What does "Sharp to the edge" actually mean?

[Stu]

I normally refrain from making those comments as I’m never sure if it’s my eyes or the eyepiece/scope combo. Everyone raves about the Morpheus being sharp to the edge, but I’ve not found that to be the case in all scopes. I do think it is important to be clear which scope is being used as it makes a big difference to the result. I seem to recall that the FS60C was a bit rough towards the edge with a Morpheus 17.5mm but it tightened up a lot when I added the CQ Extender module. I’ll dig out the report.

Probably my most consistently sharp to the edge eyepiece is my 24mm Panoptic which gives excellent results, but I know it has other distortions that allow this ie I don’t enjoy view the Moon through it off axis because it appears like an egg!

Best widefield combo is my Televue Genesis (petzval design with a nice flat field) with 31mm Nagler. 5 degrees of loveliness. The 30mm ES 82 degree did not perfrom as well, not surprising as I’m sure the Nag was designed to work with flatter field scopes.

[Stu]

Some more information here:

 

[josefk]

23 hours ago, F15Rules said:

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

 

I'm not dismissing differences and characteristics inherent in kit - not at all - but this also reminds me of commentary sometimes on HIFI forums were such and such a sonic characteristic of a hifi component comes under scrutiny - i would posit that for all our senses our own equipment - eyes in this case (and ears in the HIFI world) - are the biggest variable. i think we have very untrustworthy sensors and senses.

I agree with you binoviewers and binoculars definitely seem to help cancel out individual eye characteristics in a very positive way.

[JOC]

I forget who it is, but one of SGL's contributors (maybe one of our USA based members IIRC?) often posts pictures that he has on record from taking images of a ruler with various EP's - I think my understanding of sharp to the edge means that the image is sufficiently sharp across the whole field of view that the things on the edge of the view are not difficult to see relative to the things in the middle of the view, i.e. 'Sharp to the edge' is 'what it says on the box'.  I think the property is brilliantly demonstrated by those photos of a ruler which shows very clearly the possible differences that exist in different EP's, perhaps someone recognises my description and knows the member and could tag him to this thread?

[Stu]

1 hour ago, JOC said:

I think the property is brilliantly demonstrated by those photos of a ruler which shows very clearly the possible differences that exist in different EP's, perhaps someone recognises my description and knows the member and could tag him to this thread?

Pretty sure that’s @Louis D

[F15Rules]

4 minutes ago, Stu said:

Pretty sure that’s @Louis D

+1 for that, it's got to be Louis😊

Edited February 8, 2023 by F15Rules

[Zermelo]

I think he has copyrighted it, so no-one else is allowed do it.

This may be the most "definitive" post, but I've seen several others on specific topics.

 

[Louis D]

3 hours ago, Stu said:

Pretty sure that’s @Louis D

 

3 hours ago, F15Rules said:

+1 for that, it's got to be Louis😊

Guilty as charged. 

2 hours ago, Zermelo said:

I think he has copyrighted it, so no-one else is allowed do it.

That's right.  My lawyers are ready, willing, and able to fly around the globe to protect my worldwide copyrighted/trademarked style of eyepiece testing. 😁

I started doing it for my own edification because my eyes aren't what they used to be.  The aha moment came when I realized smartphone cameras were very small, wide field, well corrected, low linear distortion, high resolution, fixed focal length, and could take the place of my own eyeball to very accurately capture what I was seeing at the exit pupil.  Everything from SAEP,  CAEP, and vignetting to field stop sharpness and field sharpness center to edge could be examined in detail later via pixel peeping at the computer.

The only thing not accurately captured by a smartphone camera is field curvature because these tiny, wide angle cameras have vast depth of focus similar to very young human eyes.  Thus, the camera will often show what appears to be a blurry outer field to my presbyopically fixed focus eyes as sharply or nearly sharply in focus.  I generally try to note this difference in my imaging notes and reviews.

[Don Pensack]

10 hours ago, Stu said:

I normally refrain from making those comments as I’m never sure if it’s my eyes or the eyepiece/scope combo. Everyone raves about the Morpheus being sharp to the edge, but I’ve not found that to be the case in all scopes. I do think it is important to be clear which scope is being used as it makes a big difference to the result. I seem to recall that the FS60C was a bit rough towards the edge with a Morpheus 17.5mm but it tightened up a lot when I added the CQ Extender module. I’ll dig out the report.

