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Equipment Discussions >> Eyepieces

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robboski2004
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Reged: 01/14/08

Re: Eypiece transmission new [Re: ThomasM]
      #5566539 - 12/11/12 08:24 PM

Hello Thomas,

Just received and measured a 12mm Delos........96% @ 532nm.
27mm Pan..........96.4% @ 532nm.


Will possibly order a 8mm Delos next year ?

Regards.
Ian.


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Sarkikos
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Re: Eypiece transmission new [Re: robboski2004]
      #5566594 - 12/11/12 09:17 PM

Ian,

Quote:

Just received and measured a 12mm Delos........96% @ 532nm.
27mm Pan..........96.4% @ 532nm.




Thomas measured the BGO's as follows:

Quote:

Baader 18 mm Ortho I 96,3
Baader 18 mm Ortho II 97.9
Baader 9 mm Ortho 95,7
Baader 7 mm Ortho 95.8




So it appears the BGO's have very similar transmission to the 12mm Delos and 27mm Pan. I think I'll hang onto my BGO's, or are at least most of them. They are good for faint fuzzies as well as planets / lunar. I do carry some with me when I go to my dark site.

Now, I wonder how the transmission of my XW's would compare to all these?

Mike


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Starman1
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Re: Eypiece transmission new [Re: Sarkikos]
      #5566660 - 12/11/12 10:14 PM

Asked and answered:
http://pentaxplus.jp/archives/tech/xo-xw/63.html


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Sarkikos
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Re: Eypiece transmission new [Re: Starman1]
      #5567200 - 12/12/12 09:22 AM

Thanks, Don!

It looks like 96% transmission for the XW's at 550 nm. Judging from the graph, the transmission at 532 nm should be not much less than that, certainly no less than 95%. According to focal length, the XW's vary between about 95% and 96% transmission. So the XW's transmission is closely comparable to the Delos and BGO's.

A reasonable deduction from this - assuming all the measurements are correct - is that for deep sky light-transmission, there is no advantage to any of the three eyepiece lines over the others. XW's, Delos and BGO's are approximately equal in transmission.

So does this mean I should leave my BGO's home for planet / lunar and instead use my XW's at the dark site?

Up to now I haven't really had a chance to compare the BGO's against the XW's for faint fuzzies. But I have gotten the impression that the BGO's let me go deeper than my ES eyepieces and my Baader Zoom.

Mike


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Starman1
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Re: Eypiece transmission new [Re: Sarkikos]
      #5567367 - 12/12/12 11:21 AM

Mike,
We've talked about this before on the forums. It seems that there is a psychological phenomenon at work here. It's long been known that no human is capable of seeing a transmission or brightness difference of just a few percent, yet people see a difference in what they can see when comparing narrowfield eyepieces to widefields on deep sky objects.
Alvin Huey reports he can see more in the Delos than the Ethos, and more in his ZAOs than in the Delos. If the transmission of those eyepieces is within a few percent, and it appears to be, he should physiologically not be able to see a difference.
But if a psycho-physiological phenomenon is at work, then what he reports could be reality.

Here is the speculation, and it deserves a lab study somewhere:
narrowing the field of view results in an overall blacker field seen by the eye. The total amount of light entering the eye is reduced, and more of the peripheral field is seen as black. After all, a normally functioning eye has between 120 and 150 degrees of vision. If the eyepiece has a 40 degree field, then 80 to 110 degrees of that field will be black.
This may allow the dark adaptation of the eye to proceed to its maximum degree (I stare at the ground to accomplish the same thing), even though there is still light in the central 40 degrees of vision.
Or, the contrast between the black of the peripheral field and the illuminated image in the center causes an enhanced contrast in the center.
One would predict, therefore, that increasing the size of the apparent field would reduce the ability to see the faintest targets in the field, and so it seems, from anecdotal evidence.

We can test the hypothesis by using some widefield eyepieces with high transmission and some excellent narrowfield eyepieces of high transmission. IF we can legitimately see some fainter targets in the narrowfield eyepieces, then we know the psychological phenomenon is real because transmission cannot be the reason.

