Reflecting scopes bring more than just the visible spectrum of light to a single focus point, they also focus infrared and UV at the same point. Refractors don't focus these wavelengths at the same point (and some don't even get all the visual wavelengths together which is one of the reasons for big buck APOs).

So when you use a refractor with an imager like a webcam that is also sensitive to the infrared and UV ends of the spectrum unless those wavelengths are blocked (with the filter) you'll get some out of focus stuff in your shots even though the subject looks perfectly focused.