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Re: (IAAC) Planetary eyepieces
At 03:56 AM 6/17/98 -0400, you wrote:
>>Since the light transmission of Naglers is a few percent less than the
>>Orthos, and the design is not as well corrected as orthos across the field,
>>it may seem that the Nagler is "sharper, brighter, and clearer" than a good
>>ortho - especially at low power on a fast scope, where other aberrations
>>will overwhelm the correction of the ortho - but in reality the perception
>>is nothing but fantasy.
>Al Nagler has specifically stated to me quite the opposite, and I am
>surprised that he did a 180 degree reverse.
Well, let's be certain we are talking about the same thing here. Al said a
lot of good things about Naglers too, not unexpectedly since he owns the
company. However, wrt giving brighter images, Nagler explicitly agreed with
my comments about this at Riverside. Since there are more elements, and
more air-glass surfaces, and there is in fact less light making it through
the eyepiece (losses are due to absorbtion, scatter, and reflection) than
through an ortho; thus Naglers are dimmer than the well-made orthos that
were under discussion. However, you say:
>In fact, he used his work in the field of television/projection
>as a specific example of how Nagler's hold more contrast.
There are circumstances in which Naglers might provide more contrast on
specific scopes than an ortho would. A Nagler is optimally corrected for -
if I recall Al's comments right - an f/5 or longer system; orthos are
optimal at around f/7 and longer, depending on how closely one adheres to
Abbe's design (which is the only kind of ortho I am talking about). I
suppose that there are f/5 instruments in which a Nagler will degrade B-b/B
less than an ortho would, due to differences in the eyepiece design.
>He indicated that while working in the
>field, one of the sharpest pictures had only 5% light transmission, but
>such high contrast that the lgiht transmission was not an issue, as the
>contrast was far more important.
There is nothing wrong with this reasoning. In certain circumstances I
would live with a heavy light loss if I could maintain the contrast of the
image, no question. Solar observing is perhaps the quintessential example
of this. But how often are you willing to live with this heavy light loss
when viewing scotopically?
Contrast is achieved in optical systems by putting the light where it
belongs in the image. Since an ortho offers much better correction at f/7
than a Nagler does at f/7, ISTR that the ortho will win on contrast at that
ratio. You will probably begin to notice a long Nagler beating a long ortho
at f/5 or so; at f/4, the long ortho has little application. In an f/2
scope, the Nagler will clearly win (though still perform pathetically), as
an experience with a 45" f/2 photometric telescope showed.
At fast focal ratios (say, f/4.5), an ortho of medium focal length (say, a
7mm or 9mm one) will not break down badly at all. And a really short one,
assuming it does not introduce too much raw magnification, will work fine
at up to about f/3 or faster if I can extrapolate from my experience.
The Naglers are rather less sensitive to the different focal ratios that
are commonly encountered in amateur systems. Probably this stability has
fooled many observers into believing that the Naglers are superior in
certain ways in which they are not.
At any rate, the point is, yes, contrast is important. Orthos will win in
most cases over Naglers on the contrast issue; this is due to correction,
less scatter, and a relative lack of internal reflections. This only
applies if the observer is actually willing to pay attention to the field
though; as Al Nagler is fond of pointing out, "the differences are little".
If you don't care to squeeze all you can from your scope or your eyes, then
Medkeff's Corollary applies: "The differences are irrelevant".
>I personally, have tested Naglers directly against Takahashi LEs with one
>ep in one side of the binoviewer, and the other in the other side...
In my opinion, this is a bad practice, Todd. I tried to pull the same stunt
out here and found that the binoviewer optics really swamped the eyepiece
optics, causing some noticeable changes to star images and light
distribution in the field - and causing slightly different effects in each
side of the viewer. I concluded by doing my tests on a variety of local
instruments (I think you saw the list on s.a.a.), with no ancillary optics
involved (we removed star diagonals from the relevant scopes, for example).
We simply swapped the eyepieces in and out several times (in the process
almost dropping some of them, unfortunately).
>in a
>daytime test, so this does not take into acct the effect of your internal
>reflections.
In a way it does. Every point in the field of a Nagler, and for some
distance outside the field stop, any light source causes a reflection
(often below the threshold of visibility, of course). These reflections
necessarily rob contrast from the image - it creates a sort of fog through
which you have to discern the light that is falling where it is supposed to
on your retina.
This effect is, to be sure, extremely difficult to notice during the day.
>The light throughput was noticeably less in the Nagler, and
>yet the image was as clear, if not more clearly resolved in the Nagler at
>the same time.
