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What does "Sharp to the edge" actually mean?


F15Rules

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One of the many measures of an eyepiece's attributes that we visual observers often consider is a "must have" when deciding whether or not to buy it, is "is it sharp to the edge?"

I myself have often looked for, or commented upon, the "sharpness out to the edge" that an eyepiece delivers.

In my own case, and perhaps because I use a driven equatorial mount, I have always looked at this horizontally, ie panning from east to west and vice versa..

But what about from north to south and south to north?

I've recently been looking at different eyepieces which might be suitable for using with my new Maxbright II binoviewer, and during a session the other night, I was panning left to right and vice versa and estimating how far (as a percentage) from the centre of the field of view, a given eyepiece pair stayed sharp. 

I find that using the Trapezium in M42 works well, as the Trap presents a fairly small trapezoidal shapes asterism of 4 fairly faint mag 5-6 stars, which is very distinctive: by panning slowly out from the centre, it is very obvious at which point the Trap's shape begins to distort (or whether it stays sharp). 

For example, using a pair of older Meade 26mm MIJ wide angle Pseudo Masuyama eps, the view didn't start to become less sharp until about 95% from the centre - an excellent result in my F8 refractor.

By comparison, a pair of William Optics clone 20mm wide angle eps began to soften the Trap's appearance from about 80% from the edge..quite good, but noticeably less good than the Meade pair.

Now, I decided next to check for sharpness to the edge, going north to south and vice versa. The result was the same, in that the Meade pair were sharper than the WO clones moving from the centre to the edge.. however, I did notice that the "sharpness from centre to edge" for both pair of eyepieces was significantly worse moving North to South than for moving East to West..sharpness north to south definitely seems less good than east to west.

So..my question is, why does it seem that edge to edge performance is less for north to south than it is from east to west?

I have found this to be true using single eps as well as pairs..so it's not the Binoviewers that are causing this effect..

Discuss please?🤔😊

Dave

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I don't know the answer to this but am interested in the discussion. I'll have a look at this the next time I'm out and see if I notice the same thing. I guess one thing to do to that might eliminate the eyepieces themselves as suspects would be to rotate the eyepiece 90 degrees and repeat the test, then if the result is the same then it would suggest the eyepiece is not the issue. 

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I think that such measurement is best performed with calibrated instrument rather than one's eyesight.

Cause of difference in what should be otherwise rotationally symmetric view can be for example due to astigmatism in observer eyes.

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12 minutes ago, F15Rules said:

during a session the other night, I was panning left to right and vice versa and estimating how far (as a percentage) from the centre of the field of view, a given eyepiece pair stayed sharp. 

Slow night Dave? 😊

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I learned what sharp to the edge meant Dave, when at the age of 12, I decided to have a go with my dad's single blade Wilkinson Sword razer. All went well as I shaved the soap from my cheeks in a downward motion, but it was the single sideways stroke across my top lip and the lather suddenly turning red that made me think eek! That's got a sharp edge!!. 

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Perception enters into the picture.

What is "sharp to the edge"?

Is it an image that is less than the Airy disc in side at the field stop?

Is it an image that is 10' or less in size at the edge (Ernest Maratovich's criterion).

Is it a spot size of 1µ, 2µ, 5µ?

Is it in a scope that has coma?  Or in a coma-corrected scope?

And, what bothers you?

And, at what angle of looking through the eyepiece is the criterion evaluated?

I see a lot of comments about how this or that eyepiece is "sharp to the edge", when I know, from experience, the eyepiece has serious edge of field astigmatism.

So does "sharp to the edge" mean the observer simply can't see it or does it mean "the field being evaluated is sharp to the edge" when that means well shy of the edge?

It is a vague and nebulous term unless what is meant is defined.

 

Dave's comments are not unexpected, since human vision may be horizontally or vertically astigmatic.

But if direct vision was used for the evaluation, that is ruled out.

