Its been a while since I was last on here but have just picked up a new (to me) scope and want to match it with a guide scope thats properly sized.
Scope is a Willams Optics Zenithstar 2 ED Doublet F6.8 - f:545mm. I'll be using a 0.8x flttener with it and have a QHY5LII guide cam
Mounted on a HEQ5 Pro mount
Expect to use PHD guiding
What / how best to calc guide scope focal length given above setup ?