One of the main performance differences between
doublets and triplets is their color correction at the color extremes. One can
see this difference in the Chromatic Aberration graph shown on the WO website
for the two scopes. In the ED triplet all the colors intersect close to a point
about 75% up the y-axis. This means that on average all the
colors focus at the same focus position. The deviation in the x direction is the
amount this average is blurred, or spherical aberration. Ideally all the colors
would be vertical lines that all laid on top of each other. The color code for
the graph can be found under the spot diagram for the ZS105. The graph for
Megrez Fluorite doublet shows that the blue line (437nm) and the right red line
(707nm) do not intersect with the other colors like they do in the ED triplet.
This means that these two colors will be out of focal when the other colors are
in focus. The eye is not very sensitive to these extreme colors, so for visual
use, this blur is likely to not be very noticeable and if noticed, only at
high power. For photographic use these extreme colors, especially the
437nm, become more important. The photographic process is not perfect. As a
result the photographic image trends to show a brighter "fringe" than what would
be seen with our eye through the eyepiece. For prime focus photography without a
barlow, you will likely see very little difference between the scopes. It is
only when you start to increase your magnification with barlows, powermates or
eyepiece projection that the difference in the two scope should start to show.
My experience is most daylight images of birds. Here a blue fringe around a
white bird can send a otherwise good image to the recycle bin. I use a Pentax
ED80 spotting scope. This scope typically gets high marks from visual reviewer,
in fact 'better view desired" rates it as the best 80mm spotting scope.
Typically blue fringing limits the power I can use photographically with this
scope.I typically do not notice this fringing visually. For reference I am
sure that the WO fluorite doublet would have better color correction than the
If there are any astrophotographers out there that
have real world experience with doublets compared to triplets please
----- Original Message -----
Sent: Friday, March 10, 2006 6:32
Subject: [William-Optics] Re: doblet
fluorite or triplet ED???
Tom Trusak of CloudyNights has about the best
explanation that I
have seen that made any sense to me.
I'm sure others have opinions on this as well.
--- In William-Optics@yahoogroups.com,
anybody explain the pros and cons of doublet fluorite and
> The price is about the same for a Megrez in either
> Is the CA level very different? I guess the
color on the moon or
> bright stars should be pretty much the
> I'm thinking on APOgrading my Megrez but the price of
the lens is
> enough to consider selling my actual telescope
and buying a new
> At this moment I think that 3
lenses is heavier than 2 and maybe
> require a longer cooling
time. Any other thoughts?