focal lengths and light pollution

I'm considering upgrading my scope from an 8" f/6 dob to an 8" f/10 sct. I've heard that the longer focal length makes for better seeing in light polluted urban/suburban skies. How much of a difference can I anticipate?
mary

At the same magnification two 8 inch scopes of different focal ratios will have the same contrast. However, on a newtonian (dob) if you increase the focal ratio, you can use a smaller secondary mirror, which will increase contrast a little bit. The effect is lost though if you switch to a sct from a dob since the sct design requires a larger secondary mirror and more optical surfaces (which scatter light, reducing contrast). Assuming good and clean optics on both, the newtonian will win on contrast. A good 6" refractor will appear to have better contrast than either of the 8" reflectors on bright objects at the same magnification becuase the refractor gathers less light and scatters less light which will definitely darken the sky background.

Joplin

Hi Mary,

This is not correct. Magnification can help increase the
contrast by darkening the sky background on DSO. So the
using the same eyepiece, the longer focal length of the SCT
will yield higher magnification and thus **seemingly** gives
a better view. But if you use a different eyepiece on
your Dobs to reach the same magnification as the SCT, you'll
see a very similar view.

Ron B[ee]
PS Now, we've save you some $$$ trade-in to be apply to gasoline to go to darker sky ;-).

You won't be upgrading your optics if you make that switch. You can achieve the same effect by using a slightly shorter focal length eyepiece on the Newtonian.

if you want to see more, you need more aperture, not simply a slower focal ratio.

Ron Wodaski
author of "The New CCD Astronomy"
http://www.newastro.com