If it helps, I too have a similar setup.
My imaging scope is an 80mm f6 refractor, sometimes with a 0.8x reducer/flattener. I have a 65mm/500mm guidescope, and it works a treat, but of late I have been using part of a simple 8x50mm finder-scope. I unscrewed the eyepiece end and made a fitting to hold my QHY5, and this will be not unlike your DSI. Works a treat, all indications say it shouldn't, but it does. Try it and see, I am sure you will be pleasantly surprised. I have used mine with PHD, although mostly I use it with Maxim DL.
Gary
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