[rogerg (Roger)]

... sorry, just have to laugh at this mess I've created

I hope this isn't getting beyond the point where we're going to land up with one concise conclusion

Originally Posted by bird

As you increase the magnification, the airy disk gets larger (assuming a non-infinite mirror diameter). So the same amount of light is spread out over more area and it gets dimmer.

Agreed - this is where the focal ratio (so the focal length with respect to the aperture) is increasing and so the airy disk gets larger.

Originally Posted by Blue Skies

Er, can I butt in here? I'm kind of the cause of Roger's headache

Sort of, but not entirely I have had this debate with Dave (local, not DaveGee etc) and others in the past, so hence after something conclusive rather than leading people up the garden path

Originally Posted by AstroJunk

F ratio is to all intents and purposes irrelevant in this conversation

You say that, but then you say "It is also true that if you reduce the focal length, the resultant image will be brighter too.". That is exactly focal ratio - you are decreasing the focal length with respect to the aperture, you are decreasing the focal ratio and so brightening the image

And, in our case we are talking two apertures:

12" SCT @ F/7.5 vs
8" SCT @ F/4 (or there abouts - using f/3.3 reducer)

so, sorry... it "has got complicated"

... and yes, we plan to test this, but adverse weather conditions is going to prevent that for about a week.

Originally Posted by White Rabbit

Interesting discussion, and something I've thought about a fair bit.

I think it all boils down to the inverse square law http://en.wikipedia.org/wiki/Inverse-square_law.

Correct me if I'm wrong here but if you decrease the focal length to 800 F4 you may gather light faster but you spread it out over a wider surface (image scale). So the the inverse square law comes into play and diminishes your returns by a factor of 4.

Or do I have that completely wrong.

Sandy

err, sorry, I think you're wrong I think you've got it around the wrong way - F/4 will spread the light over a smaller area than F/10 and so smaller image scale and brighter image.