## Re: Colour coding on the aberration graph

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• Clive, thanks! Dmitri
Message 1 of 7 , Jan 23, 2006
Clive, thanks!

Dmitri
• OK, so here is what I believe it means. I am no expert, but I did look into this a bit some time ago. FWIW, this page may help explain the issues:
Message 2 of 7 , Jan 23, 2006
OK, so here is what I believe it means. I am no expert, but I did
look into this a bit some time ago. FWIW, this page may help explain
the issues:
http://www.vanwalree.com/optics/chromatic.html

The "Longitudinal Aberration" is showing the distance between focal
planes for light of varying wavelengths. Red light focuses in a
different place that blue light. As we visually focus on green (where
we are most sensitive), the shift in focal planes for different freqs
of light "blur" those freqs and lead to things like color halos.

So, what this graph is showing is for different frequencies (I think
we can safely ignore the 435 and 707nm) how far off focus are we? If
we find the spot where they cluster the most (about the third tick
from the top) and say that we have our focuser at this position
(y-axis here is focal plane position), we can see that we have a nice
clustering. Ignore the dark blue and the dark red (getting pretty UV
and IR) and the focal plane shift for a nice cluster in there save the
~656 (Ha-like line) is on the order of .05mm.

Craig
• Someone has already answered the color question. I thought I would answer your question on what the graph means. The graph shows how different wavelengths
Message 3 of 7 , Jan 23, 2006
Someone has already answered the color question. I thought I would answer your question on what the graph means. The graph shows how different wavelengths focus as a function of lens radial position. The bottom of the y axis is the center of the lens. The top is the outer diameter. Restated this graph shows the spherical aberration of the lens at different colors and chromatic aberration. The eye is most sensitive to light around 550nmeter, so the graphs should be about a straight line at this wavelength in a well designed lens. The graph for the Fluorite Doublet shows the lens to be very well corrected at all but the wavelength extremes. At 435nmeters the focus point is about .125mm from the 550nmeter point. (typically the focus position is calculated at 70% of full aperture). This shift in focus will cause a blur diameter of about .02mm (.125/f-stop). So at 435nm the blur is much larger than due to diffraction(around .008mm). For visual use this is of little concern because the eye has very little sensitivity at this wavelength. One of the questions floating around has been what is the difference between an APO doublet and an APO triplet. WO address this question with the longitudinal aberration chart shown in Megrez 80 fluorite triplet. It shows the curves for a doublet and a triplet. In general the triplet can bring the focus points a the 70% of max radius, much closer together over the full visual range. Also look at the curves for the Megrez 80 II ED triplet APO. Even though it uses a lesser glass than the fluorite doublet, the color correction at the extremes is much better. The comparison between a triplet and a doublet is also shown on the WO website in the 66 ED APO section. Here spot diameters are shown. As one can see the spot diameters are much small at the wavelength extremes for the triplet. The photographic process tends to be more sensitive to the color extremes. This is why triplet are often preferred for photographic work. I hope this helps.

Scott

----- Original Message -----
Sent: Monday, January 23, 2006 7:19 AM
Subject: [William-Optics] Re: Colour coding on the aberration graph

Good question Dmitri!

I want to know more than that, I also want someone to explain how to
read it and what it really means.

I want to be able to add this to the FAQs I am working on for the
group, as well as for my own understanding.

I actually went out and downloaded the Zemax demo, but even after
don't understand what the charts mean, to be honest, and how to use
them to compare one scope to another.

The real software costs \$2000-\$4000 and then you have to pay for
training, so maybe this is one of those things that you can't learn
for free, but lets see if anyone has an explanation in our group.

Anyone?  Tom?  Ron?  Anyone feel free to offer to explain these.

Also - Tom Trusak has posted his opinions and thoughts on doublets
vs. triplets, and I will try to includes some of that in the FAQs
since it comes up often as well.  I have a few sites that are very
technical, but if anyone has an easy web site that helps explain it
feel free to post it.

The majjority of companies do NOT post these types of charts, or the
glass specs, or the color correction levels, or quality control
specifications.  Perhaps WO is just more honest, or perhaps they feel
that the charts help people, or perhaps other companies just don't
want to open the kimono that far because it lets competitors see what
they are doing, and gives them a chance to try to one-up them with a
better looking chart.

Either way - if anyone has any information, let us all know.

