Review of TMB optical Monocentric eyepieces

On Fri, 25 Jun 2004 15:51:47 +0000 (UTC), (Brian Tung)
wrote:

Tom T. wrote:
I was loaned a couple for review - No astigmatism in the ones I saw,
but they do have a curved field that becomes fairly noticable on fast
scopes.

Since fast scopes often have significantly curved fields themselves, are
you certain you are seeing the field curvature of the eyepiece, instead
of the field curvature of the objective? After all, the field curvature
of the eyepiece shouldn't change from telescope to telescope, it seems
to me.

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

A good point -

The nagler and speers waler zooms, multiple types of orthos and
plossls, as well as several other types of eyepieces pretty much all
showed flatter fields (to the same off axial degree) than the monos on
the same scope(s).

Tom T.

Brian Tung wrote:

lightshow wrote:

This apparantly from Tom Back last week on The TMB Yahoo group.
Hopefully, this will explain what happened ? :-)


Yes, indeed--if true, it explains quite a bit. That's rather unfortunate
for Tom and his company.

I thought one of Tom Back's selling points was personally testing each
optic before it goes out? Maybe that only applied to Apo assemblies?

In any case, I assume that any flawed optic would immediately be
replaced; so this wouldn't be a major problem for a KNOWLEDGABLE user,
who would merely return the optic for a replacement. However, someone
like me - still very early on the learning curve - might deliberately
buy from TMB for the quality[1] but not be in a position to recognize
minor flaws and request a replacement. A catch 22 situation.

[1] for among the first things one learns is quality is important, that
quality varies, and that quality costs.

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

Brian Tung wrote:

lightshow wrote:

This apparantly from Tom Back last week on The TMB Yahoo group.
Hopefully, this will explain what happened ? :-)


Yes, indeed--if true, it explains quite a bit. That's rather unfortunate
for Tom and his company.

I thought one of Tom Back's selling points was personally testing each
optic before it goes out? Maybe that only applied to Apo assemblies?

In any case, I assume that any flawed optic would immediately be
replaced; so this wouldn't be a major problem for a KNOWLEDGABLE user,
who would merely return the optic for a replacement. However, someone
like me - still very early on the learning curve - might deliberately
buy from TMB for the quality[1] but not be in a position to recognize
minor flaws and request a replacement. A catch 22 situation.

[1] for among the first things one learns is quality is important, that
quality varies, and that quality costs.

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

On Fri, 25 Jun 2004 15:51:47 +0000 (UTC), (Brian Tung)
wrote:

Tom T. wrote:
I was loaned a couple for review - No astigmatism in the ones I saw,
but they do have a curved field that becomes fairly noticable on fast
scopes.

Since fast scopes often have significantly curved fields themselves, are
you certain you are seeing the field curvature of the eyepiece, instead
of the field curvature of the objective? After all, the field curvature
of the eyepiece shouldn't change from telescope to telescope, it seems
to me.

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


I should probably add -

I tested them in a variety of scopes, both fast and slow. Two of the
faster scopes were a 15" f5 newt (with and without parcorr) and a
Genesis SDF (f5.4). IIRC, the field curvature was quite noticable in
both.

Tom T.

On Fri, 25 Jun 2004 15:51:47 +0000 (UTC), (Brian Tung)
wrote:

Tom T. wrote:
I was loaned a couple for review - No astigmatism in the ones I saw,
but they do have a curved field that becomes fairly noticable on fast
scopes.

Since fast scopes often have significantly curved fields themselves, are
you certain you are seeing the field curvature of the eyepiece, instead
of the field curvature of the objective? After all, the field curvature
of the eyepiece shouldn't change from telescope to telescope, it seems
to me.

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


I should probably add -

I tested them in a variety of scopes, both fast and slow. Two of the
faster scopes were a 15" f5 newt (with and without parcorr) and a
Genesis SDF (f5.4). IIRC, the field curvature was quite noticable in
both.

Tom T.

Clif

what means thin lenses ?
see here the optical sheme and tell me , is it a Mono or not ?
http://www.apm-telescopes.de/images/...s/apm/14mm.jpg

Markus


"Clif" wrote in message
om

(Brian Tung) wrote in message ...
Phil Wheeler wrote:
What is a "moncentric" EP?

As I recall, a monocentric eyepiece is a three-element, one-group
eyepiece in which every surface has the same center of curvature--hence
the name. If anyone knows differently, please speak up.

THat's what I thought too. The eyepieces reviewed in the article were described
as being thin lenses. There is no way that a cemented triplet of thin lenses can
have a common center of curvature. That was the basis for the freedom from
astigmatism in the original design. I am not sure I would call these modern
eyepieces monocentric...
Clif




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Clif

what means thin lenses ?
see here the optical sheme and tell me , is it a Mono or not ?
http://www.apm-telescopes.de/images/...s/apm/14mm.jpg

Markus


"Clif" wrote in message
om

(Brian Tung) wrote in message ...
Phil Wheeler wrote:
What is a "moncentric" EP?

As I recall, a monocentric eyepiece is a three-element, one-group
eyepiece in which every surface has the same center of curvature--hence
the name. If anyone knows differently, please speak up.

THat's what I thought too. The eyepieces reviewed in the article were described
as being thin lenses. There is no way that a cemented triplet of thin lenses can
have a common center of curvature. That was the basis for the freedom from
astigmatism in the original design. I am not sure I would call these modern
eyepieces monocentric...
Clif




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Is the field curvature of the eyepiece worst at F/4.5?

I find the Televue plossls quite acceptable in fast scopes, will the SM
disappoint in otherwise high optical quality dobs?

Is the field curvature of the eyepiece worst at F/4.5?

I find the Televue plossls quite acceptable in fast scopes, will the SM
disappoint in otherwise high optical quality dobs?

wrong tools are

1, to test a planetary eyepiece at low power like he did , his scope 8"
F/4.5 ,
means focallenght 913 mm : 8 mm = 114 power, jip, thats the right power
to do Planetary
Observing in a 8" scope

2, How many out know that for Planetary Observing the scope must be 101
%
collimated. How many have hear scope of such fast f-ratio really 100%
dead on
collimated ? I mean really 100% ?

3, Dont know , but usual experienced Planetary Observers dont
use a comacorrector for observing faintest details on Planets


etc.

Markus


"Leonard" wrote in message
om

"Stephen Pitt" wrote in message news:b7061404ce8795d56ac6cc2edb1bc619.5675@mygate .mailgate.org...
Gary was wrong. It happens when using the wrong tools and asking-or
not asking-the appropriate questions.


Hi Stephen ,

Interesting post , what are the wrong tools and
questions he used and asked ? And in your opinion what are the
appropriate questions and tools ?
It seems to me he did a very competent review , no ?
Leonard




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