The emperors new Naglers

Part soapbox, part question...

Having blown so much "initial telescope budget" on five Naglers, it's
heartening to see them top every comparison or review I've read since.
The most common "con" is cost (occasionally weight), but the "pro"
column invariably bulges with clichés indicating top-notch,
second-to-none optical performance.

Take it as given that I'm absolutely chuffed to bits with my Naglers
and wouldn't swap them for anything, ever! But, while I readily
believe they knock the stuffing out of competing EPs (relative
performance) I don't believe they really have the absolute performance
attributed to them in most reviews. For example;

"Absolutely flat field across the whole FOV" - or
"No sign of any pincushion distortion"
Crap! The image in the 31mm and 17mm (with a 10" F/10 SCT) shows
negative pin-cushion distortion (the edges are "sucked in" rather than
"bulged out"). This isn't that noticeable when observing the sky, but
is obvious on terrestrial targets. Not a problem, but not what I'd
describe as "No sign of…", that's for sure!

"Perfect focus right up to the edge"
Rubbish! There is a small but obvious difference in focus between the
middle and edge of the wider FOV EPs. When stars in the centre are in
focus the ones at the edge develop a very tiny flare or two. The
converse is also true, perfect focus at the edge gives very slight
defocus in the centre. It's not that bad, very good in fact, but it
certainly isn't "perfect".

"No internal reflections or ghost images"
More rubbish. Get a bright enough source (e.g. Mars) and you'll get
all of these things if you look for them, especially when the seeing
is poor, but perhaps not as pronounced as in cheaper EPs. It doesn't
hinder observations, but there ARE internal reflections and ghost
images in certain circumstances…

"Absolutely no detectable optical aberrations"
My experience with microscopes is that "no aberrations" means exactly
that, no aberrations. Not some, not a little, not nearly none – it
means absolutely none (and I own microscopes that perform that well)!
So given the aberrations noted above, this particular "review cliché"
is just downright misleading.

So, are the reviewer's right (meaning there must be something wrong
with my SCT to cause aberrations when using my Naglers) or am I right,
the reviewers habitually overstate the (absolute) performance I should
expect from them?

Cheers
Beats

So, are the reviewer's right (meaning there must be something wrong
with my SCT to cause aberrations when using my Naglers) or am I right,
the reviewers habitually overstate the (absolute) performance I should
expect from them?


The reviewers habitually overstate the performance.Nothing is ever perfect, but
some things are good enough.

RC

So, are the reviewer's right (meaning there must be something wrong
with my SCT to cause aberrations when using my Naglers) or am I right,
the reviewers habitually overstate the (absolute) performance I should
expect from them?


My suggestion:

Stopping looking at the eyepieces and look through the eyepiece at the stars
above. There are plenty of decent eyepieces out there and whatever the
reviewers say about them, well, who cares?

What matters is what you see with the eyepiece.

jon

Jon Isaacs wrote:
Stopping looking at the eyepieces and look through the eyepiece at the stars
above. There are plenty of decent eyepieces out there and whatever the
reviewers say about them, well, who cares?

What matters is what you see with the eyepiece.

Although I certainly agree with that sentiment, I think Steve's got a
point. Reviews should be just that--reviews, not advertisements. If
you are the owner as well as the reviewer, then it is incumbent on you
to bend over backward in an attempt to remain objective. Otherwise,
the review will carry less weight, for all your conviction.

For instance, at least some of the Naglers show unmistakable pincushion
distortion, yet some reviewers claim it's not there. The Nagler is not
at all an orthoscopic eyepiece. I think it's likely that price is
translating into performance in the minds of some reviewers, rather than
the other way around. Now, do they not see it because they're the
owners of these fine eyepieces, or because they're comparing it to other,
less expensive wide-field eyepieces? Probably a little of both.

Reviewers ought to keep in their mind, at least in the back of it, the
many ways that their conclusions might be in error.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

In article ,
(justbeats) wrote:

Part soapbox, part question...

SNIP_

"Absolutely flat field across the whole FOV" - or
"No sign of any pincushion distortion"
Crap! The image in the 31mm and 17mm (with a 10" F/10 SCT) shows
negative pin-cushion distortion (the edges are "sucked in" rather than
"bulged out"). This isn't that noticeable when observing the sky, but
is obvious on terrestrial targets. Not a problem, but not what I'd
describe as "No sign of…", that's for sure!

