Quote:
Originally Posted by Atmos
For viewing, the focal ratio doesn't really mean too much as everything is determined by the relationship between the focal length and the eye piece that is being used. Visually a properly collimated F/4.5 dob with a 9mm eye piece will be pretty much identical to a F/9 dob with a 18mm eye piece.
One of the reasons long focal ratio telescopes were around for so long is due to them being less demanding. Trying to collimate a F/3 telescope is a nightmare where as collimating a F/9 telescope is a LOT more forgiving.
In refractors, field curvature and chromatic aberration is a product of not only larger aperture but also shorter focal length.
Ultimately, the f/ratio makes a big difference in photography but not much at the end of the eye piece. If you double your f/ratio (4.5-9) and you double your eye piece focal length (9-18) then there really is not much difference. The F/9 will give more corrected views around the edge of field but thats about it really.
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I would love to observe the difference between low and high f/n balanced out against eye piece selection. In particular I would really like to observe the difference in corrected view sound the edges of the visual field.
Clearly it's time for me to find my way to a IIS star party!