There are several problems with a window. First is, of course, the optical problems - you need to have optically flat glass. Remember that the window of the SCT's is a very subtlely shaped form that is called a Schmidt Corrector. There is a very slight doughnut taken out of the flat glass. The second thing is that the closed tube will probably need to have other things to be done to get rid of the heat or cold that cause tube currents. The only reason for going to a window that I can think of is to get rid of the spider holding the secondary. This will get rid of the spikes on bright objects.