On axis sharpness

[jetstream]

After using my eyepieces under some good conditions for quite a while now a question has popped up... there seems to be a small but noticeable difference in on axis sharpness in my eyepieces on high resolution targets like the moon and planets ( Saturn espc)-  what can explain this?

[ronin]

Which eyepieces and if possible which are the shrper?

However I suspect it will come down to eyepiece quality, I can see it also being slightly reliant on the eyepiece focal length. For whatever reason I would maybe expect a long focal length eyepiece to be less sharp then a shorter focal length one.

[Mr Spock]

I can see it also being slightly reliant on the eyepiece focal length. For whatever reason I would maybe expect a long focal length eyepiece to be less sharp then a shorter focal length one

There's no reason why eyepieces of different focal lengths should have different sharpness.

The factors involved in eyepiece quality will be the type of design, it's resistance to aberrations, the surface accuracy of the lenses and the quality of the glass used.

There are other factors to take into account too, such as the seeing conditions, the optical quality of the scope's optics, accuracy of collimation and other things, such as scope cool down and the magnification you are using.

Details of your scope and eyepieces would help

[jetstream]

Hi Ronin, I don't really want to name the eyepieces involved as they are all good, and don't want to predjudice people against them for some minor, but noticeable differences - to my eyes. It occurs with orthos to widefields and seeing is not the cause.

I wonder how this can be if the light is all focused on the exit pupil correctly?

Good thought about focal length but I rule this out as some widefields give very sharp views...for a widefield.

[jetstream]

There's no reason why eyepieces of different focal lengths should have different sharpness.

The factors involved in eyepiece quality will be the type of design, it's resistance to aberrations, the surface accuracy of the lenses and the quality of the glass used.

There are other factors to take into account too, such as the seeing conditions, the optical quality of the scope's optics, accuracy of collimation and other things, such as scope cool down and the magnification you are using.

Details of your scope and eyepieces would help

Hello Michael, the effect is noticed in my 2 fracs and my dob, using a mirror diag and a prism. I can't understand this either unless the accuracy of the lens figure comes into play.

My 90mm Stellarvue star tests perfectly for collimation, the SW120ED is not perfect but very close and the VX10 is collimated with a Catseye set up including an autocollimator. The effect is most noticeable under excellent seeing and shows up on solar observing as well. My scopes are always well cooled/equalized.

I can only think that some of the rays don't focus exactly at the same place in the exit disk- thoughts?

[Mr Spock]

All optical designs are a compromise. Something like an Orthoscopic would be very sharp on axis but fail towards the edge; this is why they are limited to a 40° field. Wide field eyepieces may be sharper towards the edge, but they do this by increasing the number of elements.

I have a number of different makes of eyepieces at similar focal length and they all have different levels of sharpness, contrast, control of aberrations etc.

[jetstream]

There is another thing that puzzles me- the fact that my barlow can improve on axis sharpness, for some eyepieces and never degrades the view in any. I know these may be 2 different issues, I guess I'm looking for the theory(ies) behind these observations.

[John]

Most eyepieces I've used seem to perform well on axis. The differences most often appear as you look at the view away from the optical axis. Transmission differences do show up on axis but I can't see a barlow lens altering transmission, other than very slightly reducing it, so it's not that I think.

If an eyepiece is plainly not as sharp as another on axis, and across a number of scopes, I reckon it must be to do with the figure of the lens surfaces within it and possibly the spacing of the lens elements. Using a barlow lens results in a less steep light cone entering the barlow so such optical errors could be mitigated.

I'm struggling here to be honest Gerry !

[John]

Another thought occurs that this could be due to the optical axis of an eyepiece not sitting square to the optical axis of the scope. Perhaps an undercut in an eyepiece barrel and a compression ring mismatch could cause this ?

[jetstream]

Good thought John, for some reason I don't find the eyepiece sitting exactly square to be an issue though - within reason-I've tried them "loose" in the holder to see what happens. Unless they are canted quite a bit, no effect.

