We do not recommend that
anyone prematurely have the Abbe II micrometric field marks eliminated simply
because a single negative comment has been posted on the web, long before
regular deliveries can begin and additional customers can provide feedback.
Any owner of a set of ABBE II oculars who imagines that this feature may
prove to be objectionable should take comfort in the fact that the four field
marks can be easily (and permanently) removed simply by applying four tiny
opaque droplets of flat black optical paint. Consequently, at this time we
are recommending that customers accept delivery of the Abbe II eyepieces as
they were designed and fabricated by Zeiss and simply try them. We will remove
these four marks free of charge for any customer requesting this service.
In any event, these field marks are part of a unique design that is no doubt
destined to be regarded as a collectible of historical significance. Every
owner of these eyepieces should think long and hard about (permanently)
removing them, thereby possibly detracting from the future market value of a
cherished set of unique eyepieces. In time the astronomical community may
find this feature acceptable or even embrace it as praiseworthy. History
affords many examples of new things being treated with suspicion or
Perhaps you have seen a specimen
of the old Zeiss measuring
eyepieces that featured a thick black ring centered in the field of view.
These were the Zeiss “ring micrometer” eyepieces that were used to
measure the time required for an object to pass from one side
of the ring to the other, taking advantage of the apparent
motion imparted by the Earth’s rotation. Unfortunately, this
clever device lacked accuracy because it was not possible to
determine if the object had passed through the precise center
of the field.
This deficiency has been eliminated by a new refinement called
the "mikrometrische Feldblende" that is incorporated in each
Abbe II eyepiece.
This takes the form of four very small (about 0.15 mm/ 0.006") razor sharp
triangular cutouts (field marks) spaced at 90 degree intervals in the field
of the eyepiece.
You can choose whether they are visible or not by varying the
distance between your eye and the ocular’s eye lens [add space
between eye and lens]. If you maintain an adequate distance you
won't see the field marks, but if you want to use the eyepiece for
measuring purposes or for determining the center of the field of
view you need only move your eye nearer the eye lens and use
Each eyepiece has two additional matched baffles in front of the
field stop to prevent any unwanted stray light from falling onto
the lens edges. These baffles are carefully designed so as to not
vignette the incident beam down to a telescope focal ratio of f/4.
The ABBE II micrometric field stop can be used for various purposes:
1. Virtual crosshair - to very precisely locate an
object in the center of the field of view
- without seeing any obstruction in the field -
as would be the effect of a crosshair reticle.
Within a second the observer can move the eye near
to the eye lens and precisely check if the object is at an equal
distance from the four field marks.
2. Astigmatism checker - a device for checking the effects
of your own eyes astigmatism.
Many observers suffer from astigmatism. This will
clearly show up when the observer concentrates
on observing the angular position of the field marks.
The spacing of the field marks will appear to differ from 90 degrees
from each other for an astigmatic eye. The effect gets
smaller with shorter focal length eyepieces because the
exiting beam of light decreases in diameter to the point that it is
no longer affected by deformations of the cornea.
When the field marks appear equally spaced in the 4 mm (or 6 mm)
eyepiece, the observer knows that he does not suffer from
astigmatism at this high a magnification.
3. Alignment tool – To determine whether an object drifts precisely
through the center of the field of view, simply rotate the
eyepiece until the star drifts from the left field mark and reappears
in the right one.
4. Measuring tool - Once the eyepiece has been properly
aligned as described above, the object will
drift exactly through the field of view dead center, permitting
very accurate timing measurements.
In essence the ABBE II micrometric field stop offers the same function as
ring micrometer eyepieces introduced over a century ago, but in a modern
form that doesn’t obstruct or degrade the field of view in any way.