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Beamsplitter guiding
Without worrying about UV etc a microscope glass slide at 45% will reflect about 8% of the incoming light, this can be used for guiding.
Another option is to use a "proper" beamsplitter ( Surplus Shed is a good starting point) these are usually about 25mm wide and 37mm long and come in various percentage reflection/ transmission ie 70/30 or 50/50 etc.
John H, I'm a little surprised to see the astigmatism see in the web link; beamsplitters are widely used on all sorts of sophisticated optical equipment, so I would have thought the imaging would be very good.
Although I have modified a Meade OAG ( a good starting point as the prism holder is easily removed) and incorporated a beamsplitter ( for guiding my spectroscope) my imaging performance requirements are not as critical as photographic ( I just need to maintain starlight on the slit). Silvio Mete uses a similar system for his cylinderical lens spectroscope.
Setting up and alignment of the beamsplitter is a pain! I had to include 4 X-Y adjusting screws to get mine aligned.
When I can get some observing time I'll take some photos through/ reflected from the beamsplitter and see if I can replicate the astigmatism.
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