Probably my most consistently sharp to the edge eyepiece is my 24mm Panoptic which gives excellent results, but I know it has other distortions that allow this ie I don’t enjoy view the Moon through it off axis because it appears like an egg!

Best widefield combo is my Televue Genesis (petzval design with a nice flat field) with 31mm Nagler. 5 degrees of loveliness. The 30mm ES 82 degree did not perfrom as well, not surprising as I’m sure the Nag was designed to work with flatter field scopes.

The 30mm ES 82° works fine in a flat field scope.  It's main issue, other than SAEP and CAEP, is its outer field is not well corrected for astigmatism in a fast (<f/6) scope.

Edited February 8, 2023 by Don Pensack

[jetstream]

I think the responses show that it is the combination of scope, eyepiece and eye that determines the "sharp to the edge" deal. I personally dont pay a whole pile of attention to it and just go for set up that give great views. ie the 20mm Lunt 100 has astig near the edge but all is forgotten as the Veil and other deep sky objects are viewed.

12 hours ago, Stu said:

Probably my most consistently sharp to the edge eyepiece is my 24mm Panoptic which gives excellent results, but I know it has other distortions that allow this ie I don’t enjoy view the Moon through it off axis because it appears like an egg!

And then there is the distortion issue, one type eggs out the moon, another squeezes stars together at the edge and then there is "moustache" distortion.

My Paracorr II fixes the coma in my newts but does not fix eyepiece aberrations or the type of distortion in them. We all have favourite eyepieces that work for us as individuals and there are many very good ones to choose from. To me all that matters is finding an eyepiece that I enjoy.

[Mr Spock]

Excuse poor phone photo, but, If you want sharp to the edge with minimal distortion, try an LVW 22mm. If you can find one... 

[Stu]

2 hours ago, Don Pensack said:

The 30mm ES 82° works fine in a flat field scope.  It's main issue, other than SAEP and CAEP, is its outer field is not well corrected for astigmatism in a fast (<f/6) scope.

Maybe so Don, but I found that the 31mm Nagler is sharper than it at the edges when compared in my Genesis.

[Louis D]

41 minutes ago, Stu said:

Maybe so Don, but I found that the 31mm Nagler is sharper than it at the edges when compared in my Genesis.

Someday I'll have to directly compare the much vaunted 31mm NT5 to the 30mm APM UFF.  My 30mm ES-82 has more bloated stars in the central region and spectrally challenged stars in the outer 10% of the field as compared to my 30mm APM UFF.  It would be neat if the 30mm APM UFF design could be extended to 82 degrees or more to compete more directly with the 82 degree class of eyepieces.

[Ratlet]

55 minutes ago, Mr Spock said:

Excuse poor phone photo, but, If you want sharp to the edge with minimal distortion, try an LVW 22mm. If you can find one... 

Not sure if we can trust this.  The ruler is not even made of wood!

[Don Pensack]

2 hours ago, Stu said:

Maybe so Don, but I found that the 31mm Nagler is sharper than it at the edges when compared in my Genesis.

That's exactly what I said.  I was pointing out that the flat field doesn't cause any issue with the 30mm ES.

Its problems are there in a flat field.  A scope with a curved field would make its edge of field aberrations worse by defocusing them.

Your Genesis is a fast f/ratio, which the ES doesn't deal with well.

So, naturally, you see better images in the 31mm Nagler.

It's not because the Nagler is better with a flat field, it's just better overall--less SAEP, same CAEP, and virtually zero edge of field astigmatism in fast f/ratios.

 

[Stu]

11 minutes ago, Don Pensack said:

That's exactly what I said.  I was pointing out that the flat field doesn't cause any issue with the 30mm ES.

Its problems are there in a flat field.  A scope with a curved field would make its edge of field aberrations worse by defocusing them.

Your Genesis is a fast f/ratio, which the ES doesn't deal with well.

So, naturally, you see better images in the 31mm Nagler.

It's not because the Nagler is better with a flat field, it's just better overall--less SAEP, same CAEP, and virtually zero edge of field astigmatism in fast f/ratios.

 

Thanks Don, yes of course you did, I somehow didn’t take in the last part about astigmatism.

[jetstream]

2 hours ago, Mr Spock said:

If you want sharp to the edge with minimal distortion, try an LVW 22mm

The interesting thing about distortion is that individuals may see different levels of it , many tests done including Hermann von Hemholz. Even those looking at your image might see different levels in it.

http://www.holgermerlitz.de/globe/test_distortion.html

 

[Louis D]

17 hours ago, Ratlet said:

Not sure if we can trust this.  The ruler is not even made of wood!

