An Open Letter from Thomas Baader

Open Letter Regarding The Zeiss Abbe II Eyepieces

We do not recommend that anyone prematurely have the Abbe II micrometric field marks eliminated...

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 hostility. Copernicus could say a few words about that subject.

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") razorsharp triangular cutouts (field marks) spaced at 90 degree intervals in the field stop 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 that feature.

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:

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 recticle. Within a second the observer can move the eye near to the eye ens and precisely check if the object is at an equal distance from the four field marks.
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.
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.
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 Zeiss ring micrometer eyepieces introduced almost a century ago, but in a modern form that doesn't obstruct or degrade the field of view in any way.

Thomas Baader
Baader Planetarium GmbH
Mammendorf/Munich Germany

Astro-Physics, Inc.
11250 Forest Hills Road, Rockford, IL 61115, U.S.A.
Phone: 815-282-1513 Fax: 815-282-9847