 

Far more likely is that the focal plane is tipped vertically and not horizontally, and that can easily occur in a refractor with a heavy eyepiece.

One way to test is to loosen the eyepiece and press it into a different vertical angle.  If the edge improves, it was sag.

 

 

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3 minutes ago, Mr Spock said:

Do we need eyepieces to be sharp to the edge though? Mostly we are looking in the middle so really the last 5% isn't that great a loss.

De gustibus non est disputandum.

My #1 criterion for keeping an eyepiece is that a tight globular star cluster stays in tight focus as it leaves the field.

If it doesn't, I sell the eyepiece.  Sharp to the edge in a short f/ratio scope is my main criterion.

[of course, there are lots of others, but it has to pass that before any other criterion is considered]

I use a non-driven scope and I often watch an image cross just about all the field, maybe not the exact edge to the exact edge, but close.

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2 hours ago, Mr Spock said:

Do we need eyepieces to be sharp to the edge though? Mostly we are looking in the middle so really the last 5% isn't that great a loss.

At lower powers, yes.  But as power increases in a non-driven mount, having wider field, "sharp to the edge" eyepieces helps to increase dwell time between nudges.

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The term sharp to the edge could be rooted in Al Naglers desire to design a device that allows his superb eyepieces to perform well in fast newts.

From an article years ago, on TVs website

"Then Mike [Lockwood] and I worked with Al [Nagler] in developing and testing the new advanced model of the Paracorr. The images through the Ethos eyepieces are sharp to the edge of the field with only the slightest falloff at the extreme edge when observing bright stars. When I saw that, [Lockwood] and I looked at each other and said, ‘Well, Al Nagler just gave us permission to bring f/3.3 telescopes to market.’ "

The Paracorr (parabola corrector) allows TVs eyepieces to be sharp to the edge in fast dobs.

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“Sharp to the edge” is valuable for the way myself and many others use their scope.  

For me, using non driven scopes, I use low power wide field to find the object.  Many faint open clusters would  be invisible near the edge of field in low quality eyepieces.  In a better eyepiece the object becomes visible near edge of field when searching. Having found my target I can centre it, then increase power for a better view. The same will apply for faint fuzzies.

Having found the object I could live with less good eyepieces for studying it. Thats because I can keep the object away from the edge.

Plus of course “sharp to the edge” is subjective. What for one person is ok, may not be for another. I’m fussy but try not to be over fussy.  “Fussy” and “Over fussy”…..two more subjective descriptions😊

 

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Thanks for all the replies so far🙂.

I hadn't considered astigmatism in my own eye (s)..

I am sure it's not the binoviewers as I've noticed the effect before I acquired these, both with cyclops viewing and with my previous binoviewers. I've also noticed this with ordinary binoculars, of which I have 3 pairs, all of known good optical quality.

I don't think it's the diagonal. My main one is a Baader Zeiss BBHS prism and I have also an excellent Astro Tech 2" mirror diagonal.

Don mentioned possible "sag" due to weight..I will look at this as I do use a couple of heavy eyepieces (Axiom LX 23 & 31mm), but both are currently decloaked so much less heavy now.

My binoviewers have some heft, too, but not excessive at all.

My FS128 has a very solid focuser: although I can occasionally see a small amount of image shift at high powers, I don't think the focuser is to blame for the above phenomenon, and again, I still see the "north/south" vs "east/west" difference even with binoculars and a range of eyepieces.

All this makes me think the fault has to be with my own eye(s)..

I do know that my right eye (my natural bias is to my right side) has deteriorated noticeably in the past 5 years or so, to the point that I had to train myself to use my better, left eye to be my main observing eye. I don't recall testing this perceived discrepancy one eye at a time, so I shall check that out. I am due an eye test soon, so I will discuss this with the optician also.

