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Does a long Focal Length Help Observing in LP?

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#1 aolo


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Posted 14 December 2012 - 05:00 PM

Is F/12 better than F/8 for the same aperture in LP skies?

#2 FirstSight


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Posted 14 December 2012 - 10:08 PM

No. All a longer focal ratio will do is to reach a given magnification using a longer focal-length eyepiece than a similar-aperture scope with a shorter focal ratio will, and also have a deeper, more forgiving zone of sharp focus than the scope with a shorter focal ratio. But both short focal ratio and long focal ratio scopes of the same aperture will display the same background surface brightness at the same magnification, i.e. one won't show a darker, more "contrasty" background relative to deep space objects or planets than the other.

#3 Dan McConaughy

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Posted 15 December 2012 - 09:30 AM

Indirectly yes because you can reach higher powers. What matters is the effective f-ratio at your eye. Other things equal a higher magnification gives a darker background. There is a tradeoff. You can find a magnification that gives you the best contrast of faint extended objects like galaxies which are the most problematic under LP. There is a book called "Galaxies and how to observe them" that has a good discussion of this issue.

#4 Tony Flanders

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Posted 15 December 2012 - 09:42 AM

Indirectly yes because you can reach higher powers.

Not really -- you just use different eyepieces to achieve the same high power in scope with shorter focal length. Or use a Barlow, which effectively doubles the focal length of your scope.

In any case, the OP was querying f/8 vs. f/12, and f/8. And by modern standards, f/8 is already a fairly long focal ratio.

#5 FirstSight


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Posted 15 December 2012 - 10:04 AM

Indirectly yes because you can reach higher powers.

Optically, this isn't true at all, because:
1) scopes of equal aperture have equal maximum resolution capabilities, regardless of f-ratio;
2) a shorter focal-ratio scope of equal aperture can reach any magnification a longer focal-ratio scope can, by using a proportionally shorter focal-length eyepiece.

There are a couple of potential "indirect" advantages which some longer focal-ratio scopes *might* (though not necessarily) have over *some* shorter focal-ratio scopes.
1) A longer focal-ratio scope generates a more gradually converging light cone, creating a larger, more forgiving zone of sharp focus than a shorter focal-ratio scope, making the longer focal-ratio scope easier to focus at higher powers than a shorter focal-ratio scope equipped with an identical focuser. However, the widespread availability of double-speed 10:1 focusers for shorter focal-ratio scopes has substantially reduced the inherent disadvantage of shorter focal-ratio scopes in this regard, particuarly for magnifications within the "true" resolution capabilities of many scopes.
2) it is more demanding (and expensive) to create high-quality short focal-ratio optical components than longer focal-ratio optical components; the figuring and control of potential aberrations are less forgiving at shorter focal-ratios.

BOTTOM LINE: My f/5.4 NP101 can go head-to-head with ANY 4" aperture f/10 refractor you care to put it up against, at any particular aperture under any particular conditions of light pollution. I'll need an eyepiece focal-length that's shorter by a factor of (5.4/10) than the f/10 scope to reach a comparable magnification, using either a native eyepiece or a Powermate-eyepiece combo to get there, but your f/10 scope will have *no* net advantage in mitigating light pollution.

#6 StarStuff1



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Posted 15 December 2012 - 10:57 AM

Assuming you have a refractor make sure the dew shield is generously long, say 2 to 2 1/2 times the aperture. This should add a little more contrast. Also, flocking the focuser can help with contrast.

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