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Published online by Cambridge University Press: 27 October 2009
Choice of optical combinations
In order to develop a strategy that will make you a true specialist in CCD observation, it is advisable as a first step to set up the telescope's optical assembly to obtain the desired field and resolution.
The resolution The maximum resolution we can achieve is determined by the telescope's diameter, the intrinsic quality of the images, or ‘seeing’, turbulence, and sampling. The first limitation comes from the phenomenon of diffraction caused by the instrument's diameter: the larger the instrument's diameter is, the better the resolution. For instance, a 12 cm diameter telescope cannot resolve better than 1 arcsecond, whereas a 50 cm telescope can reach 0.25 arcsecond. It is physically impossible to reach a better resolution at the diffraction limit of a given instrument.
The second limitation comes from the observation site's atmospheric turbulence. Unfortunately, atmospheric turbulence is often larger than the diffraction limit. We can assume that anything over 1 second of exposure time and with a diameter greater than 10 cm, the resolution limit caused by turbulence completely masks that caused by the diffraction limit. In terms of long exposures (above 1 second), classical amateur sites have seeing in the order of 5 arcseconds, with the better ones going as low as 2 or 3 arcseconds.
The third limitation comes from sampling by the CCD detector. The physical dimensions of the CCD's pixels limit the resolution by dividing the image into tiny tiles.
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