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I suggest you have a play with the Newt design app that is freely available on the Net, it does all the calculations for given mirror size, including recommended secondary. Visual newts can get away with a smaller secondary than an imaging newt. The difference lies in the need to fully illuminate a camera sensor as opposed to just an EP and your pupil in a visual scope. So newts do suffer a bit in terms of contrast compared to a similiar sized refractor but the cost of newts is significantly less than refractors. A good newt will be tube baffled, andvthat helps with contrast. Of course newts offer true colour rendition, something that comes at significant cost in refractors.
T-stops really don't get mentioned much in astronomy.
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