Hey Boff,
let me answer your questions a bit more selective without touching the physics and in plain english (be careful with the used term "magnification" - a galaxy is in fact several light years large, to speak of magnification if we view it in an eyepiece is technically wrong):
focal lenght; focal ratio and what follows from it:
-the focal lenght (fl) is the distance from the objective to the occular it is usually expressed in mm. The fl determines the native magnification of a telescope - the longer the fl the higher the native magnification
The light path in an SCT is folded so the tube is only half a meter whilst the actual fl is 2m
-the focal ratio (F/.. - e.g. F/5) is the result of the following calculation:
focal lenght in mm divided by objective diameter (aperture) in mm (e.g. 1000mm(fl)/200mm(aperture)=F5
!!! camera lenses are different, their f-ratios mean something else !!!
What follows:
a telescope at F10 spreads the light of a nebula e.g. over a wider area at the occular (or camera) resulting in higher maginification than a telescope of same aperture at F5 - hence a brighter image results with less magnificiation BUT: the light gathering power of a telescope depends on itīs objective diameter and nothing else! (a 2" F3 is easily outperformed by a 6" F6)
To view a small object like a planet a long focal lenght is desireble, to view an extended object like a nebula a shorter focal lenght is desireble because a longer focal lenght would only reveal parts of that object (the horsehead with flame nebula -picture attached- just fit on a APS-C sensor -Canon 450D- with 750mm fl; at 1,5m fl only one of them would fit on the sensor).
The focal lenght of a telescope can be adjusted to a certain extend with focal reducers or tele extenders/barlow lenses.
A telescope with a longer fl will give a darker sky background and a smaller field of view. The opposite is true for shorter flīs.
When it comes to imaging it is a bit like daytime photography - for architecture shots get a widefield lens, for distant animals get a telezoom - aperture rules but a minute longer exposure compensates that.
For astrophotography a good equatorial (eq) tracking mount and a rigid tripod are absolutely necessary. All eq mounts have imperfections that need to be compensated by guiding (manual, auto).
Imperfections will result in trailing stars. These are much more evident in pictures taken at long fl than at short fl as the higher magnification makes it faster visible.
It is not that easy to succesfully guide at long flīs such as the 2m of the F10 cassegrain scopes. But below 1m it is pretty easy.
YOUR TELESCOPE question:
-for observing, the best scope is what use the most: it can be a pain to set up a heavy EQ mount, pol align, get power to it and lift the scope on top and balance it. A DOB with 8" or 10" aperture is much easier to set up and use, it gives bright images and the fl can be adjusted with barlows. Second best comes the SCT as it needs no collimation and has the focuser always at the same spot. The LXD75 wouldnīt be my favourite for visual as it requires collimation and the eypiece always at another position depending on the object your are observing. I have a newton myself and wouldnīt go for another eq mounted one for visual.
-if you want to become serious about astrophotography (AP) make sure you get a decent EQ mount (like the EQ6 or NEQ6) and not one of those alt/az-fork mounts as the meade SCTīs usually have, these produce field rotation.
--If you want to shoot bright extended objects - start with the ED80 and as guidescope with an autoguider and get the LXD 75.
---if you want to shoot planets go for the 10" SCT and get an off axis guider and an autoguider (and a planetary camera as a DSLR isnīt good at shooting 30-60 frames per second).
The quality of your images depends on the conditions under which you shoot (seeing, temperature, light polution), your gear and the processing skills you can apply. But the first and the last mentioned are the most important.
So, now I have written a novel and probably didnīt answer any of your questions....
Cheers
