f ratio - eyepiece

[alro]

Hello again all .

i think i have read somewhere that you get best results from an eyepiece focul length matching that of the f ratio of your scope,

is anyone able to confirm this for me, or have i just got confsed along the long steep learning curve abd need re educated!

i know, fov, eye relief, exit pupil and even brand and design are also in the pot!

thanks again.

al

[alan potts]

Al,

So what are you saying, a 10mm eyepiece on an F10 scope? If that were the case with my Meade the power would be X304, not very user friendly.

alan.

[YKSE]

Al,

I think what you mean is the role of  exit pupils.

Exit pupil size is calculated by deviding a eyepiece's focal length by a scopes focal ratio. a 10mm eyepiece in a f5 scope gives 2mm exit pupil, while the same eyepeice in a f10 scope gives 1mm exit pupil. The most useful exit pupils are around 2mm (say 1.5mm to 3mm because of human eyes' acuity), that's what Alan means a 10mm eyepiece is not very useful in his f10 scope (1mm exit pupil, mostly for splitting tight doubles, and on Saturn, Mars).

[alro]

Alan, ykse,

thanks for pointing me in right direction. Advise as always is appreciated.

al

[russ.will]

I think Al might have been referring to the rough rule of thumb that the theoretical optimal resolution is reached when the EP's focal length equals the scope's focal ratio. This will deliver a magnification equal to the scope's aperture in millimetres and exit pupil will be 1mm, so it seems sensible enough to me.

Like all rules of thumb, it's open to bending and pushing, but I would have thought a 12" LX 200 could manage 300x quite happily, but the atmosphere may have other ideas! On a couple of really good nights, I managed to view Jupiter comfortably at about 4.5mm (Nag Zoom) in my F4.9 305mm Dob (occasionally) and 9mm on my 8" LX200 is useable on most nights.

I'm not sure it's a rule that holds up so well with refractors, as my ES 80 Apo merrily takes the 6.7mm barlowed at 140x, but I am by then in floater hell! I shall revisit that issue when my binoviewers arrive.....

Russell

[Charic]

For a Reflector telescope, I too work this theory, using the printed advice that my scope has an f/6 focal ratio. I'm finding that Planetary detail on Jupiter seems best around the 1mm-2mm exit pupil size for my eyes. This relates to EPs of 6mm ( f/6 ) and 12mm. The only issue with the longer focal length is the slightly smaller image.

[alan potts]

I find on the 12 inch that X304 is a touch too much most of the time, though I have to say I have not heard of that rule of thumb before, learn everyday. With that amount of power Russell correctly states the seeing will have a say in the matter and sometime it is loudly spoken.

Mars needs this type of power, Saturn can take it too and the Moon seems at home with even more. I don't much like these levels for star clusters finding X150-X200 is plenty and for me you get a tighter image all of the time. However sometime Al even X200 will suffer at the hands of seeing. You must remember though the size of telescope will always have a say, a 70mm refractor will not handle any of the magnifcations we talk of here well, except maybe x150 on a very good night.

We are all different and we all have our favourite eyepieces, if you are happy with what you like stick to it, don't let anyone tell you different

[John]

Over the years / scopes I've found that  better quality optics (ie: objective lens / primary + secondary mirror) with a smooth and accurate figure also help achive better performance at higher powers as well as the seeing conditions (most important !), aperture, good collimation and target selection as mentioned by Alan above.

As ever, a range of factors are involved !

[alro]

Again,

thanks alan, charic, john, russel and ykse for taking the effort in your descriptive replies, this is really appreciated and massively helpful to me. I seem to learn a little more about this each day.

al

[alan potts]

Al,

Don't ever think you are the only one learning, I didn't know of the rule of thumb that Russell mentioned. I am sure there are many more.

There was also the other night when I was like a baby trying to walk for the first time with my new Dobsonian scope, in truth I was rubbish!

alan

[alro]

Thanks Alan

onwards and upwards, every day is a school day!

i must have stumbled by accident on a brief explanation of the rule of thumb, could not remember where it was or even if i had just got confused.

So i just asked the question and i am delighted with help and responses.

i think i have nailed my goto set up now, seems much more accurate aligning true.

Your new dobsonian, it must be a good size and stunning views i bet.

Al

Al

[BVietje]

Air,

Depends on what you mean by "the best results".

Using an eyepiece FL that equals the focal ratio of the primary mirror or objective is the minimum magnification that allows you to see an Airy disc and evaluate the optical quality of the instrument.  This is the way we star test telescopes at Stellafane.  A lower magnification might not show the errors in the final figuring of the optics, so we require an eyepiece FL equal to or less than the f-ratio of the optic being evaluated.

