Notorious "SEMI-APO" term used again.

7450 From: dhennig2 dhennig2@t...
Date: Tue Nov 23, 2004 1:00am
Subject: 92mm Fluorite - clarification on definitions?


Something here is really frustrating me.

First, Tom, congratulations on getting a fine scope. The shots
through it are most interesting...

Now - it's the term "Semi-Apo" being used for this fluorite scope. I
recall many months ago [about 2-1/2 years to be exact] that Vic Maris
of Stellarvue was roasted fiercely by several parties eluding that
his 102EDT f/6.1 triplet refractor could be dubbed semi-apo. There
were rants all over SAA, the SV Yahoo! group, and elsewhere, scathing
SV for using such a term.

Interestingly enough, it was also a time when Tom Back got everyone
to think of scopes in terms of either APO or achro performance, based
on their color performance and spot behavior throughout the spectrum.
There was a LOT of talk back and forth regarding this, then a hot
topic amoungst refractor afficiandos.

Why now is the term Semi-Apo being allowed to pervade the marketing
once again, and what right does the likes of WO and BO [and for that
matter TMB] have to use this "iffy" term now, when not even 3 years
ago it was considered a false marketing term that did not mean
anything?

I just find it very interesting that all of a sudden, this seems okay
for everyone with a Chinese-made optic. It seems very hypocritical to
me. Some of the members here are ones who did the initial stuff
against Stellarvue [SV] for this, and now its okay. Not fair!

I suggest that someone here [Tom?] pin this issue down tight, else
risk flooding the refractor marketplace with seemingly meaningless
terms.

Darren.
================================================== ====================

Darren, I do understand you NOW. But, one question - were you was
at that time? You suppported "semi" camp? ;)

It was not fair in a past, it is not fair now and it will be not fair
in a future.

Note, that WO already used this notorious term to increase selling of
their plain achromat (Mrgrez-I) and they pioneered such apporach to
color correction "improvement" and hidden internal aperture stop.

It was then used in BO prototype 102F/6 - to hide poor edge performance
and improve color correction.

One of SV scope also was made with internal color correction improver
and another one with "special" formula - notorious MV cut filter inside
of it's objective.

What will be the next trick to foolish the public, which still believe in
magic in optics - in magic from peoples, who don't know the optics!


VD

Valery,

Please make Roland his eyepieces and stop trying to stir
up meaningless controversy. I don't think that 3X color
corection improvement (which is what the TMB design
on the new scope called the Burgess/TMB Semi-APO
provides) is out of the question to be discussed as a
semi-apo. I know what Vic Maris sold. Thomas Back
has designed a few lenses that have color correction far
better than the scope in question from StellarVue. The
color correction on the scope Vic called an EDT was
nowhere near the level we are discussing here. In that
case, it was a triplet, but it did not have much more that
achromat correction, due to the design and glasses used.
The term semi-apo would not really have applied there,
and the use of the term "EDT" was unfortunate, as it did
not use ED glass. In this case the term semi-apo is correct,
as it does provide enough improvement over a well-corrected
achromat (3X) to make the term have merit. Anyone
looking through this scope would readily see a marked
color correction improvement.

I really think that if you choose to address anyone on this
subject, address Thomas Back. He chose the name, and I
think he might have an idea of the difference between an
achromat and an APO. He has designed at least a few of
them that are in happy customer's hands.

Thanks, Tom Davis



"ValeryD" wrote in message
om...
7450 From: dhennig2 dhennig2@t...
Date: Tue Nov 23, 2004 1:00am
Subject: 92mm Fluorite - clarification on definitions?


Something here is really frustrating me.

First, Tom, congratulations on getting a fine scope. The shots
through it are most interesting...

Now - it's the term "Semi-Apo" being used for this fluorite scope. I
recall many months ago [about 2-1/2 years to be exact] that Vic Maris
of Stellarvue was roasted fiercely by several parties eluding that
his 102EDT f/6.1 triplet refractor could be dubbed semi-apo. There
were rants all over SAA, the SV Yahoo! group, and elsewhere, scathing
SV for using such a term.

Interestingly enough, it was also a time when Tom Back got everyone
to think of scopes in terms of either APO or achro performance, based
on their color performance and spot behavior throughout the spectrum.
There was a LOT of talk back and forth regarding this, then a hot
topic amoungst refractor afficiandos.

Why now is the term Semi-Apo being allowed to pervade the marketing
once again, and what right does the likes of WO and BO [and for that
matter TMB] have to use this "iffy" term now, when not even 3 years
ago it was considered a false marketing term that did not mean
anything?