Timm

--- In William-Optics@yahoogroups.com, "kulginoff"
<dmitri.kulginov@e...> wrote:
>
> I would like to know the colour legend on the chromatic aberration
> graph for Zenith Star Fluorite Doublet on the WO web site,
>
> http://www.william-optics.com/wowebs/prod_tel/zsfd80/chrom.htm
>
> Thankfully,
> Dmitri
>

• Thanks Scott, It does help - I will have to read it a few times over while looking at the charts when i am not at work, and being interupted by other less
Message 4 of 7 , Jan 23, 2006
Thanks Scott,

It does help - I will have to read it a few times over while looking
at the charts when i am not at work, and being interupted by other
less important topics to fully grasp it ;-)

Do you know of any web sites that explain it?

I have Googled a number of times on things like "longitudinal
chromatic aberration", but never really found anything that would
explain those charts, so perhaps there aren't any, or perhaps I am
Googling the wrong words.

Thanks,

Timm

--- In William-Optics@yahoogroups.com, "Scott Walker" <sdwalker@c...>
wrote:
>
> Someone has already answered the color question. I thought I would
answer your question on what the graph means. The graph shows how
different wavelengths focus as a function of lens radial position.
The bottom of the y axis is the center of the lens. The top is the
outer diameter. Restated this graph shows the spherical aberration of
the lens at different colors and chromatic aberration. The eye is
most sensitive to light around 550nmeter, so the graphs should be
about a straight line at this wavelength in a well designed lens. The
graph for the Fluorite Doublet shows the lens to be very well
corrected at all but the wavelength extremes. At 435nmeters the focus
point is about .125mm from the 550nmeter point. (typically the focus
position is calculated at 70% of full aperture). This shift in focus
will cause a blur diameter of about .02mm (.125/f-stop). So at 435nm
the blur is much larger than due to diffraction(around .008mm). For
visual use this is of little concern because the eye has very little
sensitivity at this wavelength. One of the questions floating around
has been what is the difference between an APO doublet and an APO
triplet. WO address this question with the longitudinal aberration
chart shown in Megrez 80 fluorite triplet. It shows the curves for a
doublet and a triplet. In general the triplet can bring the focus
points a the 70% of max radius, much closer together over the full
visual range. Also look at the curves for the Megrez 80 II ED triplet
APO. Even though it uses a lesser glass than the fluorite doublet,
the color correction at the extremes is much better. The comparison
between a triplet and a doublet is also shown on the WO website in
the 66 ED APO section. Here spot diameters are shown. As one can see
the spot diameters are much small at the wavelength extremes for the
triplet. The photographic process tends to be more sensitive to the
color extremes. This is why triplet are often preferred for
photographic work. I hope this helps.
>
> Scott
>
>
> ----- Original Message -----
> From: Timm Bottoni
> To: William-Optics@yahoogroups.com
> Sent: Monday, January 23, 2006 7:19 AM
> Subject: [William-Optics] Re: Colour coding on the aberration
graph
>
>
> Good question Dmitri!
>
> I want to know more than that, I also want someone to explain how
to
> read it and what it really means.
>
> I want to be able to add this to the FAQs I am working on for the
> group, as well as for my own understanding.
>
> I actually went out and downloaded the Zemax demo, but even after
still
> don't understand what the charts mean, to be honest, and how to
use
> them to compare one scope to another.
>
> The real software costs \$2000-\$4000 and then you have to pay for
> training, so maybe this is one of those things that you can't
learn
> for free, but lets see if anyone has an explanation in our group.
>
> Anyone? Tom? Ron? Anyone feel free to offer to explain these.
>
> Also - Tom Trusak has posted his opinions and thoughts on
doublets
> vs. triplets, and I will try to includes some of that in the FAQs
> since it comes up often as well. I have a few sites that are
very
> technical, but if anyone has an easy web site that helps explain
it
> feel free to post it.
>
> The majjority of companies do NOT post these types of charts, or
the
> glass specs, or the color correction levels, or quality control
> specifications. Perhaps WO is just more honest, or perhaps they
feel
> that the charts help people, or perhaps other companies just
don't
> want to open the kimono that far because it lets competitors see
what
> they are doing, and gives them a chance to try to one-up them
with a
> better looking chart.
>
> Either way - if anyone has any information, let us all know.
>
> Timm
>
> --- In William-Optics@yahoogroups.com, "kulginoff"
> <dmitri.kulginov@e...> wrote:
> >
> > I would like to know the colour legend on the chromatic
aberration
> > graph for Zenith Star Fluorite Doublet on the WO web site,
> >
> > http://www.william-optics.com/wowebs/prod_tel/zsfd80/chrom.htm
> >
> > Thankfully,
> > Dmitri
> >
>
>
>
>
>
>
>
>
>
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