Thanks for that info guys. Appreciated. It is another technical step that I haven't even read about yet so that is fantastic. I will look at CCDcalc and see if I can get my peabrain around it.
Meanwhile: Main Scope - Sharpstar refractor. Guide Scope: Orion StarShoot AutoGuider + 80mm Short Tube
Main scope: focal length and aperture - 690mm , F/6.5
Guide scope : focal length and aperture - 400mm , F/5.0
Imaging camera: chip pixel size: 6.42μm
Guide camera: chip pixel size: 5.2μm
Cheers
Dave
Edit: Hmmm - found
this site with this calculator.