A good test field would be the center of Abell 426 where the number of galaxies visible rises almost exponentially with aperture. Small differences in light grasp can make a big change in the number of galaxies seen. One night we saw 8 galaxies in an 8", 18 in a 16", and 51 in a 28", and the 28" field was slightly smaller than the smaller scopes.

There is also the possibility the psychological phenomenon is not universal, i.e. it might make a difference to some observers but not all. I've often said that, other than for 3 defective eyepieces, no eyepiece I've owned (out of 300+) has ever prevented me from seeing an object visible in the aperture I was using at the time. I've seen essentially zero on-axis differences among various eyepieces, where seeing a faint target is concerned, regardless of apparent field or the number of elements. But my experience is not scientific, because I did not compare them all on the same night and same targets. And memory is unreliable.

That does not relate to the personal preferences end of the equation, where an observer's reaction to the apparent field plays a role in determining whether or not an observer would actually use a particular eyepiece. As an example, even finding that the Delos eyepieces were superior to the Ethos eyepieces would not prompt me to go out and buy a set of Delos eyepieces because, for me, the apparent field of the Delos is unacceptably narrow. My personal passage to wider fields is permanent, and one way.

But this test of a variety of high transmission eyepieces with differing apparent fields could be quite informative. And, in a club or group of observers, it might be possible to put together the necessary eyepieces


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BillP
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Re: Eypiece transmission new [Re: Starman1]
      #5567442 - 12/12/12 12:07 PM

Quote:

It seems that there is a psychological phenomenon at work here. It's long been known that no human is capable of seeing a transmission or brightness difference of just a few percent, yet people see a difference in what they can see when comparing narrowfield eyepieces to widefields on deep sky objects.

Alvin Huey reports he can see more in the Delos than the Ethos, and more in his ZAOs than in the Delos. If the transmission of those eyepieces is within a few percent, and it appears to be, he should physiologically not be able to see a difference. But if a psycho-physiological phenomenon is at work, then what he reports could be reality.




Don,

I see your point of conjecture, but do not agree (having abouit the same eyepiece quantity history). Yes you could be true, but there are other possibilites just as plausable IMO.

First off, I would say the premise of your 1st statement is too generalized and therefore incorrect. I have not seen any research indicating that humans cannot see differences of a few percent. I have seen studies where as little as 9 extra photons can be picked up by our perceptions though FYI! On studies where there is the 8-10% reporting, I have seen a multitude of them where people were shown point sources and with direct vision could not tell easily when the point source was dimmed on a second look. However I have never seen a study where it was conducted using adverted vision, in which the eye is more sensitive, nor a study were it was done with areas instead of point sources...especially when the area image was only partially illumintated with a gradual decreasing illimination. So all the studys really do not in any way model astronomical observing circumstances. It is actually very easy to logic through how a dim star can be just a few photons below the eye's threshold to perceive, then with an added 1% transmission in the eyepiece or scope have that threshold crossed so it is perceived. This is the circumstance that Alvin is explaining. I have read no study properly modeling astronomical situations. The onces I have read can only be extrapolated to things like if one can tell a sub-magnitude difference between two stars if that difference is less than 5%. So not what Alvin was conveying in his observing circumstance.

I do believe that there can also be in the mix a psycho-physiological phenomenon as you suggest. Actually it is probably present there for all of us biasing our perceptions based on expectations. However, to suggest that it would only cut one way I would say makes no sense. So would posit that it is just as likely that for those observers who do not see these differences, on-axis or otherwise, that they could just as likely have a psycho-physiological phenomenon happening to them preventing their seeing.

FWIW, I can tease out on-axis differences rather easily. Not on all targets, but on subsets of targets where the smallest contrast or brightness level does move things past a threshold. based on how I do some of mu larger scale comparisons, I would say that the likelihood of an expectations bias is many times eliminated as the many eyepieces are on a platform in the dark and ordered and reordered based on what is observed through them...and after a few minutes really have no idea what the eyepiece is that I am viewing through as it is just a poistion in the lineup at this point. When done, need to bring the table insode to see who's 1st and who's last as all I have is audio notes describing the differences between the slot positions. And there are differences to be seen in some way shape or form many times.

