Thanks Daniel & also Troy & Gary
OK so there is a difference in the last-element-to-sensor distance for different focal length OTAs - I wonder where could we find these details from?
That would then perhaps be what I need too - an extension tube of "X"
length to give the correct spacing - certainly it is best to have the FF to attach to the CCD and if requiring an extension would solve the issue then that's great! Experimenting with my present lot of accessories is necessary; it's only for the 80mm that I need a solution for with the HoTech FF since I'm able to get round stars to the corners - see
here for an example
Will need to wait for the rain to clear before being able to do any testing now
Will post results after
Cheers
Bill