min). The signal is often in a very specific wavelength and not always in the visible spectrum (Ha nebulae) so the camera has to have a wider detection range (up to 700nm or full spectrum) and narrowband filters must be used to discriminate the signal.
Planetary photography has very small targets (max 2-3 seconds of arc) which requires very high focal lengths (up to meters) but are relatively bright (Mag -1). Exposure times are considerably smaller (up to thousands of photos per second); the light is by definition in the visible range since they are lit by the Sun. Filters are used to accentuate surface features.
Mounts can be equatorial (one axis follows latitude) or alt-az (altitude-azimuth). Equatorial mounts are generally motorised and are used for photography.
The tracking mount moves at the same relative speed as the sky in order to allow long exposure times and avoid star trails. It needs to be polar aligned, meaning it should point at the north at an angle corresponding to your latitude.
Tracking errors (imperfect alignment and mechanics) can be corrected with an autoguiding camera pointed at a star that gives real-time instructions to the mount if the star starts to drift. This requires a second instrument mounted in parallel.
The sensors used in astrophotography are very varied and range from niche monochrome CCDs to point-and-shoot and vintage analog cameras. There is even Pinhole astrophotography, at the very top on the scale of geekiness.
Sensors that have the same size as 35mm film are called “full frame” and coexist with many other sizes, generally smaller. In principle, larger pixels tend to give you more signal, but there are many other variables.
Consumer cameras have a filter installed in front of the sensor that cuts unnecessary information from regular daytime photography, such as the UV light and the infrared. These can be removed to access the full spectrum of the sensor, or be replaced with another filter for a specific range.