What is the design of these Tak LE's? I am not too familiar with them.
It sounds as if the Nagler, in these tests, was the better corrected
eyepiece for your system. This reminds us that there is no *inherent*
superiority of any one eyepiece; only superiority on defined criteria and
matched to specific optical systems and uses.
Interestingly, the Large Binocular Telescope on Mt. Graham will be using
Naglers. (8.4 meter f/very fast, f/2 or thereabouts.)
>At night, similar performance was noted, to the degree that
>I was not able to perceive the loss in light.
Subjectively, I assume? Did you set out with some deep photometric fields
and actually compare limiting magnitudes?
I find that subjectively, I never notice light loss due to an eyepiece
during an evaluation. However, I do note wide scatter (a half magnitude or
so) in the telescopic magnitude limit during the same tests, between the
best and the worst eyepieces in this regard. The Naglers fared rather well
in these evaluations, being middle of the road when compared to other
eight-element eyepieces. However, in all cases I experience the best
limiting magnitude by using a set of <ahem> cheap-o Edmund RKE eyepieces, a
Kellner derivative. That's a great eyepiece if you simply want to observe
very faint stars....
>HOWEVER in a quick test
>against a Pentax XL last week, I noted that the view seemed "cleaner" in
>the Pentax 7, as opposed to the Nagler 7, but I couldn't put my finger on
>it due to poor seeing and limited time.
The single time I used a Pentax XL seven, I felt that the internal
reflections were a good deal less than in the Nagler 7. I did no
side-by-side, however. I felt it might have potential as a demonstration of
what internal reflections, even "invisible" ones, can do to an image.
>Similarly, I sold my 9mm Pentax Ortho and kept a 9mm Nagler after a quick
>back and forth swap at the eyepiece last year.
Uhhhh, being familiar with the Pentax orthos, I think you might have been
hasty there! :-)
>Overall, I have seen very clearly the light throughput difference, and
>agree there.
>I also have seen internal reflections at times in the Naglers.
I have recently done differential photometry on those reflections, in a
20mm Nagler, in eyepiece projection mode, using an ST-8 camera, with
Arcturus as a target behind a V filter. They (the reflections) are rather
bright. Just in case anyone wondered.... (Yes, I was bored silly, we had
sucker holes and I have been doing photometry lately - only desperation
conceived that experiment!)
>I cannot 100% for sure state at this point that I have noticed
>performance differences to date on Jupiter (as an example) and
>globulars (my other test target) through other eps over Naglers,
>but do believe it to be true. As a person that tests so many eps,
>I have found it frustrating not being able to perceive an immediate
>difference in this regard
I also had trouble quickly perceiving differences lo these last twenty
years or so. For this reason I spent practically no money on eyepieces
since my first set as a beginner, until about 1995. For the last five years
or so, I have been an empirical studies nut, a lot of folks around here (on
this list, I mean) have helped me out on that.
Anyway, here are a few things to consider. First, IMO globulars are not
good targets for evaluation. The thing to do here is to look at large,
diffuse galaxies, and estimate how far you can trace them out. Since there
are considerable halos in galaxies, their apparent extent is an excellent
indicator of contrast. If you can, compare the galaxy size using a
starfield. If you can't, let the galaxy drift across the field stop and
time it. (Several timings should be done if you want a definitive trend.)
Currently, there is a group that is studying the scatter in published
visual cometary coma diameter estimates and its relationship to the optics
used - including the eyepiece used. These results should be quite
interesting - I have looked at some of the early numbers, and it interested
me at least. Similar theories apply.
Jupiter is also a tricky target - it rotates quickly, is rather large, is
full of distracting detail, and is really only suitable for higher power
eyepieces. I think most any decent eyepiece will show you 80% or more of
what Jupiter has to offer. Its in that last 20% that the gains occur. When
I use Jupiter as a target, I pick a small area of the planet that will be
rotating through the CM during the test, and concentrate on that little bit
of Jupiter exclusively. One of the things I look at very carefully is color
contrasts, and I find that many observers forget this is an important way
to discern detail. (In fact, I have heard many observers call Saturn's
globe "bland"; perhaps, but note that most of the detail other than limb
darkening is noticeable as color contrasts rather than changes in intensity
of light in the image. Food for thought.)
--
Jeff Medkeff | Acting Assistant Coordinator
Rockland Observatory | Association of Lunar and Planetary
Sierra Vista, Arizona | Observers, Solar Section
On the web at http://shutter.vet.ohio-state.edu/