Anyway, food for thought. In the end though, I am alwaysof the opinion that the root cause why a difference is seen is really not important. All that is important is knowing that one eyepiece shows better or more for a particular observer with their optical chain.

Edited by BillP (12/12/12 12:08 PM)


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Paul G
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Re: Eypiece transmission new [Re: BillP]
      #5567637 - 12/12/12 01:51 PM

AAVSO has crunched large numbers of observations and came up with the same 8-10% difference needed to perceive a difference in brightness. That's directly applicable. This is not surprising given that for one light source to be perceived as twice as bright as another it has to be 9 times as bright (response compression). It's also important to distinguish between absolute threshold and difference threshold.

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Starman1
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Re: Eypiece transmission new [Re: BillP]
      #5567708 - 12/12/12 02:24 PM

In long conversations with AAVSO people, it has been noted that even the best observers cannot reliably detect a magnitude difference less than 0.1 magnitudes, which represents a 10% difference in brightness. Only by getting a large number of reports can they statistically narrow that to about 0.05 magnitude.

Lab studies I've read show that brightness differences of less than 10% fall into the "random chance" range of accuracy, with a "floor" of about 8%. That doesn't mean no observer could see a difference smaller than 8%, but it implies there is a floor to human perception as a statistical thing.

When we are talking the visibility of a faint star at the limit of averted vision, it's fair to ask, "With what percentage seen?". The limiting magnitude calculators seem to settle on 10% visible/90% invisible as a standard for determining visibility, but that is sort of arbitrary. Why not 5% or 15%? And I think the interpretation of "visible" varies from person to person. What difference is present between "invisible" and "visible with averted vision 10% of the time"? I've never read anything on that, ever.

One thing I've experienced over and over again through the years is the "you need a large scope to discover X, but not a large scope to see X after it's discovered". What I mean by that is that I may have difficulty seeing something in, say, my 12.5". I walk over and easily view it in a 28". Then I walk back and discover I CAN see it in the 12.5", only it's dimmer--but still there.

So what I distrust is the "now I see it, now I don't" type of statements concerning eyepieces. Nothing is simply visible or simply invisible; it's visible a certain percentage of the time, and that percentage varies, perhaps. But if it's invisible in a particular scope, I simply don't believe that changing eyepieces can make that "visible/invisible" difference. I've never seen it and every time I've thought I've seen it, looking longer proved me wrong.

So when talking about visibility of certain things at the limit, I believe that extreme differences reported between eyepieces of similar transmission are psychological. That doesn't mean they're not real. I don't think we really know the causes. Which is why I think a test of a number of individuals with a number of eyepieces of differing apparent field on the same target in the same scope on the same night could be interesting. And it might illustrate some psychological phenomenon worth studying in a more controlled environment.

I should also state that measured differences in transmission between eyepieces can exceed the 8-10% threshold. I've seen transmission percentages in the low 80s and the high 90s, and that difference is noticeable to nearly all observers.
But when the differences are 95-98%? Sorry, no way. Human vision has a logarithmic response. Or should I say no statistically significant way because random chance might score correctly for a particular individual and only indicate true randomness in a large sample.


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Sarkikos
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Re: Eypiece transmission new [Re: Paul G]
      #5567717 - 12/12/12 02:26 PM

Gus,

I was thinking in terms of mere detection of a faint object - star, galaxy, BN, whatever - rather than comparative brightness. But if that 8-10% differential is toward the LM of a particular optical train - absolute rather than difference threshold - it should still be important.

Mike


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Sarkikos
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Re: Eypiece transmission new [Re: Sarkikos]
      #5567757 - 12/12/12 02:43 PM

If two eyepieces have virtually the same transmission - let's say an XW and a BGO - and an object near the LM of an optical train can be seen in the BGO but not in XW, I'd say the difference is in the eye of the beholder ... literally.