Finally, one thing that I am very sure of is that with my new binoviewers my good left eye helps to lift the perceived performance of my poorer right eye. I am certain of this. Viewing Mars the other night, the view through both eyes was much clearer, more comfortable and more satisfying than the cyclops view through either eye - no doubt about it.

The new Maxbright IIs are excellent..but that's another story..😊

Dave

 

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What about _scope_ field curvature, in, say, a short focal-length refractor? Obviously one shouldn’t condemn an eyepiece on the basis of a feature of the scope.

I recently managed to achieve visual focus with my SW Evoguide 50, which has 240ish-mm focal length. It has strong field curvature. With my Panoptic 24, the centre was lovely and sharp, a match for my Leica 10x50s. However the edge was markedly blurred. But a slight turn of the focus-knob brought the edge to equally nice sharpness.

Whereas a poor eyepiece will still look horrid at the edge after a refocus attempt.

So in this context I would forgive the “de-focus” aberration when judging an eyepiece and consider only the others. Is that valid?

Magnus

Edited by Captain Scarlet
condemn rather than reject
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Field curvature often gets forgotten as a source of edge distortion. A ST120 with an ethos 21 sounds like the perfect combo for widefield views, but FC really takes the edge off the experience. 

I have 2 scopes that are designed primarily as astrographs; one is a TS65Q and the other is a Pentax SDHF75. Both have a flat focal plane built-in for the camera. When teamed up with an eyepiece also having a flat focal plane the view across the whole field is just insanely sharp without any need to refocus. Optically they are the perfect match for Delite and  the smaller Ethoses . 

The tragedy is that it only works reliably for 1.25" format eyepieces with a 1.25" diagonal and reducer. Using a 2" diagonal requires too much in-travel for many fine eyepieces......why, oh why can't they make an astrograph that will handle both? A 13mm ethos will focus but the 21mm won't...

Similarly on Newts, the better coma correctors also flatten the field as well as sorting the coma, getting the best out of flat-plane eyepieces.

Edited by rl
spelling
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14 hours ago, F15Rules said:

Now, I decided next to check for sharpness to the edge, going north to south and vice versa. The result was the same, in that the Meade pair were sharper than the WO clones moving from the centre to the edge.. however, I did notice that the "sharpness from centre to edge" for both pair of eyepieces was significantly worse moving North to South than for moving East to West..sharpness north to south definitely seems less good than east to west.

Did you rotate you whole head 90 degrees to either side to eliminate the possibility it was something to do with your eyes or perhaps even your visual processing complex?  That way, you're looking side to side along both axes of the eyepiece/diagonal/telescope combination.

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57 minutes ago, Louis D said:

Did you rotate you whole head 90 degrees to either side to eliminate the possibility it was something to do with your eyes or perhaps even your visual processing complex?  That way, you're looking side to side along both axes of the eyepiece/diagonal/telescope combination.

No, I didn't Louis, but good idea, I will try it..as mentioned above, I do think it must be something to do with my eyes, not my equipment..🥴

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Many years ago, I DARED to ask what eyepiece astigmatism *really* meant. Someone
(on CN) produced an excellent "flower" pic! Very different from eyeball astigmatism! 😉

[Unfortunately, I cannot find it now!] 😉

Astrigmatism... Field curvature? A lot of us bandy terms? I'm "guilty as charged" too!
Some of this might be a problem with the "Sharp End" of the telescope? 😛
It seems difficult to illustrate/quantify "sharp" in a visual scope sense.


 

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11 hours ago, Captain Scarlet said:

What about _scope_ field curvature, in, say, a short focal-length refractor?

Great you brought this up Magnus.

Al Nagler designed the PC to give a parabolic scope the same field characteristics as a 4 element refractor, which has a flat field.

Mr Nagler, from his site:

"In 1989 I decided to try to achieve the same field quality for parabolas as reached by my Nagler eyepiece and 4-element refractor designs."

Its kind of hard to be sharp to the edge using a scope with field curvature IMHO.

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