The Quark is supposed to be sensitive to this and I find it isn't either. I checked my focuser on the dob by using the sight tube and Cheshire to see if racking it in or out changes the angle with respect to the optical axis... it doesn't.

I noticed this sharpness difference a while back and again during some very good seeing on Saturn's rings, an interesting thing is that at the same mag with a particular eyepiece, the barlow brought out additional features including that faint thin line past Cassini's division. Solar sharpness on axis also shows some EP differences as well. I've concluded it must be the individual eyepieces or how they interact with the scope or... yes I don't really know lol!

[YKSE]

Spherical Aberration (SA) is an aberration affects on-axis sharpness, Eyepieces, even the best ones, has either under or over correction of SA, depending on telescopes used has the same sign of SA correction or not, SA can be reduced or increased. Difficulty in finding best focus is a sign of presence of SA in the system(telescope+eyepiece).

Different design of eyepeices has intrinsic SA, some has more SA, others less, and the quality of components in an eyepiece (lens, housing, spacers) can make SA worse than design goals.

PS:

Spherical Aberration at Exit Pupil (SAEP) is a total different from SA, SAEP doesn't affect image sharpness, if only cause eye placement difficulty, resulting in kidney bean effects.

[Charic]

Hi Ronin, I don't really want to name the eyepieces involved as they are all good, and don't want to predjudice people against them for some minor, but noticeable differences - to my eyes. It occurs with orthos to widefields and seeing is not the cause.

I wonder how this can be if the light is all focused on the exit pupil correctly?

Good thought about focal length but I rule this out as some widefields give very sharp views...for a widefield.

..........I've told folk here I don't  like  a certain TeleVue Plossl...........hasn't dented TeleVue  sales!

As with most EP's, they are all different in their type, construction and  image result! not one is 100% suitable for everyone, or suitable for the scope?

The advice given above is probably in the right direction, but I feel a  description / name of the said eyepiece would be of more help to those trying to explain why 'A' particular EP may be causing such an issue.

But that is your choice.

[YKSE]

..........I've told folk here I don't  like  a certain TeleVue Plossl...........hasn't dented TeleVue  sales!

As with most EP's, they are all different in their type, construction and  image result! not one is 100% suitable for everyone, or suitable for the scope?

You've drawn the conclusion that eye relieve of 8mm Televue plossl is too short for you. Have you compared optical performance to your other 8mm? For planetary, you don't need to see the field stop, you can back off a bit, still have the object nice framed in the center of FOV. Look closely on axis sharpness, plossls might be sharper than your other eyepieces.

[jetstream]

Spherical Aberration (SA) is an aberration affects on-axis sharpness, Eyepieces, even the best ones, has either under or over correction of SA, depending on telescopes used has the same sign of SA correction or not, SA can be reduced or increased. Difficulty in finding best focus is a sign of presence of SA in the system(telescope+eyepiece).

Different design of eyepeices has intrinsic SA, some has more SA, others less, and the quality of components in an eyepiece (lens, housing, spacers) can make SA worse than design goals.

PS:

Spherical Aberration at Exit Pupil (SAEP) is a total different from SA, SAEP doesn't affect image sharpness, if only cause eye placement difficulty, resulting in kidney bean effects.

So spherical aberration of the eyepiece can effect sharpness, I will look into this thanks Yong The eyepieces I'm talking about still have "snap" when focusing and all have no difficulty finding best focus.

I must learn more about out this SA.

If this is the case, the sharpest EP's I have interact well with my scope or have very little to none present. Back to school...for some lessons...

[jetstream]

..........I've told folk here I don't  like  a certain TeleVue Plossl...........hasn't dented TeleVue  sales!

As with most EP's, they are all different in their type, construction and  image result! not one is 100% suitable for everyone, or suitable for the scope?

