Do telescopes have a 'sweet spot' that works best with a certain eyepiece?

[Nova Load]

I was reading an article on the Cloudy Nights website that says that telescopes produce their clearest images when used with a particular sized eyepiece. Does anyone know if this is true?

The 'sweet spot' is calculated by multiplying the focal ratio by 2, in other words an F/10 telescope should produce it's clearest images with a 20mm eyepiece, an F/8 with a 16mm, F/5 with a 10mm eyepiece etc. 

[ronin]

That would be the 2mm exit pupil, the problem with that is that on CN about 3 years ago someone was explaining that a 1mm exit pupil produced this "sweet spot". They even supplied the maths behind it.

So do you want 2mm or 1mm exit pupil?

Seems that both/either are valid from the same "source".

I would suggest you "ignore" these rules.

The common one is max mag = 2x diameter in mm.

I guess that 80-90% of magnification problems asked here are that the user gets nowhere near this 2x amount.

At that failure rate it is a pretty suspect rule.

Bit like "We have this cure for your ailment, only 90% of the patients have died."

[LeeB]

its a fine question, at 1-2mm exit pupil (for a number of reasons) the image will be nice and sharp, eye lens defects will be reduced etc.

However when at a dark site and wanting nice bright wide images I like a 6mm exit pupil , as an added benefit it also helps reduce eye floater effects.

I sometimes think these magazine articles are written for something to write, as above stick to 1-6mm exit pupils (assuming your pupils are not smaller) and you will be fine. I push to 0.75mm or even 0.5mm occasionally when skies allow.

[rwilkey]

I always find that an eyepiece that produces the optimum image for me is one that gives magnification near or equal to the telescope aperture, so for my 100mm ED, the optimum mag would be 100x, just a 'rule of thumb' guide that I use.

[damnut]

Hi There,

I cannot find anything in relation to the 'sweet spot' when dividing the focal ratio by 2.

What I have found applying that rule to my scopes is, .... strange??

st120   F5 FL 600                  (F/2) = 2.5  apply that to the sweet spot   600 / 2.5  = 240x

MAk 150mm F12  FL 1800   (F/2)= 6                                                    1800/ 6      = 300x

Ev 120 F10 FL 1000              (F/2)=  5                                                  1000/ 5      = 220x

Ph 127 F9.5 FL 1200             (F/2)= 4.5                                                1200/4.85  = 266x

Meade 152mm F5 FL 760     (F/2)=  2.5                                                 760/2.5     = 304x

What the calculation is giving is the theoretical maximum magnification useable for each scope. This is usually  diameter *2 in mm.

So whilst this is consistent in providing the scopes limit, it would not be what I would call a sweet spot for an ideal focal length for an eyepiece.

[alan potts]

I don't take a blind bit of notice about such things, if we stuck to 1mm rule it would give me such a massive magnification on the LX 12 inch that it would be unuseable. The two time rule is also rubbish on the same scope, imagine using X610, not a lot of good I would have thought. I think Robin's maths works very well on refractors and the like but even X305 on the LX is really hard work in anything but excellent seeing but that is another issue altogether.

I personally also feel it differs a lot depending upon what you are looking at, take Jupiter, for me optimum is between X160-200, maybe a little above. I tried Juptier on X325 in excellent seeing the other week on an 18 inch scope, there was absolutely nothing to gain by me doing this, if anything the image was degraded somewhat and that type of power is well within the scope.

I tend to agree with Lee, magazines looking for things to write.

alan.

[damnut]

Agreed Alan,

That is my point above,  formulae are all well and good, but take no count of atmospheric conditions at any one time. Rarely do I use more than 150x. Usually 40-100x.

[YKSE]

I was reading an article on the Cloudy Nights website that says that telescopes produce their clearest images when used with a particular sized eyepiece. Does anyone know if this is true?

The 'sweet spot' is calculated by multiplying the focal ratio by 2, in other words an F/10 telescope should produce it's clearest images with a 20mm eyepiece, an F/8 with a 16mm, F/5 with a 10mm eyepiece etc. 

The 2mm exit pupils is likely based on research results of our eyes, i.e. larger than 2mm exit pupil, the eyes are aberration limited, and smaller than 2mm, our eyes are diffraction limited, as mentioned here:

Since we're different, certain variations do exist, so a general 1-3mm exit pupil range should cover mostly, also it depends on what're you're observing, doubles stars and faint extended nebulae are at the extremely ends of usable exit pupil range.  RN. Clark's page worths to spend some time on.

