I've got a SkyWatcher 120 BD ED Doublet Refractor that I connect to my PC using ASCOM. I've entered all the properties data correctly, but I'm not sure what my 'Obstruction (mm)' is -- see attached screenshot. It's 75 by default. Any ideas?
Since it has something to do with the optics I'd guess it refers to the central obstruction of a SCT or Mak. Yours is a refractor and does not have an obstruction. Nothing to worry about. Enter 00 if you can.