You certainly need to ensure that the instrument has fully cooled before making judgeement in a star test. Strong light and dark rings and uneven secondary shadow can all indicate zones and spherical aberation, although the star test for zones is incredibly sensitive: a star test showing zoning may well be within a 1/10 wavefront tolerance. An easier test for a beginner to interpret is a Ronchi grating in place of the eyepiece. At F10, it is fairly sensitive and can show at a glance what the problems are and if they are significant.
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Originally Posted by casstony
I can say from experience that a crappy sct will still show more detail than a good refractor half its aperture simply by virtue of the greater resolving ability of the bigger aperture
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The larger aperture will only give better resolving power if the light is constrained in or near it theoretical airy disc diameter. This also ensures that the visual limiting stellar magnitude delivered by the scope will be appropriate for its size.
Larger apertures , even if not diffraction limited can impress on extended nebulous objects. Stellar limiting magnitude will suffer though. Considering just pure spherical abberration, a largere telescope of 2/3 wavefront error will show the same faintest stars as a diffraction limited one of half the aperture.
Mark
I'll have a read at it too.