I just find it very interesting that all of a sudden, this seems okay
for everyone with a Chinese-made optic. It seems very hypocritical to
me. Some of the members here are ones who did the initial stuff
against Stellarvue [SV] for this, and now its okay. Not fair!

I suggest that someone here [Tom?] pin this issue down tight, else
risk flooding the refractor marketplace with seemingly meaningless
terms.

Darren.
================================================== ====================

Darren, I do understand you NOW. But, one question - were you was
at that time? You suppported "semi" camp? ;)

It was not fair in a past, it is not fair now and it will be not fair
in a future.

Note, that WO already used this notorious term to increase selling of
their plain achromat (Mrgrez-I) and they pioneered such apporach to
color correction "improvement" and hidden internal aperture stop.

It was then used in BO prototype 102F/6 - to hide poor edge performance
and improve color correction.

One of SV scope also was made with internal color correction improver
and another one with "special" formula - notorious MV cut filter inside
of it's objective.

What will be the next trick to foolish the public, which still believe in
magic in optics - in magic from peoples, who don't know the optics!


VD

"Tom Davis" wrote in message ...
Valery,

Please make Roland his eyepieces and stop trying to stir
up meaningless controversy.

1. Eyepieces were not stopped. In fact a lot were made and
waiting the papers form local customs. We have very good
manufacturing rate, especially if consider the fact, that
two of three lenses in this design have very steep curves
and can be made only one by one, not in blocks.
If consider their very small diameters, then you can imagine
how difficult to make them.

And I do not stir up the water here - some other companies
did and doing now.



I don't think that 3X color corection improvement (which is
what the TMB design on the new scope called the Burgess/TMB
Semi-APO
provides) is out of the question to be discussed as a
semi-apo. I know what Vic Maris sold. Thomas Back
has designed a few lenses that have color correction far
better than the scope in question from StellarVue. The
color correction on the scope Vic called an EDT was
nowhere near the level we are discussing here. In that
case, it was a triplet, but it did not have much more that
achromat correction, due to the design and glasses used.
The term semi-apo would not really have applied there,
and the use of the term "EDT" was unfortunate, as it did
not use ED glass. In this case the term semi-apo is correct,
as it does provide enough improvement over a well-corrected
achromat (3X) to make the term have merit. Anyone
looking through this scope would readily see a marked
color correction improvement.

Did I said, that 3x color correction improvement is not a
"semi-apo" ??? Indeed, even 2x correction can be called as
"semi-apo".

However, I still have to see a real spot diagram for that
102mm F/6 doublet vs C-F achromat of the same size and F/D
to conclude if this is really semi-apo.
Note, that even this is not convinceable! If a scope has
3x smaller spot diagrams (in diameter), this absolutely
does not mean, that this design has 3x better color correction.
Untill I see a graph of longitudinal color aberrations, I
can't say, that this objective has 3x better color correction.
I also know, that it is quite difficult to find glasses for
_doublet_, which will deliver 3x better color correction and
will not use real ED glass. This may be even impossible.
I should remind - 3x better color correction means, that
longitudinal color aberration is 3x shorter, not 3x smaller
spots!!!!

I should remind also, that Meade ED apos have only about
4x better color correction, than ordinary achromat.
But they use true ED glass FPL51 with matched crown-flint
KF-3. Not one doublet can deliver 3x better color correction,
than achromat being F/6 and use only short-flint as ubnormal
glass. Even if some combinations are possible, they have very
steep surfaces and very sensitive to collimation and lenses
decentering.

Therefore, I think, that that SEMI-APO in question does not
has 3x shorter secondary spectrum and just has 3x smaller
spot diagrams - in other words - the next in a row trick.


I really think that if you choose to address anyone on this
subject, address Thomas Back. He chose the name, and I
think he might have an idea of the difference between an
achromat and an APO. He has designed at least a few of
them that are in happy customer's hands.

Tom, were did you saw, that I adressed these poits to YOU???
You just speak instead of Bill and Tom Back.


VD

In article ,
RichA wrote:

Why don't you refractor makers simply tell the rest of
us what the colour correction is? That way, we'd have
a way of automatically comparing all scopes?

Because it's not at all clear how you'd *measure* colour correction;
it's not really a single number. I think it's at least a
four-dimensional function, maybe five; you want the shape of the point
spread function at multiple wavelengths at multiple distances from the
optical axis, and I'm not completely sure that you can ignore the
angle of the image within the field of view.

