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Addendum
In adding to the previous post, provided is the following extract to explain my dilemma.
"A well-made conventional 90-degree prism star diagonal can transmit as much or more light as a mirror, and do so with higher image contrast since there is no possibility of light scattering from a reflective metallic surface as in a mirror diagonal. Also a prism will never degrade over time as a mirror will since there is no reflective metal coating to degrade from oxidation. However prism diagonals may introduce chromatic aberration when used with short focal-length scopes although this isn't a problem with the popular Schmidt-Cassegrain and Maksutov Cassegrain telescopes, which have long focal lengths. On longer focal ratio telescopes a well-made 90-degree prism diagonal is the optimum choice to deliver the highest image contrast short of using the telescope without a diagonal entirely. However prisms seem to be falling out of favour probably due to marketing forces which have been favouring short focal length instruments which tend to function better with a mirror diagonal. In some special cases however, the colour dispersion effects of a prism diagonal can be used to advantage to improve the performance of undercorrected refractor objectives (regardless of focal length) by shifting the spherical and colour correction of the objective closer to the design optimum. The natural colour dispersion properties (overcorrection) of the prism works to lessen or nullify the undercorrection of the objective lens."
Of course, if money were no issue, would just buy a mirror diagonal, try it out, and either continue to use it or then buy a decent prism.
Regards,
David
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