Thinking in exit pupil is a better way than in magnification, Alan's Jupiter observing is a very good example. 325x in 18"(i.e. 450mm aperture) means an exit pupil 450/325=1.4mm, for any F10 SCT (8", 9.25", 10" or 12" whatever) it is a 14mm eyepiece, I'd think it's a perfectly good selection of eyepeice in good seeing. The same eyepiece Alan used in 18" should work perfectly all right in any f5 scope for viewing Jupiter with the same exit pupil, but different magnifications. And this, is what I see one of the good reasons to understand the exit pupils. 

A lighter but very good link for those interested.

[Lowjiber]

As magnification increases, surface brightness decreases.  It's generally accepted that an eyepiece with a focal length of two times the scope's focal ratio (expressed in millimeters) gives the optimum combination of brightness and magnification.  There are other ways to arrive at that figure, but it works fast for me as I change scopes.

[alan potts]

Yong,

I didn't stop and do any maths myself as I tend to let conditions decide what I can use but 14mm on the 12 inch with it's 3048mm of F/L is about as high as I ever go on a very good night. As you probably know more aperture often means worse seeing as they are more affected by poor conditions, I have before now closed the obsey with the 12 inch and got out the 115mm only to find I could get 80% of the power with no seeing problems as such.

Lowjiber,

again there we are back to in my case around X150 that being the case I could not agree more with that statement. if I understand correctly, the 18inch is F4.33 so X2 is 8.66 that give a power of about X265 give or take and that works pretty well too

Alan

[YKSE]

Alan,

Yes, bigger scopes are more seeing limited than small ones, that's why 8" SCT can use smaller exit pupils than 12", yet, we're still talking about 1mm(maybe down to 0.7mm, 0.8mm) to 2mm exit pupil range for Jupiter.

BTW, calculating exit pupil of a f10 scope is a little easier than calculating magnification an eyepiece given, you just need to move the decimal of eyepiece focal length one step to left.

[Nova Load]

As magnification increases, surface brightness decreases.  It's generally accepted that an eyepiece with a focal length of two times the scope's focal ratio (expressed in millimeters) gives the optimum combination of brightness and magnification.  There are other ways to arrive at that figure, but it works fast for me as I change scopes.

I think this is what the article is suggesting. There is also something on the Tele Vue website about how high magnification darkens the background and improves contrast. Perhaps this 'sweet spot' is about finding the perfect balance between brightness, contrast, size and detail.

[gooseholla]

I just put in an eyepiece until I find one that makes images look good on that night. It then pretty much stays in there all night. Forget the theoretical - practical is of more use!!

John

[chiltonstar]

I'm with John on this! Suck it and see - rules are probably written for average conditions, average eyes, average optics. A bit of experimentation may produce a far better view.

Chris

[YKSE]

I'm glad that we have choices to find suitable ways to the best of all of us.

[jetstream]

The theoretical, which has been proven by observation, is of great importance when choosing eyepieces if views want to be maximized. Of course we could buy eyepieces of every focal length and try them to find which is best, for what.... and of course a good zoom eyepiece could be purchased to give infinite choices within its range...

YKSE's post #8 is an excellent resource and so are the links in it.IMHO.

[jetstream]

As magnification increases, surface brightness decreases.  It's generally accepted that an eyepiece with a focal length of two times the scope's focal ratio (expressed in millimeters) gives the optimum combination of brightness and magnification.  There are other ways to arrive at that figure, but it works fast for me as I change scopes.

This is a good way to end up at the 2mm exit pupil, which has been recommended for galaxies (M31 excepted) and confirms the theory behind this choice.  After a while we just reach for the right eyepieces for whatever objects, at least I do personally.

I agree extended objects surface brightness decreases with increasing mag, but the increased object size in the eyepiece more than offsets this up to a point, re Clark, etc.

[alan potts]

Gerry

The only down side of that is the other night my 15mm TV Plossl was superb on Juptier in the 12 inch, better than the 14mm Delos. The night before it was the other way around. In the same scope also the 14mm D was better than the 17mm E the other night, so never the same as far as I can see, everything else was the same on given nights.

Alan

[jetstream]

Gerry

The only down side of that is the other night my 15mm TV Plossl was superb on Juptier in the 12 inch, better than the 14mm Delos. The night before it was the other way around. In the same scope also the 14mm D was better than the 17mm E the other night, so never the same as far as I can see, everything else was the same on given nights.