As Don suggested, it makes sense that this detection difference is due to a difference in the level of light adaptation of the eye when looking through the two eyepieces. An eyepiece with a narrower AFOV will shield more of the surrounding background light from the observer's eye. I think that this alone could make a barely detectable object detectable in the BGO but not in the XW.

I have seen variations in my own level of dark adaptation produced by globular clusters and moderately bright stars when I've been trying to detect faint galaxies and nebulae. If a narrow-field Ortho keeps us from seeing these brighter objects, it makes sense to me that it could help us to see a very dim object - even if the transmission of the Ortho is no better than that of the wide-field eyepiece.

Of course, the observer could keep a bright star or glob just beyond the edge of an 82 or 100 degree eyepiece. But then the object we want to detect would probably not be near on-axis, where we would want it to be for optimally aberration-free viewing. Besides, the natural compulsion - at least mine - is to place the object we want to observe toward the center of the FOV.

Mike


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Eddgie
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Re: Eypiece transmission new [Re: Starman1]
      #5567808 - 12/12/12 03:13 PM

Quote:

Sorry, no way. Human vision has a logarithmic response.




I agree that the likelyhood that even the very very best observer can reliably differentiate between much less than 10% brightness variation is unlikely.

On the other hand, I think that I have to interject this. I believe that some of the discrepency is they way we look at the numbers, and we tend to do this with an "Absolute" scale of "100%" so that any eyepeice that is between 90% and 100% would appear as bright.

Quote:

Since the highest transmission is 97.9%, you kind of want none of your eyepieces to fall below a loss of 0.1 magnitude, or 10%, which says it should be at least 87.9%




But that is not the way it works. You really have to start the differential from the lowest transmission eyepeices and add 10% from the bottom.

In other words, if you started with an eyepice with 88% transmission, you would have a "Baseline" brightness as compared to the 100% absolute.

But to see a difference, you would only have to be 10% brighter than this baseline.

So, if the eyepeice started with 88% transmission, and you added 10% to that, you would add 8.8% transmission.

Now my math may not be perfect so this is more about the idea. So, this means that if I can see a difference of 10%, if I use an eyepeice with 88% + 10% of that baseline brightness, I would come up with a relative transmission of the sum of the two. So, a 10% brightness increase from the baseline would be 88% plus 10% of that (8.8%) for a total of 96.8%. So, the step between an eyepeiece with 88% transmission and one with 96.8% transmission would perhaps be at the edge of the range of detection for some individuals, but perhaps not all.

Anyway, we can't look at it as a fraction of 100%, but as the amount of light increase from the baseline eyepeice and add 10% to that number.

And using this, I could indeed see people detecting a tiny brightness difference. But these two eyepieces would have to be seperated by between 8.8% or more in "absolute" transmission.

My own experience is that the difference in brightness is so tiny and difficult to see that it is basically meaningless and no one should worry about this with modern eyepcies.

But some older eyepecies did seem dim to me. Most notably, the Meade 8.8 UWA and a Meade 6.4mm Plossl (Contamination in the cementing between the lenes on the plossl. In other words, a defective unit. I think the Meade 8.8 just didn't have modern enough coatings).

Mostly, modern wide field eyepces though seem to be on par with simpler types to me in brightness. If there is a difference, it is very, very subtle.

Anyway, this explains has at the extremes of transmission, it could be possible to see without 10% seperation on an absolute scale, but I still believe that while there may indeed be individual cases where we see an eyepeice with below 90% transmission compared to some of the eyepieces with the highest possible transmission, most eyepecies today have better than this. Not all, but most.

But using the logic above, I can see how an 88% transmission eyepeice could be detected when compared to a 96.8% transmission, and clearly some of the better eyepcies have this level of transmission.

If somone measured a T6 Nagler and it had 88.7% transmission, then yes, an eyepeie with (88.7 + 8.7) 97.4% transmission should be able to be discerned by someone doing very careful comparisons.

