Paper Synopsis: Lens Centering using the Point Source Microscope Robert E' Parks

1
Paper SynopsisLens Centering using the Point
Source MicroscopeRobert E. Parks

• by Wenrui Cai
• Opti521

2
The Importance of lens centering
Modern lens design, Interferometric testing of
optical surfaces, and Computer controlled
polishing make it possible to produce excellent
optical elements.

• However, all the potential gain in performance
  can be lost at the last step of assembly.
• Thus it is important that there is a method of
  assuring the lens elements are well centered in
  their lens cell.

3
Definition of Centering

• The optical axis
• Centering is moving (tilt and decenter) optical
  surfaces until their axis lies on the mechanical
  axis defined by the lens barrel.
• We will use an air bearing rotary table to help
  with centering as well as the PSM.

4
Point Source Microscope (PSM)

• Objective focus at CC. of the surface
• A return spot from the surface focused on CCD
• Viewing spot on a monitor.

If the alignment is less than ideal, the spot
will defocused or decentered. When the cell is
rotating about its axis, the return spot will
move in a circle on screen.
5
Viewing both centers of curvature
The alignment of one center may disturb the
centering of the other. It is more expedient to
view both centers of curvature simultaneously
With a convex surface we need to add a relay lens
to find the center of curvature. Use Zemax to
calculate the distances among the lenses and the
PSM the location of the upper surface center of
curvature and the rear surface center after
refraction in the front surface.
6
Viewing both centers of curvature
Separate two centers of curvature Only light
from the correct conjugate is well focused and
bright on the detector of that PSM. All
calculations (locations of beamsplitter and PSMs)
are first order. what we are ultimately
interested in is whether the relayed return spots
from the centers of curvature move or not as the
rotary table is turned.
7
Sensitivity of Centering

• Spot motion (rotate 180)
• is 4 µm for a 1 µm decenter
• from the rotary table axis.
• there is another factor of 2.5 for 5X objective.
• image motion at the detector is 10 µm.
• The detector has 4.65 µm pixels and can centroid
  to 0.1 pixels.
• A theoretical sensitivity to surface decenter of
  about 50 nm.

8
Conclusion

• The paper showed how to implement such a method
  to assure the centering as lenses are assembled
  using a precise rotary table and a PSM.
• The method can sense the CC. of both surfaces of
  an element so that any errors in tilt and
  decenter can be corrected during assembly to the
  sub-micron and sub-second level.