Regarding focal length, trying to come to terms with your equation. My tak fl is1000, it is a F7.9 I think from memory, and I have 2 powermates, a 2" 4x and 1.25 5x . In the equation what is Tel FL? I understood using a 5x powermate would bring the FL to about 5metres and using the Toucam which is equivalent to a 6mm ep (I have been told) gives a magnification of over 800x..Based on this assumption one would need absolutely perfect seeing to see anything, I would reckon. All very confusing stuff at times methinks..