Will it make a difference in viewing? Maybe. I an not inclined to worry about it though. Given that every eyepiece design in production today is diffraction limited at the center of the field, to me personally, all of the meaningful difference in eyepiece performance is to be found off axis. Here, differences are titanic and easy to see, and for me personally have a huge impact on how I perceive the field. But a barely detectable change in brightness (.1 magnitude) provokes a big yawn from me personally.

But someone saying that they can see the difference between a .92% and a .97% transmission? I for one don't believe it.

I do feel as if seeing the difference between an eyepiece with 88% transmission and 97% is indeed possible even thought they are not seperate by 10% of the total brightness because it is the relative brightness differece starting with the lower transmission unit.

Edited by Eddgie (12/12/12 03:17 PM)


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Eddgie
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Re: Eypiece transmission new [Re: Eddgie]
      #5567860 - 12/12/12 03:47 PM

Here is an example from this thread that corresponds to the post I just made.

If the TV Nagler 13 mm II has 87.3% transmission, to be seen as 10% brighter, you would add 10% to that figure, or 8.73% in relative terms. So adding 10% of the 87.3% means that any eyepice with 96.03% transmission would be 10% brighter on a relative scale.

And I have no doubt that someone looking for it could see this difference.

But this is an old eyepeice, and modern eyepeies tend to be better than this.

I would put the cutoff at 89% though. If add 10% to this, you get 97.9%. Still possible that there are some eyepieces out there with this level of transmission difference, but for the most part, most modern eyepices will indeed have transmission that is within 10%, but you are looking for a 10% improvement in brightness over the one with the lowest transmission, and not as an absolute of the highst possible transmission.

In other words, If I started with 50% and added 20% transmission to this, I would still have only 60% of the total.

This is important for marketing because sometimes you rea that "Transmission has been improved by 8%!"

But suppose you have a Baader Mark V bino that already has 93% light transmission. If you improved it by 8%", then it would have 101% light transmission, and of course that can't happen.

But if it starts with 88% and baader improved it by 10%, then the unit passes 10% more light than it did in the past. But it is only working at 96.8% transmission.

It didn't go from 88% to 98%, but the transmission could very honestly be said to have improved by 10%.

And this was common in the great SCT coating wars. People thought that a 7% transmission inprovment meant on an absolute scale, but it was relative to the amount of light that was going through the system. From a marketing standpoint, it sounds better to say that "Transmission has improved by 10%" rather than say "we went from 70% transmission to 77% transmission. To someone not thinking about it, it would sound like 80% of the light is getting though, but that is not what a 10% increas means at all. It means 10% more from where you startetd.

Hope this example helps.

Edited by Eddgie (12/12/12 03:50 PM)


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slack
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Re: Eypiece transmission new [Re: Eddgie]
      #5567877 - 12/12/12 03:58 PM

I didn't see information about the current ES series in this thread, but regarding the Nagler T6 line (and 13mm in particular), info here may relate to my experience evaluating them against comparable focal lengths in the ES line. I have posted my observations before.

I could not claim that there were faint stars that I could not see in the T6 but did see in the ES. However, I could definitely discern faint stars more quickly in the ES than the T6.


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Jim Curry
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Re: Eypiece transmission new [Re: slack]
      #5568197 - 12/12/12 07:59 PM

I subscribe to the physiological, not psychological theory.

Talking about diffuse, threshold objects. When you observe the Flame Nebula do you look for detail and veining with Alnitak in the field? If you do you're missing a lot of detail. Move Alnitak out of the field to get rid of that "flashlight" shining in your eyes and it's a whole new object you're viewing. Perhaps an extreme example so let me continue.

Now for me a threshold subject would be Stephan's Quintet. Tackle that with a Nagler, Delos or any other wide field and there's not a prayer of detection. I know, I've tried. Drop in a 40-45 deg. ortho and they will shimmer into view. Why? Because many, many "flashlights" dispered across the wide field ep are shining in my eyes. The ortho cuts the view down to a couple dozen, maybe(I've never counted). When your eyes are fully dark adapted even a 9-10 mag star in the field will seem bright and make a difference between detection or not.