The advice given above is probably in the right direction, but I feel a  description / name of the said eyepiece would be of more help to those trying to explain why 'A' particular EP may be causing such an issue.

But that is your choice.

Well Charic, as I pursue knowledge about eyepieces much is learned, and some of what is learned may be interpreted as "this brand is poor" or "that eyepiece distorts like this".A lot of these eyepieces work extremely well, even if some are sharper or more contrasty on the Veil etc., or bring out fine detail on Saturn.

Take pincushion distortion for example and the term sharp to the edge....well..are they really sharp to the edge with lots of pincushion? But pincushion is a common and effective distortion to choose.

So for this thread I will remain neutral and not reveal the EP's involved, I was hoping for open discussion about what the cause is (causes) and what design features can promote the existence of this condition. I don't want to debate individual eyepieces in this thread...we can do it in another one if interested.

[Charic]

jetstream....its still just an open discussion as fas as I can see, I just had the same thoughts as the second and third replies, in that simply a little more detail is/was required.  Thats all!

[Moonshane]

Your eye is of course part of the optical train and perhaps an aberration of one's eye could cause odd effects with certain designs? I get an odd effect with certain eyepieces on the moon (splashes of blue and pink) which not many people see.

[Charic]

YKSE......I was even warned about the TV? But  was so kindly loaned one for testing, and looking back now to that windy night, the BST just felt more comfortable for me, and of course, with  the extra 10°afov over the TV.

I don't doubt the quality of the TV itself, it just wasn't for me. 

Surprisingly now, I have a Plossl here that is perfect ( for me anyway ). Its a 9mm with 2° more afov than the TV, and the same eye relief. This EP  feels so much more comfortable, yet so wrong ~ £10 vs. £80+? Should I name that EP? I have elsewhere!

But were heading off route here, I was simply stating that naming doesn't always prejudice people away, but merely helps with their informative decision making. After all, most of us can  advise from the experience of using the said items. Hearing several folk tell me that 'X' is somehow better than 'X II' is more informative to me, for whatever the shortcomings,  in preference to just reading the specifications from the manufacturer

In the end our eyes and our eyes alone, can be the only deciding factor. We learn from experience, but rely on our own!

[Charic]

jetstream.....Your largest aperture gathers more light, making the image formed by the telescope, brighter and possibly sharper!

Does the sharpness you perceive decrease when you drop down in aperture? If no, then rule out aperture.

You mentioned.......... I've concluded it must be the individual eyepieces or how they interact with the scope or... yes I don't really know lol!

Could just be your eyes? I'm just a little older than you, but my eyes are not as good as they were a few Years back. Is there someone else with you or local to you, that could view  your image just to see if they too can see any difference?

You sure have picked a difficult one?

[jetstream]

My eyes have tested very well 20/10 or so and the Doc said all's good.  I have considered my eye being an issue as well, but judging from the fine detail that I see, I don't think its an factor.

Aperture/resolution can be a factor but I notice the same effects with all scopes, at different scales. I have used different observers to try, but there are no experienced ones here that I'm aware of. Believe me for lunar/planetary some of these differences are immediately noticeable...

And why can a barlow allow more detail to be seen? at the same mags... I would guess because the effective focal length is longer, but how does this effect the eyepiece on axis?

I'm fine if people want to name eyepieces when discussing things, in this case I want to understand what is going on, irrespective of brand or model.

[jetstream]

Your eye is of course part of the optical train and perhaps an aberration of one's eye could cause odd effects with certain designs? I get an odd effect with certain eyepieces on the moon (splashes of blue and pink) which not many people see.

Good thinking Shane, I can be effected by the "globe" effect for instance when panning with some eyepieces. This sharpness thing seems static and always shows when conditions allow.

[Charic]

 I want to understand what is going on, irrespective of brand or model.

Agreed.........the theory makes for better understanding. The problem here is unique at present, and were still searching?