SNIP_

Just a minor point, and somkeone will undoubtably correct me if I'm
wrong. Pincushion, by its very name, means 'sucked in.' When the edges
are bowed out, that's known as barrel distortion.

Starry Skies,
Rich

Richard DeLuca wrote:
Just a minor point, and somkeone will undoubtably correct me if I'm
wrong. Pincushion, by its very name, means 'sucked in.' When the edges
are bowed out, that's known as barrel distortion.

That's as I've always understood it. So pincushion distortion means
that magnification scales faster than linearly with off-axis angle,
and barrel distortion means it scales slower than linearly.

Also incidentally, I understand that distortion doesn't necessarily
imply that the star images are aberrated (although they might be).
It just means that they are formed further from the center (pincushion),
or closer to the center (barrel), than they "should" be.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

The Nagler is not
at all an orthoscopic eyepiece. I think it's likely that price is
translating into performance in the minds of some reviewers, rather than
the other way around. Now, do they not see it because they're the
owners of these fine eyepieces,

They don't see it because they don't know what all these aberrations really
are. People confuse distortion with astigmatism, atsignamtism with coma, etc.
In fact, the original poster complained about stars not being in focus at the
edge of the field, little realizing that the field curvature of his SCT is
really to blame.

The problem with almost all reviews is that people lack measuring equipment.
They cannot measure what they are reviewing.

Roland Christen

"Chris1011" wrote in message

They don't see it because they don't know what all these aberrations
really
are. People confuse distortion with astigmatism, atsignamtism with coma,
etc.
In fact, the original poster complained about stars not being in focus at
the
edge of the field, little realizing that the field curvature of his SCT is
really to blame.

Roland Christen

How about a quick primer? Let me know if this is true:

Field curvature - you can focus to correct the apperance of stars at the
edge of field, but only at the cost of the focus (sharpness) of on-axis
stars. Caused by?

Coma - An abberation resulting from the use of a parabolic mirror in a
newtonian telescope, worsens as the focal length gets shorter, if its a
problem, get a paracorr.

Astigmatism - resulting from the eyepiece, worse with faster scopes, live
with it or get a new eyepiece (or slower scope).

Ok, let me have it!

Ed

Coma - An abberation resulting from the use of a parabolic mirror in a
newtonian telescope, worsens as the focal length gets shorter, if its a
problem, get a paracorr.

To be precise, I think one should replace "Focal Length" with "Focal Ratio" and
probably "Shorter" with "Faster." But if the mirror diameter is kept
constant, it doesn't matter...

jon

Jon

Field curvature - you can focus to correct the apperance of stars at the
edge of field, but only at the cost of the focus (sharpness) of on-axis
stars. Caused by?

Can be caused by the telescope or the eyepiece. Not easy to tell which one
causes it. Short refractors and commercial SCTs have significant field
curvature. Newts and Mak-Newts have very low field curvature even at fast focal
ratios. Petzval refractors typically have no detectable field curvature.


Coma - An abberation resulting from the use of a parabolic mirror in a
newtonian telescope, worsens as the focal length gets shorter, if its a
problem, get a paracorr.

Coma can be positive or negative. Can be found in all types of telescopes,
depending on design. Coma is an open-ended or seagull shape.


Astigmatism - resulting from the eyepiece, worse with faster scopes, live
with it or get a new eyepiece (or slower scope).


Almost every telescope will show astigmatism off-axis, including coma-free
scopes like Petzvals and RCs. Even Newts will have a mix of astigmatism and
coma off-axis. Astigmatism is a closed shape resembling a football or oval.
However, most eyepieces have even more astigmatism off-axis than a typical
telescope, so their aberration will dominate. This includes flat-field and
orthoscopic ocular designs. The amount of astigmatism exibited will generally
be higher with faster F-ratio light beams.

Distortion is a change of magnification across the field. It will misplace the
stars in the outer part of the field, but not change their individual shape.
They can still be quite sharp.

Orthoscopic oculars are free of distortion and thus can be used with reticles
in applications where you are measuring tiny distances (measuring microscopes).
Orthoscopics are not necessarily free of astigmatism and can show mishaped star
images at the field edges. An eyepiece that has no distortion and is free of
astigmatism and coma at the edges of the field is orthoscopic, anastigmatic and
aplanic.The same would be true of a telescope that had all these
characteristics. Usually, one or two of these is sacrificed to achieve the
other(s).

Roland Christen