1. Lab 1 Paraxial Optics
Lab in 106A

2. Look at paraxial optics rules
Use a bi-convex singlet at 1:1 conjugates
Do it double pass so can see image
Lateral magnification
Axial magnification
See aberrations due to misalignment
Measure optical thickness
Placement between pupil and image

3. Test setup Use an autostigmatic microscope
Produces “perfect” point source of light (stigmatic)
Views reflected image on CCD camera
Is calibrated in real units
Autostigmatic mode Autocollimator mode

4. Hardware used

5. Details of set up
Object will be point source projected by an autostigmatic microscope
Lens will be a biconvex singlet used at roughly f/8 cone at 1:1 conjugates
The image will be located at the center of a ball, or convex mirror, so the light will be reflected back into the microscope
The video camera allows precise centering and focus of the return image
The ball reflector can be tricky since a Cat’s eye reflection can be gotten off the surface. How can you know you are focused at the center of the ball?
For those interested there is a Zemax file called Lab 1 1to1 relay with CB’s to permit tilts and decenters

6. Initial alignment
Set the laser source to MAX so the light from the object is visible
See that the light is going roughly through the center of the lens
Find the focused spot and move the ball until the spot is centered
Find the reflected light and move the ball until it goes through the lens
Find the focused spot near the objective and move the ball until the spot returning overlaps the spot coming from the objective
The return image should be visible on the video screen
Move the ball axially to find best focus. The screen will be saturated
Turn the intensity to MIN and continue the focus and centering
Move the ball back until the spot is focused on the surface (Cat’s eye)
Center the reference cross using the SET REF button

7. Initial alignment (con’t)
Move the ball toward the lens until the spot is again focused
Move the ball laterally to bring the spot on the reference cross
Does this suggest how to measure a radius of curvature?
Does the image look symmetrical? We’ll learn how to align things better later
How sensitive is the adjustment of the ball?
Touch the reflecting surface of the ball (lightly) What happens to the image?
Clean the ball surface with alcohol and a swab (gently)
Now we are ready for some measurements of image motion

8. Nominal optical setup Point source at object position
Biconvex lens at roughly 1:1 conjugates (Thorlabs LB4941) BK7
Ball centered on image to act as a convex mirror to return light to detector

9. Image and wavefront metrics
Defocus (mm)

10. Effect of lens decenter
Decenter lens 0.05 mm in +Y direction
Look at spot, now aberrated with astigmatism and coma
Let Zemax move ball sideways so image at center of ball
Look at spot again
Moved lens .07 mm, then moved return mirror .135 mm to restore image
.07/.135 ~ 200/380 or s/(s+s’)

11. Effect of lens tilt Tilt lens 5 degree in +alpha direction
Look at spot, now aberrated with astigmatism and coma
Let Zemax move ball sideways so image at center of ball
Look at spot again
Tilted lens 5 degrees, then moved ball -.06 mm to re-optimize image
Tilt has less effect but alignment doesn’t work well to restore image

12. Effect of adding plane parallel window
Total track increases by 1.87 mm, while first order (n – 1)t/n = 2.16 mm
However, window adds spherical aberration that affects focus
Spot before window after Wavefront before after

13. Effect of tilting window 15 degrees
Wavefront no longer symmetric Image shows astigmatism
Ball moved mm near to theory but not much bigger

14. Effect of thin poor quality window
Insert a microscope slide into the beam in place of the window. The slide in a poor quality window
Move the window by hand laterally. Position the window close to the lens and close to the ball. What is the difference in effect on the image?

15. Conclusion Record your results in your lab book
Photograph some of the test setups for later viewing
When finished, return the setup to its original condition so the next group is set to go right from the beginning
If there is not time to finish all the suggested experiments it is better to do and record what you can in a professional manner rather than rush through the experiments
The goal is to learn, have fun while doing so and be proud of your work