[Drac0 (Mark)]

Hi all,


Trying to get my head around the light capturing abilities of different scopes and if it can in some way be used for comparison, especially taking into account different focal ratios & how it all affects exposure times.

So I have 102mm F7 & 150mm F8 refractors. Now from calculations, if I'm correct, the 150mm will gather a little over twice the light of the 102mm. So if they both had the same focal ratio I could expect to need exposures of half the time on the 150mm? But how is this affected by the different focal ratios? And if I stick a x0.8 reducer on the 102mm, reducing the focal ratio to ~F5.6, how does that affect things?

I'm used to the photography version of things like F-stops where doubling/halving the light changes things by one stop of light (in this case the 102mm @ F5.6 would ~= 150mm @ F8) but can't figure out what it means when it comes to telescopes. Would the 150mm @ F8 collect about the same amount of light as the 102mm @ F5.6?

Anyone know where I can find some information that won't be too confusing?

Cheers,
Mark