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Figure 10-11.  Lens Bench
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TM-9-254 Gearcase Transfer M548 M548A1 (
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Uses
TM 9-254 10-10. Lens Bench - Continued (4) The  observing  microscope  provides  the  high  magnification  that  is  required  to  observe  and  study  any image formed by the lens being tested, and is provided with a focusing adjustment. (5) The  optical  axes  of  the  microscope  and  the  collimator  must  be  alined  accurately  and  pass  through  the geometrical  center  of  the  chuck  on  the  nodal  slide.    In  addition,  the  ways  on  which  the  microscope slides,  must  be  sufficiently  precise  so  that  this  alinement  will  not  be  affected  by  any  change  in  its location along the ways.  The alinement of collimator can be corrected by adjusting screws. b. Uses.  The lens bench is used to test lenses for the following: (1) Equivalent focal length.  The lens to be tested is mounted in the chuck of the nodal slide, using a light clamping pressure in order to avoid straining the lens, and the azimuth of the chuck is adjusted to 0.  A mark, with soft wax crayon, is placed at the center of the surface of the lens facing the microscope.  The microscope is focused on the mark and the chuck is oscillated with the lens through a few degrees of azimuth   on   each   side   of   0.      This   oscillation   will   probably   cause   the   image   of   the   crayon   in   the microscope to shift laterally.  Next, the chuck is moved longitudinally along its own slide, the microscope is  refocused,  and  chuck  is  oscillated  again.    The  process  is  repeated  until  a  position  of  the  lens  and chuck is found which will permit the image to remain stationary while the chuck  with  the  lens  is  being oscillated.  When this location has been found, the image of the crayon mark is focused as sharply as possible and the location of microscope on the ways of the bench is read by means of the vernier and scale.  This reading is often known as the constant of the lens bench, because it is the same  for  any lens tested on it. CAUTION The  microscope  must  not  be  moved  by  its  own  focusing  adjustment.    Move  the  microscope support stand along the ways of the lens bench when focusing the microscope. Move the microscope along the ways until it is focused on the image of the target.  This setting should be  corrected  by  oscillating  the  lens  in  the  chuck,  and  moving  the  chuck  and  microscope  in  their respective ways in the same manner used previously, until the image of the target is stationary.  When proper position of the chuck and the microscope is determined, the new location of the microscope on its  ways  is  read  on  the  scale,  again  using  the  vernier.    The  difference  between  this  reading  and  the constant that was previously obtained is the equivalent focal length of the lens. 10-11

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