F ratio performance factors - teach me please

[SteelRat]

Greetings learned folks A little education from you experts-in-field please;

From what I have read/picked up here - would I be right in thinking that with good seeing conditions an f8 newt makes a better planetary and lunar scope than an f5 of equal aperture? Where the f5 will be better for wide field views for obvious reasons.

(i) Could someone explain to this 'igno' (in easy-speak if pos) just why the longer scope gives more detailed views?

(ii) At what size aperture telescope would the difference really come into play for visual use.

(iii) How would the two scopes compair when viewing globular clusters?

(iv) And how would they compair when viewing such nebulae as M57?

Cheers in advance :?

[geppetto]

SR

For viewing/imaging planets you need to get close in (higher magnification)

For instance, a 200mm F5 scope has a focal length of 200*5 = 1000mm

The magnification of the scope with a given eyepiece is focal length divided by the focal length of the eyepice.

So, with this scope and say a 20mm eyepiece, the magnification is 1000/20 = 50 times magnification.

With a 200mm F8 scope, focal length is 200*8 = 1600 so with the same eyepiece, you get 1600/20 = 80 times magnification.

So the higher the F number of the scope, the more magnification you can achieve

[SteelRat]

So the only difference in performance at the same mag would be down to the different eyepiece needed to reach that mag? That answers all of em really - think I must've tripped myself up on this one long ago. You know I REALLY DID THINK there was more to it than that. Too much SCANNING and not enough actual READING going on perhaps?

[SteelRat]

Am pretty sure that there IS a difference in performance with achromatic refractors of varying f ratios but is that ONLY in the amount of false colour and that 'purple halo'? Nothing at all to do with detail? Or is that just down to eyepiece quality and characteristics too? :?

[Andrew*]

well, as I see it, if you use an f# that's well suited to the object, you're putting less strain on the other optics and the focuser. An f/5 is best suited to DSOs because you're getting more light and smaller magnification. For planets, this extra aperture is not needed - you rather need the higher magnification. An f/5 will be okay on planets, but you will need to use very high power eyepieces, thus putting the strain on the optics, rather than the natural length of your scope.

Also, I think there's something about difficulty of focus when using the "wrong" f#

I hope that makes some sort of sense. I'm not at all sure of myself, but that's just what I've worked out.

Andrew

[FLO]

There are a number of factors at play but perhaps the most significant is that, for a given aperture, a fast scope (about f5 and below) has a shorter focal length than a slow scope (around f8 and above) so it's mirror/objective must bend the light into a steeper light cone that requires more correction at the eyepiece. Also, the mirror or lens that produces the light cone requires more curvature so is more difficult/expensive to make. And, fatter lenses with more curvature tend to act as prisms that separate the lights individual colour components - which is where the false colour comes from.

[SteelRat]

Thanks folks

So, longer (SCOPE) has many benefits in use and during manufacture - until it's time to ask it for real wide views that is; tough/expensive to get x20 mag from a 1000mm length scope?

Steve - that curvature issue... does that mean that a longer newt would be easier/less critical to collimate?

[FLO]

Steve - that curvature issue... does that mean that a longer newt would be easier/less critical to collimate?

Yes

Probably worth mentioning that mirrors don't act as prisms so have no false colour

[SteelRat]

S'rite mate - got that bit But might a longer newt be easier/less critical to collimate?

[geppetto]

It's noticably easier to get my F6 tweaked than my F5 that's for sure

[FLO]

S'rite mate - got that bit But might a longer newt be easier/less critical to collimate?

Sorry, I must have been ambiguous.

Yes, definitely, the longer focal length, slower Newtonian will be easier and less critical to collimate

[GazOC]

Another factor is that in scopes with a central obstruction (Newts, SCTs, Maks etc), the larger the f# is, the smaller the secondary mirror needs to be so the views should have more contrast.

For instance a f12 Mak has around a 34% CO but a f15 Mak only 'needs' 24%.

[GordonCopestake]

Dont forget that the slower a scope the more forgiving it is on eyepieces too. You dont need an eyepiece case full of naglers with an f/10 'scope but you do with an f/4 'scope.

[Andrew*]

stop it! STOP ITT!!! You're making f/5 scopes sound like nightmares! I'm just about to get one, and I hope it won't lead to hundreds of pounds spent on very fancy eyepieces and collimation tools. So the only advantage of a low focal ratio is shorter exposure times for imaging and more light for a given magnification??

Oh well

Andrew

P.S. sometimes you really shouldn't take me seriously!!

[GordonCopestake]

You can't have your cake and eat it unfortunately

[geppetto]

You can't have your cake and eat it unfortunately

Too true..

This is one of those threads that inevitably leads to the fact that just

one scope aint good enough.

You need lots of different scopes 8)

[GordonCopestake]

I've slowly come to the conclusion that I need a 4" f/5 APO and an 11" f/10 SCT.... and maybe a 24" f/4 dob (with step ladder)

I think that combination would cover all angles

fortunatly i can't afford any of the above at the moment

[John]

stop it! STOP ITT!!! You're making f/5 scopes sound like nightmares! I'm just about to get one, and I hope it won't lead to hundreds of pounds spent on very fancy eyepieces and collimation tools. So the only advantage of a low focal ratio is shorter exposure times for imaging and more light for a given magnification??

Oh well

Andrew

P.S. sometimes you really shouldn't take me seriously!!

Don't worry Andrew - I think the solution for you is to stick to standard field eyepieces like plossls which work well enough at F5 and are reasonably priced - you will run into problems if you start using budget wide field eyepieces in your F5 scope - the short focal ratio shows up why they are in the budget category. I think you will need a collimation tool of some sort though if you want to get the best out of your newt - plenty of advice on SGL on which are the most useful and they are not too expensive - I picked up a cheshire collimator for about £20.

John.

[GazOC]

stop it! STOP ITT!!!

Well you did ask mate.......

[barkis]

Steelrat,

Resolving power is a function of the diameter of the mirror.Along focus Newt by definition would require a smaller secondary and there,fore less central obstruction, which is the bane of definition especially in planetary detail. If the Newtonian is designed from the outset to function as a planet scope, it will perform that job very well. The mirror will bring all the light to the same focus, therefore no chromatic abberration. Also if the mirror is of a sufficient focal length, it can remain a sphere. That means it will not require parabolising.

Hope this answers a little of your questions.

Barkis.

[tony4563]

Andrew. You can still get great performing EP's for your f5 scope without spending a fortune. Have a look at the Baader Hyperions which are clones of the Vixen LVW's. Performance is said to be very similar...65 degree FOV, 20mm eye relief and stars pinpoint to near the edge. The Burgess Optical TMB's are having great reviews too. Check 'em out!

[GazOC]

I agree with Tony, I have a few Hyperions and they are very good in my f4.5 Dob.

[SteelRat]

Thanks people. Longer (higher f#) Newt = smaller central obstruction = higher detail = better planets.

Cheers

[FLO]

Almost

A longer focal length and higher f-ratio allows greater magnification and a smaller central obstruction - which increases contrast.

Detail (actual resolution) improves as the aperture size increases (which, unless you also increase the focal length, lowers the f-ratio).

[SteelRat]

Cheers. Suspected there'd be something more to it than just eyepiece characteristics.