Laser Tracker Optics

The ltoptics package allows accurate placement of flat mirrors and spherically mounted retro-reflectors on an optical table to locations specified in a Zemax OpticStudio prescription.

Getting Started

Dependencies

• Assumes a scientific python installation including numpy, tkinter, etc.
• This software has been run on Ubuntu, but in principle can be run on any platform
• The code currently supports the Leica AT4xx laser trackers. Usage with other laser trackers will require development of a light interface to support connection, initializaion, measuring at a specified location, and measuring the current location. See the file leica_at4xx.py for an example.
• If using the Leica AT4xx laser tracker you will need to download and install the CESAPI package from https://pypi.org/project/CESAPI/

Installing

• Download and unpack this repo.
• Copy the configuration file wfpt.yaml to your_project.yaml
• Edit ltoptics.py and change the config file name to your_project.yaml

Configuration file explained

laser_tracker:
    type: leica_at4xx
    host: 192.168.1.243

The laser_tracker type controls which module will be imported for communications with your laser tracker. If you have a different type of laser tracker you will need to write your own interface module, e.g. super_lt.py, modeled on the one in leica_at4xx.py

Zemax_origin_in_benchcoordinates: [155.63, 1537.35, -171.1]

These are the coordinates of the Zemax global coordinate reference surface measured in your bench coordinate system. You'll likely need to measure this in your CAD software.

Zemax_unitvectors_in_bench_coordinates:[
             [0, 0, 1],
             [0, 1, 0],
             [-1, 0, 0]]

This means that the Zemax X coordinate direction is congruent with the bench Z coordinate, Zemax Y with bench Y, and Zemax Z with bench -X.

smrs:
    0:
        name: Origin SMR
        layout_column: 0
        layout_row: 0
    1:
        name: X-axis SMR
        layout_column: 1
        layout_row: 0

These define the SMRs that are on your optical bench and the names and where they appear on the GUI. The final smr is the one that is used to make your measurement.

If you change the number of SMRs you also must add or delete lines from two files: permbench.txt and tempbenchcoords.txt

Executing program

cd your/path/ltoptics
python ltoptics.py

First steps

The first thing that needs to be done is to measure the positions of all the SMRs that are permanently attched to the bench. Aim the laser tracker at each of the SMRs and press the corresponding button in the GUI. Once all of them have been measured, press Save. Use the filename permbench.txt. This step needs to be done only once but must be done with the laser tracker in a position where it has a sightline to all of the SMRs.

In Zemax OpticStudio you must generate and save a Prescription report in the ltoptics folder. This should be done in ascii format (the default is Unicode).

Let's get going!

Mirror positioning

• The laser tracker is positioned where it can see three fixed retros as well as the positioning retro and its reflection in the mirror that is being aligned. With a crowded bench this may take some iteration. The positioning retro nest is secured to the bench.
• Start ltoptics
• Manually point the laser tracker in turn at exactly three of the fixed bench retros and measure by pressing the corresponding button.
• Aim the mirror measurement retro at the laser tracker and measure.
• Rotate the measurement retro within its nest to face the mirror so it can be seen by the laser tracker in reflection off the mirror.
• Press the "Measure mirror" button, which will cause the laser tracker to remeasure the three permanent retros, and then repeatedly measure the positioning retro. The mirror position is computed, and the x-tilt, y-tilt, and axial positioning errors are displayed on the screen updating at roughly 1,Hz.

Focal point retro positioning

• Sometimes in the alignment process there are times where a retro sphere has to be placed in the correct location so it can be used by an interferometer.
  The SMR is positioned using the laser tracker in a process identical to that for positioning a mirror except that the positioning retro is repeatedly measured directly, rather than in reflection. Once the SMR is positioned with the laser tracker, it is rotated in its nest so that the interferometer beam will hit the spherical surface.

Authors

Brian McLeod, Peyton Benac, Grant Meiners
Center for Astrophysics | Harvard & Smithsonian

Version History

• 1.0
  • Initial Release

Further reading

McLeod, B. et al 2022, Proc SPIE, etc etc.