9 replies to this topic

[AnthonyP]

Thanks in advance.

[Lawrence Sayre]

The technical definition of the optical design compromise which leads to "Kidney-Beaning" is "Spherical Aberration Of The Exit Pupil".

Take the original 13 mm Nagler for instance. The smallest cross-sectional area of the principal rays exiting this eyepiece (when taken as a whole) is listed (by Rooten and van Venrooij) as being a rather broad 3.5 mm in diameter, and I believe this would be the actual measure of its "spherical aberration of the exit pupil". In an F/5 telescope the "exit pupil" is 2.6 mm (being that 13/5 = 2.6). The sum total of the "exit pupil" and the "spherical aberration of the exit pupil" is thus 6.1 mm in diameter. Therefore, whenever the pupil of the viewers eye is less than 6.1 mm in its full measure of dilation, there is going to be kidney-beaning. This results in a literally kidney-bean shaped area of black-out, which is most evident when the eye drifts even slightly off of the central axis. This would be horribly noticeable in daylight viewing, as the effect is progressive in nature with regard to how much less than 6.1 mm the eye is actually dilating. At close to 6.1 mm of pupil dilation, but only somewhat less than 6.1 mm, the effect (though most certainly present) may hardly be noticed, particularly if the viewer keeps his eye on axis. However, at ~2.5mm to 3 mm dilation (as in daylight viewing) the effect is not easily tolerated.

[Lawrence Sayre]

Continuing with the above example of the 13 mm Nagler T1, and with much introspection on my part as to what is actually taking place here:

Though Rooten and van Venrooij (in the book "Telescope Optics") do not make this abundantly clear (to my mind at least), I take this as being that an eye with a pupil dilation of 5 mm would, if kept on axis, encompass all of the 3.5 mm area of "spherical aberration of the exit pupil", and thus it would not be apparent. If the eye glances to one side of the FOV, then the additive nature they mention (I.E. exit pupil of the eyepiece + its spherical aberration diameter) comes into play, and the opposite field of view from where one is looking actually vanishes into a kidney bean shaped black hole, since the bundle of light is no longer falling within the viewers pupil diameter. To ensure that this would not ever happen, one requires a pupil dilation of a full 6.1 mm.

[SaberScorpX]

'Kidney Beaning' Quickie:

A dark region created by spherical aberration of an eyepiece's exit pupil requiring the observer's eye to be placed at different distances from the eyepiece to view different regions of the image.



Stephen Saber
PAC/Astronomical League
http://www.geocities...corpx/home.html

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[caableguy]

What I experienced was, when I looked through my X-Cel 2.3, there is such a small area of visible light coming through the ep, that unless you are looking directly down the absolute center of the ep, you will notice a "kidney bean" blackout effect appearing along the outer edges of the field of view, which generally consume the entire filed of view, until the eye is re centered in the ep.

Again this is my experience.

Jeff

[Lawrence Sayre]

Caableguy,

There is "black-out" and there is "kidney-bean black-out". The first case (black-out) is due to factors such as insufficient eye relief, or just as likely too much eye relief, and/or too little eye lens aperture (often combined with either of the two extremes of eye relief problems). A narrow eye lens diameter requires much dilligence in maintaining the eye directly on axis. Too little or too much eye relief can lead to an ideal eye position which differs from where your brain is telling you to position your eye. This causes the entire perimiter to black-out all at once, or (in particurlarly bad cases) leads to a complete inability to acquire and/or hold an image at all. I've stepped up to unfamilliar eyepieces whereby I could not acquire an image at all with the first go or two at it, because my brain was guessing quite wrongly. This is very frustrating, and is a bit of a downer on the psyche.

The second case (kidney-bean black-out) is due to factors discussed in posts above this one. What I feel you are describing here is first case "black-out".

[caableguy]

Very good, I am still trying to learn all this. As I stated, it was my experience I was describing. It looked like little kidney beans around the whole perimiter of the ep, and I was chasing it around I would loose the whole image.

Thanks for the correction, I will remember and try not to confuse in the future the two.

Thanks again, Jeff.

[Psa19one]

Jeff,

You were not incorrect in your description. No need to apologize! Your experiences and opinion are valued here!

[Lawrence Sayre]

Yes, certainly no need to apologize for describing what you see. However, spherical aberration of the exit pupil is an exit pupil of the eyepiece vs. dilated pupil of the viewers eye phenomenon, so for eyepiece designs where kidney-bean is likely (due to the design intentionally [or otherwise] permitting a large degree of spherical aberration of the exit pupil), the odds of experiencing "kidney-bean" decrease as the focal length of the eyepiece shortens. Thus for the first generation Nagler's, the 13 mm (the longest FL in the Gen 1 series of Nagler's) is renowned for exhibiting this problem, and though it is occasionally mentioned for the 11 mm, it becomes pretty much a non issue for the 9 mm, 7 mm, and 4.8 mm Nagler's T1 (though it is present in these to the same "scaled" degree). By the time an eyepiece reaches a focal length of 2.3 mm, I would assume it to be impossible for this particular aberration to make its presence known to the end user. However, I have been wrong many times before (witness my wrong thinking that the Tak LE's of 7.5 mm and 5 mm were 7 element designs, for but one example), and I will no doubt be wrong again (the bane of taking a stab at offering insight into a difficult subject, I guess), so I encourage that if this info is not correct, someone please step to the plate and correct it.

PS: For those wondering about it, spherical aberration of the exit pupil is not a sharpness or contrast robbing aberration like classic spherical aberration, so it does not degrade the view from that perspective.

[Macavity]

Thanks for the illuminating discussion, Lawrence. Helped me recover some composure, after discovery of kidney beaning, during daytime testing of a 90mm/1250mm MAK. To my general chagrin, a worst case being my(!) cherished Televue 24-8mm Zoom. But even that seems reasonable-ish with dark-adapted pupils.

Never bothered me with the "old" 102mm/F5 refractor, but, for some reason, it does with the MAK. Maybe putting a few numbers into the formulae may prove enlightening - Or maybe just down to retraining my own posture? (Differs quite a lot between the refractor on an Alt-Az and a MAK on an EQ!) Certainly a phenomena of lateral shift wrt the eyebeam and seems (subjectively) to worsen as one gets "nearer" to the lense. This may explain why the default 10mm eyepiece does so well - Eyelashes PREVENT me getting too close!

As you suggest, nothing below 10(?)mm (even the above Zoom) seems to be much of a problem, in this particular respect. "Psychology" notwithstanding, my 4 & 6mm Lanthanums seems to be easiest to use - Probably the inherently long eye-relief! Sadly they are a bit "high power" for a 90mm MAK. Maybe I will have to invest in another one (or two) at longer focus? I sometimes feel I inherited "Friday Afternoon" eyeballs...

Chris