Focal length pros and cons

[mbo (Pat and Kirk)]

The focal length thing has me a bit confused. This is what I gather so far, please correct any misconceptions.

Short F leads to shorter, more manageable tubes.
Short F gives greater 'Fully illuminated field' at the eyepiece (good for photography)
Long F gives greater magnification / narrower FOV for a given eyepiece
Long F tends to hide edge field flaws in eyepieces.
Long F mirrors are a bit easier / cheaper to make

Some things I don't get:
Does F value influence Apparent Field of view (or is this just the eyepiece)
Why don't I see many F8 or bigger 16" scopes? I realise they would be essentially untransportable, but they would seem pretty good for a home observatory.

[astro744]

Short focal length means smaller tubes provided there are no multiple internal reflections as in a SCT. eg an 8" SCT has a f.l. of 2000mm but is about 1/4 of that in legth.

Short focal ratio may still mean a long tube if the mirror diameter is large.

Fully illuminated field is influenced by many factors namely secondary size, tube size, focuser height (distance from secondary to focal plane). f ratio does not really come into it but a short f ratio usually means a larger secondary to get a better fully illuminated field. The cone of light from a short f ratio mirror is wider than that for a long f ratio mirror so the light fall off from centre to off axis is faster.

Long f length does give greater mag. than short f length for a given eyepiece. Note long f ratio doesn't necessarily do the same unless the aperture is the same.

Some non-corrected simple lens eyepieces do perform better at long f ratios, say around f15 perhaps lower. Note f ratio is the determining factor not f length.

Long f length mirrors are easier to figure but a little harder to test.

Apparent field of view is a function of the eyepiece.

A 16" f8 mirror would have a focal length of 3.25m and the focuser height would simply be too high off the ground for comfortable viewing when at zenith. However, such a telescope would indeed produce some fine images since it could have a much smaller secondary for better contrast. It would be a visual only instrument as the mount would have to be very large and sturdy to cope with imaging.

[torana68 (Roger)]

Originally Posted by mbo

The focal length thing has me a bit confused. This is what I gather so far, please correct any misconceptions. .

Assuming were discussing your std Newtonian, dont stress it, get an F6 as its a good in between, F4 is more specalised and you probably wont find anything much more then F7 in most shops anyway. Yes the longer the focal ratio the easier to make. Yes an F8 16" would be good but somewhat difficult, not impossible, to use ( ladder probably, would be awesome on planets)

[mbo (Pat and Kirk)]

Yes, I suppose you would really need to put big mirrored, long F Newts, into a special observatory sort of arrangement to be able to use then comfortably. It's slowly starting to make sense. F8 is great, but above 6'ish inches it's also very long, Cassegrains fold the long focal length into a short tube, but they have a greater number of critically shaped optical bits so they're more expensive. Refractors have no secondary obstructions and use all lenses, no mirror coatings, so are 'clear' but tricky to make in large apertures. All just physical compromises to achieve a given result.
If humans were all 10 feet tall we would see a lot more 12" F10 Newtonians aimed at the amateurs!

[mac (Matt)]

Originally Posted by mbo

If humans were all 10 feet tall we would see a lot more 12" F10 Newtonians aimed at the amateurs!

That's as good an argument for genetic modification as any I've heard!

[torana68 (Roger)]

Originally Posted by mbo

F8 is great, but above 6'ish inches it's also very long,!

8" f8 still usable on a GEM unless your a bit on the shorter side

[NorthernLight (Max)]

Originally Posted by torana68

8" f8 still usable on a GEM unless your a bit on the shorter side

Has anyone actually seen an 8" F8 newton in a shop recently. since photography became the focus for future use F5 or even 4 has become sort of a standard.
other point is mounting issues, balancing and wind load. if you´re alone at night and try mounting a big, long tube on a gem - can be tricky (though I´ve seen guys here mounting 12" newtons on an EQ6!) and the longer tubes introduce more vibration issues (esp with wind)- every touch of the focuser lets the image shake in your eyepiece for a while.

[Jeffkop (Jeff)]

Originally Posted by mac

That's as good an argument for genetic modification as any I've heard!