Alan

Yes, seeing and the refractive index of the atmosphere can work better with some glass/coatings than others at times. I have never had a plossl beat my other good EP's really- other than use of the plossl's narrow FOV to keep bright stars out of the scene.

I do find, like you, that planetary views can change a bit ( more than other objects) with different EP's near or at the same fl. The Docter 12.5mm UWA barlowed has beat the Leica zoom a few times now on planetary/lunar and yet they are both excellent and better than most other eyepieces I own or have owned for this use. Strange how individual eyepieces work isn't it?

[bingevader]

It's always interesting to discuss these things and understand the physics behind them, but as others have said, I wouldn't get to hung up on it, there are just too many other variables.

Whilst I may not have an eyepiece at every possible FL, or a zoom EP to fill in the gaps, conversely, I'm highly unlikely to ditch them all, except my 12mm EP, to get the "optimum" from my f5.9 'scope.

I rarely go below 14mm on DSOs or above 8mm on planets, therefore I hardly ever use my 'scope at its "optimum".

[YKSE]

Gerry

The only down side of that is the other night my 15mm TV Plossl was superb on Juptier in the 12 inch, better than the 14mm Delos. The night before it was the other way around. In the same scope also the 14mm D was better than the 17mm E the other night, so never the same as far as I can see, everything else was the same on given nights.

Alan

That's just another case for understanding exit pupil, IMHO.  As mentioned before, under 2mm exit pupil, our eyes are diffraction limited, i.e. it's affected more by the seeing than larger exit pupils, therefore it calls for tighter spacing of eyepieces for under 2mm exit puil, a zoom is a good tool for this purpose.

Another way to see it is for people wearing glasses, in low power eyepieces, the astigmatism(an aberration) in the eyes will show up to mess up the view, but under 2mm exit pupil, the aberration the our eyes will not be dominant, i.e. we can take off our glasses without having astigmatism ruin the view.

[alan potts]

Yes, seeing and the refractive index of the atmosphere can work better with some glass/coatings than others at times. I have never had a plossl beat my other good EP's really- other than use of the plossl's narrow FOV to keep bright stars out of the scene.

I do find, like you, that planetary views can change a bit ( more than other objects) with different EP's near or at the same fl. The Docter 12.5mm UWA barlowed has beat the Leica zoom a few times now on planetary/lunar and yet they are both excellent and better than most other eyepieces I own or have owned for this use. Strange how individual eyepieces work isn't it?

Gerry,

Indeed they are both excellent eyepiece and you are very lucky to have them. Like myself having a few half decent eyepieces we are able to make comparisons though sadly many are not. I am absolutely sure anyone would be more than happy with any view from the type of glass we keep in our cases.

Alan

[alan potts]

That's just another case for understanding exit pupil, IMHO.  As mentioned before, under 2mm exit pupil, our eyes are diffraction limited, i.e. it's affected more by the seeing than larger exit pupils, therefore it calls for tighter spacing of eyepieces for under 2mm exit puil, a zoom is a good tool for this purpose.

Another way to see it is for people wearing glasses, in low power eyepieces, the astigmatism(an aberration) in the eyes will show up to mess up the view, but under 2mm exit pupil, the aberration the our eyes will not be dominant, i.e. we can take off our glasses without having astigmatism ruin the view.

Yong,

I would have thought the difference between 15mm and 14mm in a 3.0m scope was not that important, in that range eyepieces only really come in 1mm steps at best then not in the same design, however the 17mm E and the 14mm D I can agree there is a difference, but I would not call it massive, I think it is more down to size of the FOV and general eyepiece design. As you know almost all of mine are from one manufacturer.

Alan

[-Aldebaran-]

I just put in an eyepiece until I find one that makes images look good on that night. It then pretty much stays in there all night. Forget the theoretical - practical is of more use!!

John

Simple and to the point, seconded.

[jetstream]

The OP asked a good question in this thread and is important especially if purchasing a few eyepieces to start with. Many will recommend 3 eyepieces to begin with and if well chosen, following some of the guidelines presented here, will ensure early success in astronomy.  It is possible to take any scope of any aperture and focal length and use some of this info to pick eyepieces well suited for widefield, DSO and planetary- these guide lines will come very close to providing near optimum eyepiece choice, regardless of brand and design. IMHO.