raschemmel:
How canbot be correct as shown ?
( the SCR will short the 230V to GND.
The scr should be in series with the load or power source. ( not in parallel)
I think you will find that the 230Vdc is what the OP has measured on that wire, which is the trigger wire that has to be shorted to gnd.
The "flash contacts" in the schematic.
I think that not worrying about an opto will be okay, as the flash is battery powered, and its gnd is basically floating.
Your old metal cased pentax/nikon/cannon had its chassis at flash gnd potential when you plugged into the "hotshoe" socket.
Tom... 