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As others have said its not worth the time and effort for the minimal gain involved.
You need to get the secondary obstruction under 20% and ideally down to 16% to 18% to notice any worthwhile gains in contrast for lunar and planetary observing. To do this you need to change the focuser, reposition it and reposition the spider or move the primary down in the tube, all to hard. As Mark Hodgson has said these are design considerations that need to be thought of prior to building the scope, not an afterthought as a bandaid fix. That 50mm secondary would improve planetary contrast by 3% to 5% but would reduce the size of the 100% and 75% FIF and would affect the scopes performance on DSO's. The 3% gain in planetary contrast would really only be detectable under the very best of observing conditions.
In short a complete WOFTAM IMO. If you want a specialist planetary scope get someone to build you a 10"/F8 equatorially mounted newt with a 12% CO, then you notice a difference.
CS-John B
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