Thanks for your input, but I realise there's plenty of other factors and reasons which I'm well versed with, I'm keen to keep this conversation to the question at hand
Quote:
Originally Posted by Wavytone
Roger,
The real reason people choose fast aperture reflectors is simply size, and the issues associated with the mounting. A short scope is lighter, far more transportable, and more easily mounted.
For visual use you don't use the objective alone, there is also an eyepiece. What matters is the working magnification and the size of the exit pupil, which should be smaller than the entry pupil of your eye. That's the theory.
There are other issues:
- achieving high magnification with a short objective (f/4 or less) implies very short eyepieces that are less than satisfactory; a long focal ratio objective (f/15) coupled with a larger eyepiece will produce far better images.
- achieving very low magnifications with a long focal ratio objective (say f/15) implies using eyepieces that will give an exit pupil too large for your eye.
The optimum turns out to be f/7 where you can achieve a 10:1 range between lowest and highest magnification and still have an exit pupil less around 6mm at low power. But f/7 'scopes aren't small and pose real problems if you want an equatorial mount.
For astrophotography, the focal length and focal ratio are important in different respects:
- the limiting magnitude (faintest stars) that can be recorded is determined by focal length, not aperture or focal ratio;
- increasing the focal length makes the background sky darker, along with extended objects (nebulae etc) whereas the apparent brightness of point sources (stars) remain the same.
- for a given aperture, a shorter focal ratio means a shorter exposure time, brighter nebulae. it also means a brighter background sky, so the longest possible exposure is also limited.
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