2020.01.27

Steps for alignment set-up for Laser Truss

Stephanie and Heejoo

Equipment: Laser Tracker, Spatial Analyzer Software, Etalon unit, Allen key (two types, forget the sizes) for unlocking universal adjuster and fine tip tilt on collimator mounts. Laptop with Teamviewer connection to Etalon Unit. SMR holders a) Right angle bracket holder and b) 10 mm “pin-nest” holder.

1) Lock telescope at Zenith

2) Power on Etalon unit and alignment lasers. Make sure to email that laser is on.

3) Authorize LBC binocular and “collimate” telescope so that the primary mirrors are raised to their collimation model position for Zenith pointing.

4) Select collimators/retroreflector pairings such that angles between collimator beams onto a retroreflector are maximized. In our initial configuration try to ensure that at least 3 collimators illuminate each retro-reflector.

5) Rough aim collimators to retroreflectors using universal bearing and alignment laser. For

collimators with “old” monochromat lens, will need a dark dome.

6) Fine-tune adjust collimator pointing with tip/tilt adjusters on collimators, looking for return in the Etalon software on the “alignment” window. Maximize signal, repeatedly going through peak in both adjustment axes to ensure good “centering”.

7) Lock locking screws while checking alignment is maintained.

8) Record

a) Collimator position/number,

b) Etalon channel.

9) Laser Tracker Survey:

a) Power on Laser Tracker and connect with Spatial Analyzer software.

i)Go to Instrument > Add> Select OmniTracker 2

ii) Click on the drop down menu to run instrument interface and select API Device Interface

iii) Insert the ip address located on the back of the laser tracker and ping it to make sure you can communicate.

iv)The instrument control window should pop up as shown below. Set the units to millimeters.



b) Since this is a relatively infrequent task, maximize accuracy by preforming a QVC (calibration) of the laser tracker prior to beginning measurements.

c) Using the right-angle SMR holder, perform “stable point” measurements of the primary mirror with at least 30 points, in an arc around the primary mirror of at least

100 degrees. Clicking on the reflectors and add offsets to compensate for the use of the edge nest

With the Spatial Analyzer software set to “stable point measurement” move the SMR holder around the primary mirror at each measurement point pushing the vertical face of the SMR holder against the mirror edge and the horizontal face down onto the mirror surface.

d) Fit a circle to the mirror surface measurements. Click construct> circle then when Geometry Fit Interface window pops up press F2 to select points. Click ok when done selecting and press ENTER to generate circle

Go to the points list and discarding the most “noisy points” by unchecking them and hitting the update button until a good low-residual circle fit is obtained.

e) Produce a line in SA software through the centre of the circle at right angles to the circle. This will be the measurement frame “z axis”. Click on the cardinal points button in the Geometry Fit Interface window as show below. Click ok.

f) Make the “pin-nest” measurement at the “x-location” on the primary mirror cell top flange. This will be the point to “clock the x-axis” in the reference frame.

g) In spatial analyzer produce the measurement frame. Go to Construct>Frame > Frame Wizard. Using the object (circle, line) origin point (centre of circle) and x point to clock the x-axis.

h) Calculate the sagitta of the primary mirror (y^2/2R) and offset the origin point in the negative z direction by the right amount (~ 450 mm), taking into account the pin nest SMR Z offset (10 mm).

i) Offset the newly-made frame in Z by this sagitta, placing now the origin of the reference frame on the primary mirror vertex. Go to frame properties and click on transform and move the z

j) Measure the 3 retroreflectors on the LBC into a point group “LBC points”. You can use the Drive Head/OVC button on the device panel to steer the laser tracker towards the retros. Use Search/Lock to lock onto the retros. Take a single point measurement.

k) Survey the collimator locations. Set the laser tracker to Stable Point measurement, make a new point group “Collimator points”. With the Etalon unit alignment laser on, hold an SMR in front of the collimator and centre the alignment laser on the SMR (visual observation of the alignment laser reflection off the SMR surface). Stabliizing arm on mirror flange, hold the SMR still enough to obtain a stable point measurement. Note the point number against previously recorded collimator/Etalon channel identifier. Once measurements are complete, for each measurement, produce a line using the collimator stable point measurement as one point and the associated retro-reflector point as the second point. Get the Etalon distance measurement for that channel. Take the magnitude Etalon length for that channel. Go to LineName >Properties and check Ray Method and change the Length to the magnitude of the Etalon distance and press Transform. The Length will be added onto the end of the line so make sure you are adding the length to the right end (if it adds it to the side on LBC just undo and click the Reverse button to swap the Begining and End points). You will see that the line extends beyond the collimator point. Now go to Construct> Point > End of Line and make this coordinate is recorded as the collimator launch point (Fiber Tip) for that channel.

l) Once a set of collimator launch points is thus obtained together with the retroreflector

measurements, the alignment set up is complete and coordinates can be entered into Etalon software.

Topic revision: r1 - 11 Jun 2020, HeejoChoi
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