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Re: (IAAC) FWD: What should I buy? (rich field scopes)

At 11:03 PM 6/18/98 -0400, you wrote:

>All three of us were quite
>surprised to find that we found the views of both M-13 and 
>Jupiter were best in the Naglers.  M-13 was brighter and 
>detail on Jupiter was somewhat easier to see.  This was 
>not an illusion.  

I never said that the ease with which you saw detail on Jupiter, or the
perception that M13 was brighter through a Nagler, was an illusion. This
is, in fact, the first I have heard about it. Let's back up even further
than you did, Alan, because I am not sure you are seeing this discussion
the way I am seeing it and vice versa. What I said was:

>....it may seem that the Nagler is "sharper, brighter, and 
>clearer" than a good ortho ... but in reality the perception
>is nothing but fantasy.

This was in response to Sue's statement that:

>...I find the veiws in Naglers generally
>sharper, brighter, and clearer on both planetary and 
>deep sky targets.

Which Sue wrote in response to this exchange:

>>	As far as image brightness goes, an 8 element eyepiece, 
>>assuming each coated lens element has a transmision of 98.5%, 
>>will have a total transmission of 88.6%.  A 4 element eyepiece has a
>>transmision of 94%.  In a 12.5" scope used at full aperture a 
>>5.5% reduction will probably not be noticeable, but if an aperture 
>>stop is used it could become a problem.

>As you suggest, the lost light from a multielement ocular will 
>be largely irrelevant to the view through the scope. OTOH, contrast-
>reducing effects such as scatter (caused by air-glass surfaces and 
>increased by the use of coatings) and internal reflections (which, 
>though decreased because of coatings, are still quite obvious with 
>the Naglers), will be far more deleterious - and obviously so to 
>the observer.

Now, there are several facts to be noted here. On my side of the
discussion, my original comments state that (1) light loss is mostly
irrelevant, and (2) the only thing about the Naglers that I said was that
they had obvious internal reflections, and (3) I stated that contrast
robbing defects (in general, not in a specific design) will be obvious to
an observer. Another fact is that Sue posted her comments, about the Nagler
seeming brighter, sharper, and clearer, in the context of what I thought
was a general comment about all eyepieces. The poster who compared the
transmission of an eight element eyepiece with that of a four element one
did not seem to me to be talking about any particular eyepiece design.
Therefore, I concluded that Sue's comment was in classic 'Cult of Al'
tradition, saying the Nagler was brighter, sharper, and clearer than all
other eyepieces.

I think you and I can agree this is not so, whether that was what Sue meant
or not. If this is not what Sue was saying, then I misinterpreted her
remarks. I can only defend that misinterpretation by pointing out how brief
the remarks were, and how they appeared in the context of the discussion at
that time.

Certain facts still remain. The transmission of a good ortho is more than
that of a Nagler. (Al says so, on this I am prepared to believe him. I have
never measured it, but I could easily do so if anyone has any interest.)
The Nagler cannot, therefore, be "brighter". 

If we want to talk about apparent brightness, or image contrast (B-b/B), or
better yet MTF (or other rigorously treatable attributes like PSF), in a
given system, then the Nagler can beat out an ortho or vice versa as
conditions change. However, apparent brightness, image contrast, and MTF,
do *not* mean the same thing as "brightness".

>The fact that our perceptions differ
>from yours does not mean they are wrong.  

I'm not certain your perceptions do differ. I have no idea what criteria
you used to determine that it was easier to see detail on Jupiter with your
Naglers than with your unspecified other eyepieces; I have no idea how you
determined that M13 appeared brighter in your Naglers than in your other
eyepieces, and I have no idea what telescopes you used to perform this
comparison.

I can tell you what a Nagler vs. an ortho vs. an Ultima (etc, etc) have
done to my visual telescopic limiting magnitudes, and I can tell you what
using a Nagler for projection does to colorimetry and color (Johnson)
photometry. I can tell you mathematically how it affects oversampling in
images. These are quantifiable measurements. I don't have any measurable
quantities from your group, but I believe that - since the laws of physics
do not change much from place to place - your results in similar tests
would be similar (with due allowance for variations in instrumentation and
eyeballs). I'm not really sure it is relevant to the discussion either way,
because quantities that one group finds significant might not be found to
be so by another group.

