You might try my fishing line model which works very well on 100mm F8 lense. As shown in the attached image. Also see a version made by the Avertons which is the same thing fine tuned and they provide a dxf file. I will se if I can find a PDF.
http://www.iceinspace.com.au/forum/s...7&postcount=27
With a refractor the bars and spacing needs to be as fine as you can, 1 mm bars or fishing line is ideal. I posted a dxf in that thread.
When I say 100mm that is the diameter of my old manual NIKON F8 800mm lense and also my ancient Pentax 67 f4 400mm. Have not tried a smaller diameter lense but maybe your 500 is big enough. I think live view full zoom focus is good enough for shorter focal length wide field work. I think my live view zoom is 10x