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This is only my theory so I could be wrong However
The use of an artificial star for fine collimation is to get around the tracking problem with a real star.
The object of a fine collimation is to make all the rays that form the images on the focal plane to arrive in phase from all parts of the mirror. This means extremely fine adjustment (micro distances - angstrom units).
The way we do it on a SCT is to adjust the secondary mirror to produce uniform Airy discs caused by diffraction of the light around the central obstruction. It is the phase difference between the diffracted rays and the direct rays that cause the Airy discs.
To get defined Airy discs from an artificial star the light from the artificial star needs to be a point source that subtends an angle (size in arc seconds) that is less than the real resolution of the telescope otherwise the disks will be blurred beyond recognition.
The distance will therefore need to be suitable for this to happen at a focal range that will allow the main mirror to be locked in position.
Barry
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