Zeiss Abbe Micrometric Field Marks


 

 


 

 

 

A few words from Thomas Baader, regarding the new Abbe II micrometric Field Marks:

 

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 hostility.



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 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:
 
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 Zeiss
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.
 

 


last updated 07/26/08

 

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