>        You claimed at one point that Sue put words in your mouth, but you
>have an entire paragraph of doing so to Sue.  Nowhere did she say Naglers
>were better than any other eyepiece no matter what the design, 

She jumped into the middle of a general discussion about unspecified types
of eyepieces with varying numbers of elements, and said:

>...I find the veiws in Naglers generally
>sharper, brighter, and clearer on both planetary and 
>deep sky targets.

I interpreted this to mean that she felt Naglers were better than any other
design, for all applications. If this is not what was meant, then so be it.

>nor did she
>ever say she hated all narrow angle eyepieces.  

That is correct, she did not say it, and I never claimed she did say it -
as your statement implies. It was *I* who said that it seems as if she has
never met a narrow angle eyepiece she didn't hate. That is certainly the
implication she is fronting on this mailing list, or so it seems to me.

>And what the blazes does any
>of this have to do with a previous discussion about Deep Sky 
>observing and small telescopes?

It does not. Topics of conversation tend to drift on these forums. I don't
consider that to be a big deal; it has been happening for years. Its called
'thread drift' in the culture, and it is best to just live with it. Lew has
blessed the content as relevant to the group, as equipage and technique do
not appear to be off-limits for the announce list. Lew's the boss, so I
will bow to General Lew's statements.

>You seem to have the idea that if someone has different perceptions
>about eyepieces than you they must be wrong.  

As does Sue, or you, for that matter.

>My impression is that there is
>a great deal of subjectivity in comparisons of telescopes 
>and eyepieces.

Yes, I believe there is. And there is nothing wrong with that, especially
if the observer has subjective experiences in mind, like casual stargazing.
But this is one reason why I have turned to more empirical tests, in an
attempt to attenuate subjective impressions.

>I've often been behind someone who was raving about 
>a view I found quite disappointing.  

Similarly here.

>I once heard a well
>known TN rave after viewing through one of his favorite designs 
>under some of the most horrid seeing conditions I have ever seen.  

TN = TM? If so, this I can understand; some TM's I know (unlike you) are
not terribly skilled observers, though they are tremendously talented
instrument builders.

>(I don't know how
>you run your comparisons, but we kept our mouths shut until 
>we'd all decided individually what we thought).

Similar here. We recorded everything on a form. No-one got any results
until the next morning, in fact. Our tests included deep field
observations, drift timings, and other non-subjective, quantifiable
performance indicators; and we also evaluated subjective criteria. An
example of subjective criteria might be how the perception of pincushion
affects the observer; pincushion drives me nuts aesthetically (so I
personally don't find, e.g., the 20mm Nagler or any of the Edmund RKE's to
be terribly pretty), but it doesn't bug most of my buddies. 

>        What aberrations are Orthos better corrected for on axis than
>Naglers?  

Color, for one, if the raytraces I have seen are to be trusted. This is
certainly supported by published observations. I can't say I can see the
difference casually, but I can perceive there is a difference even when
visually quantified color estimates are looked at (there is a larger
scatter, roughly to a factor of 1.8 or 2); OTOH I can point to large
differences when a Nagler is put in an instrument pack (this last is not,
incidentally, one of the applications for which a Nagler was designed, so
this is not a huge damnation of the Nagler here).

>What's wrong with Sue's statement that Orthos are not corrected
>for astigmatism (off axis, of course) while Naglers have distortion?

Things change when you put the words "off axis" in there, which Sue did
not. As you know, the definition of 'orthoscopic' (in this context)
contains some specific words about how orthoscopic eyepieces are to be
corrected for astigmatism. (It may be worth reminding again that
orthoscopic is a generic term; for example most plossls are also
orthoscopic designs, but I have been talking about Abbe orthoscopics in my
comments about orthos. Some amateur eyepieces marketed as Abbe orthoscopics
are actually plossls.)


--
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/
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