Hi Chris
Thank you for your reply.
What generated my interest was a recent image I took of a Quasar BR 1202-0725 in Virgo with z = 4.69.
Whilst trying to establish its position within my image, I identified a star at mag 20.56 and began to ponder what is the faintest star I could reliably identify.
Using "Astrophotography CAD" has helped me understand some of the physics and important HW/SW/Environmental considerations when imaging faint objects, and has likely saved me much trial and error in the field.
Cheers
Dennis