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Connecting a camera to a telescope:|
There are two common ways of connecting a camera to a telescope. In principle, the simplest way is to use a telescope as a (very) large long focus lens. The focal plane of the camera is placed at the focus of the telescope. No eyepiece is used with the telescope. This is known as "prime focus" operation. Only cameras that have removable lenses can be used for prime focus photography. Single lens reflex cameras are ideal for this purpose, but compact cameras that have non-removable lenses cannot be used for prime focus photography.
An alternative way of connecting a camera to a telescope is by means for "afocal projection". In this case a camera complete with lens is held so that the lens looks directly into an eyepiece that is fitted to the telescope. Depending on the subject to be photographed, most kinds of camera can be use in this way, including compact cameras. Afocal projection does have an advantage that very high magnifications can be achieved, effectively combining magnification resulting from the long focal length of the telescope with magnification resulting from use of an eyepiece.
A disadvantage of afocal projection is that most eyepieces cause significant levels of optical distortion, so image quality is unlikely to match that which can be obtained from via prime focus. For operation via prime focus, the camera body is connected via a special adaptor to a tube which can be inserted into the eyepiece focuser of the telescope.
Afocal projection can be done by manually holding the camera up to the eyepiece, but for most purposes, it will be better to use a special mount for the camera that can be clamped directly onto the eyepiece. Manual support is suitable only for very short exposures as might be used to photograph bright subjected like the Moon.
Cameras for Astrophotography:
Film cameras are no longer much used for astrophotography, but can still produce some good results. So if you still have a quality 35mm SLR (Single Lens Reflex) camera that has a "B"/time exposure setting, pull it out of the cupboard and try it out.
For good results, it is necessary to use a suitable film. A problem with films is that they show reciprocity failure. This means that with increasing exposure period, they effectively become less sensitive. One of the best colour films for astrophography is Kodak Elite Chrome 200. This film shows reasonably low reciprocity failure and good sensitivity to deep red light, ie hydrogen alpha radiation. This can be an advantage over most (unmodified) mass market digital cameras which have poor sensitivity to hydrogen alpha radiation.
Exposure periods of several minutes at an aperture number of F4 can reveal many gaseous nebulae. Unfortunately, optimum exposure period must be found by experiment.
Another disadvantage of using film is that it is likely that the film image will need to be copied to digital media.
Mass market digital cameras:
Recently made digital cameras are generally very much more sensitive (much greater than ISO 1000) than photographic film. A limiting problem with mass market digital cameras is digital "noise". Even so, useful exposures of several minutes or so can applied with most basic digital cameras.
As a rule, small digital sensors (as used in compact cameras) are most prone to digital noise. Digital SLRs use larger sensors. In some cases full 35mm sized sensors are used. Some top quality SLRs can now operate at sensitivities of around ISO 100,000 !!! However, for astrophotography settings of ISO 1000 to 2000 (or less) are often prefered.
Sensors used in digital cameras respond to most visible light. Unfortunately, they also respond to infra-red light, which for normal terrestrial photography is undesirable. For this reason infra-red filters are always included directly over the sensors. An undesirable result for astrophotography is that these infra-red filters also reduce sensitivity to deep red coloured light of the wavelength that is known as "hydrogen alpha". This kind of light is very common around the Universe. Even so, good quality digital cameras are very widely used for astrophotgraphy.
CCD astro cameras:
These are specialist cameras that are made specifically for astrophotography. They are of little use for normal terrestrial photography. Their main features are:
--- Absence of any form of infra-red filtration over the sensor. The user is free to use various specialist filters and even to image ONLY in the infra-red region.
--- Provision of cooling systems for the sensor. Digital noise levels reduce considerably as the operating temperature of the sensor is reduced. Depending on the type of sensor used, typical operating temperatures are from -25 degC to -40 degC.
--- 16 bit analogue to digital converters are used. Normal digital cameras use 8, 12, or 14 bit conversion which results in lower tonal density resolution. This means for example that shadow areas of images form a CCD astro camera can be "stretched" to a greater degree than is useful from normal digital cameras.
--- Can be obtained in either colour or black & white only versions. The very best results are usually produced via black & white cameras while using special red, green, and blue filters to synthesise colour images.
Astrophotgraphy via one of these dedicated astro cameras can produce truly stunning results, but a lot of practice and effort is needed. And these cameras are relatively expensive.
Web cams can be used for astrophotography. Refer to the section about web cams.
Certain kinds of video camera are used for astrophotography -- (not covered on this site).