Quote:
Originally Posted by mura_gadi
Hello,
A little more basic, the F ratio is the multiplying factor that is based on the size of the mirror by the length of the tube. The lower the F ratio the faster the scope is, lower number = faster the scope.
Visually faster scopes need to be corrected with additional hardware and require far more expensive eyepieces to handle the steeper light cone. Slower scopes tend to be longer therefore harder to balance, move around and generally need a larger mount/tripod than a faster scope with the same aperture.
Imagine the same size mirror with a longer tube, your field of view(FOV) reduces as the tube length increases. With a newt you will often see a 4 line spike associated with stars, at F9+ that effect diminishes to the point that you won't see them at all much like a refractor scope.
Generally slower scopes of the same aperture allow you to see more detail, in the bands of Jupiter, the rings around Saturn etc, but won't allow for really wide fields of view.
At F5/6 your closer to an all-rounder scope, the F6 will be slightly better optically, the F5 will have a wider FOV with the same EP and still be a good optical scope. Below F5 you start to have problems with optics from a visually point of view. Faster scope at F4.5 or less are great for digital image collection, but would really suck visually with cheap eyepieces for instance and no correction hardware like a parracorr.
Like most things in life one shoe doesn't fit all, I'd stay above F5, considering your location most DSO's are going to be a stretch unless you really know what your doing with your camera. Some DSO's like M42 are just big and bright and like planets show up well despite surrounding light pollution. Having said that, there are plenty of DSO's you could capture in a Bortle 6+ area. (Bortle is a reference to your light pollution levels)
You can overcome most problems with deep enough pockets though.
Steve
Ps. Get a copy of the "The Southern Sky", cheap as chips 2nd hand and plenty available, nice section at the back of the book with the best 25 DSO (deep space objects) at the back and plenty of good advice even if some of it is now very out dated(re: imaging with film!!!, omg I remember when you had to worry about your photo's being public as they shot out of the film processing unit... ).
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Thanks for the excellent write up especially the difference between F5/6 - those are my choices right now. And I'll definitely look for "The Southern Sky" book too.
After your write up I'm leaning F6 and with imaging (when I get to it) I'll be doing smaller brighter objects; both because of where I live and experience level.
Finally, while deeper pockets can overcome challenges, alas even deep pockets can't buy more time