Jim


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Sarkikos
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Re: Eypiece transmission new [Re: Jim Curry]
      #5568223 - 12/12/12 08:18 PM

Jim,

Yes, exactly. The physical eye plays a larger role in visual astronomy than many observers seem to realize. Why wouldn't it? The telescope is important. The eyepiece is important. But what about the eye of the observer?

Just try to detect a faint fuzzy at the limiting magnitude of any telescope, even with an eyepiece which has the highest possible light transmission, when the eye is not optimally dark adapted. Good luck with that.

Mike


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Dave Ittner
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Re: Eypiece transmission [Re: Sarkikos]
      #5568259 - 12/12/12 08:52 PM

Since I have been getting out observing with others I have noticed a big difference among folks in what each can see (or say they can see) with their naked eye. It's interesting as there are quite a few reasons as to why that happens. One thing that bugs me is the folks that refuse to get glasses or corrective lenses.

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Starman1
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Re: Eypiece transmission [Re: Jim Curry]
      #5568262 - 12/12/12 08:54 PM

Quote:

I subscribe to the physiological, not psychological theory.

Talking about diffuse, threshold objects. When you observe the Flame Nebula do you look for detail and veining with Alnitak in the field? If you do you're missing a lot of detail. Move Alnitak out of the field to get rid of that "flashlight" shining in your eyes and it's a whole new object you're viewing. Perhaps an extreme example so let me continue.

Now for me a threshold subject would be Stephan's Quintet. Tackle that with a Nagler, Delos or any other wide field and there's not a prayer of detection. I know, I've tried. Drop in a 40-45 deg. ortho and they will shimmer into view. Why? Because many, many "flashlights" dispersed across the wide field ep are shining in my eyes. The ortho cuts the view down to a couple dozen, maybe(I've never counted). When your eyes are fully dark adapted even a 9-10 mag star in the field will seem bright and make a difference between detection or not.

Jim



Well, I see the point and I agree. That's not the ONLY explanation, though it works in the case of NGC2024 or NGC404 or even the Horsehead. It also partially explains why raising power can make something more visible--it narrows the field of view.
But it doesn't explain why sometimes the reverse occurs (e.g I see fainter things with the 100 degree 13 Ethos than I did with the 82 degree 13 Nagler) or why some observers claim to see fainter in one 45 degree eyepiece than another.

But I'll grant there may be both physiological and psychological factors at work.


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Starman1
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Re: Eypiece transmission [Re: Dave Ittner]
      #5568267 - 12/12/12 08:57 PM

Quote:

Since I have been getting out observing with others I have noticed a big difference among folks in what each can see (or say they can see) with their naked eye. It's interesting as there are quite a few reasons as to why that happens. One thing that bugs me is the folks that refuse to get glasses or corrective lenses.



In some cases, as my optometrist and I have discussed, it's simply impossible to correct the person's vision because there are multiple cylinders of astigmatism and variable correction needed across the retina.
But vanity, poverty, and convenience all play roles in human lives.


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Sarkikos
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Re: Eypiece transmission [Re: Dave Ittner]
      #5568283 - 12/12/12 09:08 PM

Dave,

Quote:

Since I have been getting out observing with others I have noticed a big difference among folks in what each can see (or say they can see) with their naked eye. It's interesting as there are quite a few reasons as to why that happens. One thing that bugs me is the folks that refuse to get glasses or corrective lenses.




... or wear good sunglasses or clip-ons during the day to protect their eyes. Can folks really expect to have their eyes scoured by full sunlight over a lifetime and not have it affect their ability to see the faint stuff at night?

Mike


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Dave Ittner
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Re: Eypiece transmission [Re: Sarkikos]
      #5568304 - 12/12/12 09:19 PM

yup, something I now try to bring to everyone's attention more often.

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