[on the other hand you're only losing one dimension in this confusing
function when looking at reflective optics; but the other N-1
dimensions kick in nastily if you're trying to compare different
multi-mirror wide-field designs, or designing things to compensate for
coma in short-focus reflectors. I'm not sure why those debates are so
much less loud than the refractor one. Central obstructions, which are
entirely a matter of the _shape_ of the PSF, are I suppose one of the
other major debates]

You could compute "assuming perfect manufacturing and neglecting
diffraction, the diameter of the disc encompassing 90% of the energy
in the PSF is below [mumble] arcseconds between 350nm and 700nm"; you
could probably compensate for known imperfections in manufacturing;
but people would still debate whether 90% or 95% is the right energy
to use, and ask whether it's better for the widest PSF to be at 450 or
at 650 nanometres.

It's intrinsically multi-dimensional, and there's no good ordering on
such things. Hence debate.

Tom

It's intrinsically multi-dimensional, and there's no good ordering on
such things. Hence debate.

Tom

Hey Tom, how about joining the ATM design group on Yahoo:

http://groups.yahoo.com/group/ATM_Optics_Software/

Rolando

On 25 Nov 2004 00:37:39 +0000 (GMT), Thomas Womack
wrote:

In article ,
RichA wrote:

Why don't you refractor makers simply tell the rest of
us what the colour correction is? That way, we'd have
a way of automatically comparing all scopes?

Because it's not at all clear how you'd *measure* colour correction;
it's not really a single number. I think it's at least a
four-dimensional function, maybe five; you want the shape of the point
spread function at multiple wavelengths at multiple distances from the
optical axis, and I'm not completely sure that you can ignore the
angle of the image within the field of view.

[on the other hand you're only losing one dimension in this confusing
function when looking at reflective optics; but the other N-1
dimensions kick in nastily if you're trying to compare different
multi-mirror wide-field designs, or designing things to compensate for
coma in short-focus reflectors. I'm not sure why those debates are so
much less loud than the refractor one. Central obstructions, which are
entirely a matter of the _shape_ of the PSF, are I suppose one of the
other major debates]

You could compute "assuming perfect manufacturing and neglecting
diffraction, the diameter of the disc encompassing 90% of the energy
in the PSF is below [mumble] arcseconds between 350nm and 700nm"; you
could probably compensate for known imperfections in manufacturing;
but people would still debate whether 90% or 95% is the right energy
to use, and ask whether it's better for the widest PSF to be at 450 or
at 650 nanometres.

It's intrinsically multi-dimensional, and there's no good ordering on
such things. Hence debate.

Tom

How about starting with the visual? Lets say given the average
human's colour perception this ( ) measurement will mean no
in-focus colour on Venus? Seems pretty straight forward.
As for manufacturing variances, the apo mfgs won't agree there
ever ARE any, except with competitor's lines.

Thomas Womack wrote:
Because it's not at all clear how you'd *measure* colour correction;
it's not really a single number. I think it's at least a
four-dimensional function, maybe five; you want the shape of the point
spread function at multiple wavelengths at multiple distances from the
optical axis, and I'm not completely sure that you can ignore the
angle of the image within the field of view.

I think most of us would settle for the longitudinal chromatic aberration,
C to f (or whatever the usual standard is). One part in 2,000? In 4,000?
In 8,000? In 16,000? That sort of thing. No, it would not capture the
shape of the focal length vs wavelength curve, and it says nothing about
the spherochromatism, but it's one piece of information we don't often get
for anything beyond an achromat. (I'm willing to stipulate that any achro
will get in the neighborhood of 1 part in 2,000.)

When people talk color correction, I think they usually mean on-axis and
geometrically, so that we can ignore diffraction effects and the off-axis
distortions.

Of course, I agree that color correction has, in its entirety, many
dimensions. But I also think it's somewhat disingenous to suggest that
that is why manufacturers do not give longitudinal CA in parts, say.
I think they don't do it because it leads to arguments and returns. I
suspect that most people wouldn't lose their cool, but it only takes a
few before manufacturers just get fed up with being helpful.

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

Thomas Womack wrote:

In article ,
RichA wrote:

Why don't you refractor makers simply tell the rest of
us what the colour correction is? That way, we'd have
a way of automatically comparing all scopes?

Because it's not at all clear how you'd *measure* colour correction;
it's not really a single number.

Preposterosis! It's totally clear to me and to others, and has
been for decades (centuries?).
M.