Now, as a practical matter, that could be way too much magnification for the seeing conditions on a given night, but when seeing is steady, you might approach the optimal performance with an eyepiece (or eyepiece-Barlow combination) of that effective FL.  If not, there could be other factors at work, like tube currents, colllimation issues, or even -- and hopefully NOT -- optical defects.

You'll find many of the entry-level telescopes show minor defects when critically star tested, as you might expect.  Most casual observers would never notice.  When you purchase a high-end instrument and spend some real money, you expect great optics, so if you can detect zonal issues, a turned down edge, or other correction issues, you may want to explore getting the manufacturer to come clean, and replace the instrument with one that won't fail the test.  If you make your own mirror, things can look pretty good until you get a night with steady air, and very critically evaluate a star at pretty high power -- thus the guideline for a minimum magnification for a star test, which could reveal very subtle figuring issues, if present.

Clear skies,

Brad

[ronin]

I tend to use f number=focal length simply as it means that the magnification = Objective diameter and as a reasonable guide you can expect a magnification equal to the diameter without too much trouble. It additionally gives an exit pupil of 1mm, which is convenient.

Have read elsewhere that this is "optimal" and whoever said supplied a stack of maths supporting it.

I didn't bother to read it or take it in.

The 1mm exit pupil is fair, but others will say that 2mm is optimal, so then the magnification equals half the objective diameter.

That seems a little low, I do not expect my 90mm ED to be at it's "best" at 45x. Actually would be very much unamused by it. Don't think Alan would like the idea that his Sumarian was limited to 230x before it all went down hill.

Going the other way have read that at 0.5mm and smaller exit pupil size then problems of the eye appear, floaters are more apparent. And magically a 0.5mm exit pupil means a mag of two times the objective diameter and that is the commonly quoted mag magnification.

So which one do you want?

As said I use focal length = f number for the simplicity and it also seems to be fairly realistic.

It is easy to work with and is I find a reasonable approach to selecting, or at least initially thinking about, eyepieces for a scope.

Again the eyepiece needs consideration, I would say a 4mm unnamed plossl was asking for trouble but a 4mm TV Ethos would I hope work well.

I have no idea if TV make a 4mm Ethos, but you can see what I mean.

[alro]

Thanks again Ronin for your detailed guide.

i think i like it for simplicity!!

al

[BVietje]

Again, if I may w/out offense, "Ideal" is pretty subjective, and depends on what you are viewing.

Jupiter/Saturn/Mars:  1mm - 2mm exit pupil may be ideal.  Most would say as much magnification as possible w/out a loss of resolution, which depends on the seeing.

Comets, the Rosette Nebula & such:  shoot for an exit pupil that comes close to your dark-adapted eye -- corrected for the light pollution present, and what pupil size you can achieve.  As much as 7mm in a dark site/young eyes, probably 5.5 for me in a dark place, but in my mid-50's, and possibly 4mm or less from light polluted skies.

Clear skies,

[Stu]

I agree with the advice given, with the caveat that small, high quality refractors will take an exit pupil down to around 0.5mm for planetary and sometimes more for doubles. I would not like to be limited to x85 in my 85mm, it will handle x160 or x180 depending upon conditions

[Alien 13]

I have often wondered about exit pupil size of a scope/eyepiece combination when for example the result is an exit pupil size of say 6-7mm giving the optimum for averaged surface brightness of a set of objects but does this in effect cause the iris to get smaller.

I think what I am trying to figure out is whether anyone has measured their pupil size when observing with various eyepiece combinations so they can effectively hit the sweet spot when observing.

Alan

[John]

I agree with the advice given, with the caveat that small, high quality refractors will take an exit pupil down to around 0.5mm for planetary and sometimes more for doubles. I would not like to be limited to x85 in my 85mm, it will handle x160 or x180 depending upon conditions

Quite true. I frequently use 257x and even 300x with my ED120 on Saturn, Mars, the Moon and binary stars.

I was aware though that the theoretical maximum resolving power of a scope is reached at much lower magnfications than this and I think that is roughly the same as the aperture in mm. My ageing eye just seems to need the "helping hand" of a bit more image scale to detect what the objective has delivered though

[bingevader]

Rules of thumb would be great if we all had the same sized thumbs!

For me, there're just too many factors involved too be worrying about any rules.

You just have to pop in what works best on a particular evening.

I know that doesn't help much if you are just setting out on your 'scope adventure, but that's why the general advice here is to get to know your 'scope first before rushing into any eyepiece purchases.

I've taken a bit of time to select mine, and I'm glad I did.

[A-N-Y]

Hi Al,

I'm still at the starting line of the learning curve, and through my reading I found this interesting article discussing the topic in hand, which do agree with what some members have noted.

https://www.astronomics.com/eyepiece-exit-pupils_t.aspx