Funny you mention this.  I've noticed that the low contrast printing on my wooden rulers does make for a more stringent test than would (wood?😄) high contrast markings.  Also, the subtle wood grain is also a good indication of ability to render low contrast details.  The bar code on one of them is also handy for showing chromatic aberrations.  I suppose I could print up some sort of high contrast target and attach it to the rulers to be able to track chromatic smearing center to edge.  Ideally, a series of backlit pinholes in foil would also be a good artificial star test across the field.  I've just not had the time to work out the details yet.  That, and I'd have to do all my testing in near darkness.

[Mr Spock]

18 hours ago, Ratlet said:

The ruler is not even made of wood!

I've not seen a wood ruler here for donkey's years. Mostly they are plastic 

[Stu]

12 minutes ago, Mr Spock said:

I've not seen a wood ruler here for donkey's years. Mostly they are plastic 

You can still buy them!

[Ratlet]

In all seriousness though, the photogrpahing a ruler thing is amazing when you are starting out.  It would be a lot of work, but if a retailer had it for their eyepieces for a couple of scopes (say a fast newt and a mak) it would make comparing eyepieces a dawdle.

[F15Rules]

Well, today I set up the binoviewers in daylight and focused the scope on our nearby church tower steeple 

I just used a 26mm plossl eyepiece in the left hand eyepiece holder (my better eye these days) and panned left to right across the field. By chance, and very helpfully, there was a Collared Dove sitting on the top of the steeple, and as I panned left to right the image of the dove stayed sharp virtually to the edge of the fov.

Next, without changing the orientation of the binos, I panned up and down, or north-south, and saw a definite blurring of the image of the dove at each extreme north or south point..not huge but there..

I then, as per @Louis Ds advice, rotated the binoviewer by 90 degrees, so that the former left right horizontal view was now viewed as effectively an "up-down" view. The image of the dove again stayed sharp virtually to the edge. I then rotated the binoviewer to the opposite side, ie 180 degrees from the first rotation, and again the image stayed sharp.

From the above results, I can only conclude that the blurring I see when panning up and down (north-south), but don't see when panning right to left (east-west) can only be as a result of a fault in my eyes.

I am making an eye test appointment tomorrow for sometime in the next 10 days. I'll report back what the findings are!😏

Dave

[JOC]

So pleased you found Louis D's photographed rulers masterclass.  Per my original comment, though I know precious little about the subject, if the photos of the rulers are pin sharp across the whole view then that is what I think it meant by 'sharp to the edge' 🙂 

[JOC]

On 08/02/2023 at 20:32, Mr Spock said:

Excuse poor phone photo, but, If you want sharp to the edge with minimal distortion, try an LVW 22mm. If you can find one...

Oooo....look at the optical illusion there - the grid paper shows perfect squares everywhere, but the eye says that the ruler looks thinner in the middle, but it is isn't LOL!

[michael.h.f.wilkinson]

On 09/02/2023 at 21:05, F15Rules said:

Well, today I set up the binoviewers in daylight and focused the scope on our nearby church tower steeple 

I just used a 26mm plossl eyepiece in the left hand eyepiece holder (my better eye these days) and panned left to right across the field. By chance, and very helpfully, there was a Collared Dove sitting on the top of the steeple, and as I panned left to right the image of the dove stayed sharp virtually to the edge of the fov.

Next, without changing the orientation of the binos, I panned up and down, or north-south, and saw a definite blurring of the image of the dove at each extreme north or south point..not huge but there..

I then, as per @Louis Ds advice, rotated the binoviewer by 90 degrees, so that the former left right horizontal view was now viewed as effectively an "up-down" view. The image of the dove again stayed sharp virtually to the edge. I then rotated the binoviewer to the opposite side, ie 180 degrees from the first rotation, and again the image stayed sharp.

From the above results, I can only conclude that the blurring I see when panning up and down (north-south), but don't see when panning right to left (east-west) can only be as a result of a fault in my eyes.

I am making an eye test appointment tomorrow for sometime in the next 10 days. I'll report back what the findings are!😏

Dave

A simple way to test whether the EPs where at fault is to rotate them in the binoviewer, which is easier than rotating the whole binoviewer. I have never noticed any difference in sharpness between top and bottom and left and right edges (until I started using varifocal glasses, obviously). EPs are generally very close to perfectly rotationally symmetric, so I would not expect any difference between the north-south and east-west edges