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The principle requirement of the drive is that it be able to position the 
carriages to a repeatability of approximately 0.1 micron or less, such 
that the overall accuracy requirementof +0. 5 microcan be realistically met. 
The dead zone width of the positioning system must be equal to or less 
than the repeatability to avoid continuous servo hunting or jitter. The 
drive must also be capable of exerting large linear (inertial) forces to 
meet the traverse rate requirements and the servo demands. 
To accomplish this, the AHAC drive system will use the controlled dis- 
tortion of a flexible element to couple the drive motor rotating shaft to 
the table. Based on the performance already demonstrated by the develop- 
ers of this technique, this will provide essentially zero fit clearances be- 
tween the critical parts of the mechanism, and at the same time will mini- 
mize sliding friction with its accompanying stick-slip and wear effects. 
Optical System 
1. General Description 
The optical system for the fine measuring device will use the 
same principles as that for the previewer except that a battery of normal 
microscope objectives will be available for the initial image pickup. The 
relatively short conjugate distances for the microscope objective requires 
that an extra stage of image formation must be present in the horizontal 
tube. The extra stage consists of a power telescope between relay lenses 
at either end. Again, the use of unit magnification eliminates all transverse 
aberrations in this stage of the imaging process. The lenses making up 
the objective and eyepiece ends of the unit power telescope, as well as the 
two relay lenses, must be well corrected for spherical aberrations, longi- 
tudinal color, and curvature of field. The correction problems that are 
faced in this case are also encountered in all military instruments in which 
image relaying takes place and the means for correction are well under- 
stood. 
  
All magnification change in this system will take place at the objective end. 
If, for any reason, it is found that a different range of magnification is 
needed, the change will be effected by either switching binocular heads or 
merely the eyepieces themselves. 
The condensing system can be adjusted automatically to accommodate the 
fields and numerical aperture of the microscope objectives mentioned a- 
bove. Provision will be made in the condensing system to adjust the light 
intensity and for insertion of color filters and diffusion glasses. 
  
The binocular eyepiece end will provide for adjustable interpupillary dis- 
tance and individual eyepiece focus. The eyepieces themselves will be | 
generally of the Ramsden or Kellner types so that the specified reticles | 
may be used, The reticle insertion slot will be such that the reticle will |