jet. 
al 
he 
‘es 
verse 
the 
1gi- 
hich 
end. 
he 
| - 
zht 
  
  
be located precisely at the microscope focus and means will be made avail- 
able for adjusting the reticle position to the plate fiducial marks both in 
X, Y, and rotational motion. 
2. Positioning Sensing Device 
The position sensing device will automatically seek the density 
center of the image by adjusting the position of the plate carriage until the 
quantity of light from each of four quadrants in the image plane is indis- 
tinguishable by quadrant. Pulsed electrical signals from a photomultiplier 
which scans each quadrant in sequence are sorted by commutating gating 
signals and compared for signal difference between quadrants. Motion of 
the plate carriage in the X or Y direction results from an unbalanced sig- 
nal between the quadrants after they are summed and compared in the pro- 
per combination. Means are also provided for automatically changing the 
magnification of the spot image at the quadrant ring position so that pre- 
cision centering is obtained. The scanning system has two principle fea- 
tures. One is the removal of the sensing elements from the photographic 
plate area by a special optical design; so that effects of heat and vibration 
produced by the sensing head do not disturb the plate. The other is the 
employment of techniques to prevent the formation of defined images on the 
sensitive surface of the scanning photomultiplier. The latter must be 
avoided, because the well-known variations in local sensitivity of present- 
day photomultipliers would result in erroneous weighting of star-image 
densities according to their position in the quadrants. 
Metering System 
The AHAC automatic metering system is incorporated into the basic 
machine as has been done with the Plate Previewer. The digital display 
is presented in millimeters rather than in wavelengths of light. Micron 
units displays can readily be obtained by shift of decimal points on the 
display, if desired. 
The proposed AHAC interferometric metering system will initially provide 
both absolute accuracies and comparative accuracies of better than +0. 25 
micron continuously over the full range of 200 mm. and simultaneously on 
both the X and Y axes. It may be possible to further extend the measure- 
ment range considerably and increase the accuracy to approximately +. 01 
microns ata later time. "Thus, larger photographic plates could be meas- 
ured, and the AHAC might also be used for measurement purposes other 
than photographic. 
The AHAC will also incorporate a manually reading, integral metering 
system, This system is presently under development and information con- 
cerning the system is of a propriety nature. As with the Plate Previewer, it 
will be possible to cross-compare measurement readings with two entirely 
independent and separate metering systems. 
13