I think it's at least a
four-dimensional function, maybe five; you want the shape of the point
spread function at multiple wavelengths at multiple distances from the
optical axis, and I'm not completely sure that you can ignore the
angle of the image within the field of view.

[on the other hand you're only losing one dimension in this confusing
function when looking at reflective optics; but the other N-1
dimensions kick in nastily if you're trying to compare different
multi-mirror wide-field designs, or designing things to compensate for
coma in short-focus reflectors. I'm not sure why those debates are so
much less loud than the refractor one. Central obstructions, which are
entirely a matter of the _shape_ of the PSF, are I suppose one of the
other major debates]

You could compute "assuming perfect manufacturing and neglecting
diffraction, the diameter of the disc encompassing 90% of the energy
in the PSF is below [mumble] arcseconds between 350nm and 700nm"; you
could probably compensate for known imperfections in manufacturing;
but people would still debate whether 90% or 95% is the right energy
to use, and ask whether it's better for the widest PSF to be at 450 or
at 650 nanometres.

It's intrinsically multi-dimensional, and there's no good ordering on
such things. Hence debate.

Tom

Thomas Womack wrote:

In article ,
RichA wrote:

Why don't you refractor makers simply tell the rest of
us what the colour correction is? That way, we'd have
a way of automatically comparing all scopes?

Because it's not at all clear how you'd *measure* colour correction;
it's not really a single number. I think it's at least a
four-dimensional function, maybe five; you want the shape of the point
spread function at multiple wavelengths at multiple distances from the
optical axis, and I'm not completely sure that you can ignore the
angle of the image within the field of view.

[on the other hand you're only losing one dimension in this confusing
function when looking at reflective optics; but the other N-1
dimensions kick in nastily if you're trying to compare different
multi-mirror wide-field designs, or designing things to compensate for
coma in short-focus reflectors. I'm not sure why those debates are so
much less loud than the refractor one. Central obstructions, which are
entirely a matter of the _shape_ of the PSF, are I suppose one of the
other major debates]

You could compute "assuming perfect manufacturing and neglecting
diffraction, the diameter of the disc encompassing 90% of the energy
in the PSF is below [mumble] arcseconds between 350nm and 700nm"; you
could probably compensate for known imperfections in manufacturing;
but people would still debate whether 90% or 95% is the right energy
to use, and ask whether it's better for the widest PSF to be at 450 or
at 650 nanometres.

It's intrinsically multi-dimensional, and there's no good ordering on
such things.

Baloney.

Hence debate.

Hence obfuscation, ambivalence, commercialistic gobble wobble
.............. since there's money at stake! The Chinese will settle
it all for you. Good Chapter-7 luck.

Mark





Tom

They will NEVER answer you - there is $$$$$$$$$$ at stake.



RichA wrote:

On 25 Nov 2004 00:37:39 +0000 (GMT), Thomas Womack
wrote:

In article ,
RichA wrote:

Why don't you refractor makers simply tell the rest of
us what the colour correction is? That way, we'd have
a way of automatically comparing all scopes?

Because it's not at all clear how you'd *measure* colour correction;
it's not really a single number. I think it's at least a
four-dimensional function, maybe five; you want the shape of the point
spread function at multiple wavelengths at multiple distances from the
optical axis, and I'm not completely sure that you can ignore the
angle of the image within the field of view.

[on the other hand you're only losing one dimension in this confusing
function when looking at reflective optics; but the other N-1
dimensions kick in nastily if you're trying to compare different
multi-mirror wide-field designs, or designing things to compensate for
coma in short-focus reflectors. I'm not sure why those debates are so
much less loud than the refractor one. Central obstructions, which are
entirely a matter of the _shape_ of the PSF, are I suppose one of the
other major debates]

You could compute "assuming perfect manufacturing and neglecting
diffraction, the diameter of the disc encompassing 90% of the energy
in the PSF is below [mumble] arcseconds between 350nm and 700nm"; you
could probably compensate for known imperfections in manufacturing;
but people would still debate whether 90% or 95% is the right energy
to use, and ask whether it's better for the widest PSF to be at 450 or
at 650 nanometres.

It's intrinsically multi-dimensional, and there's no good ordering on
such things. Hence debate.

Tom

How about starting with the visual? Lets say given the average
human's colour perception this ( ) measurement will mean no
in-focus colour on Venus? Seems pretty straight forward.
As for manufacturing variances, the apo mfgs won't agree there
ever ARE any, except with competitor's lines.